Black Holes: Going to Extremes

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Black Holes: Going to Extremes

From the Editors of Scientific American

The Science of Black Holes

If any object defies belief, black holes are it. The idea that any object could be so massive that even light can't escape was once considered impossible on its face, as light was a wave, massless and gravity wouldn't affect it at all. But black holes distort space itself, bending and stretching light around them. They can destroy stars and even might point the way to the long-sought link between relativity and quantum mechanics.

The idea of black holes is not new, though the term wasn't coined until the 1960s by physicist Robert Dicke. John Michell first proposed the idea of a star so massive that light would fall back to its surface in 1784, but after some initial interest it was dismissed as a mathematical curiosity. It would take Albert Einstein's theory of general relativity, and the work of physicists such as Karl Schwarzchild, Arthur Eddington and Georges Lamaître to show that yes, it was possible to have a region so dense that matter would collapse and nothing, not even light, could escape. Once again, it was difficult for physicists to believe that such an object could exist in reality–surely it was a mathematical quirk that would soon go away, because in real life we don't see regions where physical phenomena take on infinite values, a condition called a singularity. However, by the 1960s it was clear the singularities weren't going away. Clearly there was something out there, maybe, that was unlike any astronomical object then known.

Then, finally, in the 1960s, surveys of x-ray sources showed an object called Cygnus X-1 that seemed to have the characteristic radiation emission expected from a black hole taking up matter. The question was no longer whether black holes existed, it was what the nature of these strange objects–what can they tell us about the universe. Which brings us to the works in this eBook.

We begin with seeing back holes in the first place. John Matson, in "Evidence Points to Violent Past for the Milky Way's Black Hole" takes us through the evidence that a black hole containing the mass of millions of suns lies at the center of our galaxy. In "Ripples in Spacetime," Clara Moskowitz recounts the detection of gravitational waves, confirming a key part of Einstein's century-old theory. Davide Castelvecchi, in "Here Come the Waves," introduces us to the new vistas that gravitational wave astronomy opens up–including the behavior of black holes. And while we cannot see a black hole directly, Seth Fletcher writes the story of the first direct imaging of the "shadow" of a black hole; the light from the matter falling into the event horizon, the point of no return.

The second part of the book includes stories of where black holes come from. We know they can form from collapsing stars, but that isn't the whole story. Yasemin Saplakoglu, in "Zeroing In on How Supermassive Black Holes Formed," outlines the research trying to answer the question of how the black holes at the centers of galaxies got so large. And the history of black holes in the universe may go back to the very beginning, as Nola Taylor Redd writes in "Did Astronomers Just Discover Black Holes from the Big Bang?" Priyamvada Natarajan also tackles the mystery of where the very first black holes came from in "The First Monster Black Holes," while Charlie Wood in "Black Hole Factories May Hide at Cores of Giant Galaxies" asks why so many of the black holes we see are more massive than expected.

In part three we look at the ways black holes can open up new vistas in physics–offering science-fictional possibilities such as wormholes to distant parts of the universe, as in Juan Maldacena's "Black Holes, Wormholes and the Secrets of Quantum Spacetime." Here black hole might offer a path to unifying general relativity and quantum mechanics, a goal that physicists have sought for decades. Steven Giddings, in "Escape from a Black Hole" and Yasunori Yomura in "Have We Solved the Black Hole Information Paradox?" examine one of the biggest physical mysteries: what happens to the information encoded in an object when it falls into a black hole, since that information can't be destroyed as far as we know?

Part four delves further into our ignorance. In "Black Hole Pretenders Could Really Be Bizarre Quantum Stars" Charles Q. Choi writes about the possibility of a new kind of astrophysical creature, similar to a black hole but one in which gravity becomes repulsive. Clara Moskowitz covers the discovery of what happens when black holes gorge on matter. Linking black holes to another ongoing area of research, Juan García-Bellido, and Sébastien Clesse ask if black holes might be the mysterious dark matter that makes up most of the universe.

Finally, Abraham Loeb asks a question beloved of science fiction fans: what would happen to life near a black hole? Could a civilization survive? And might we be able to send an expedition to one?

Black holes are a journey to the very edges of physics and even the universe itself–and in this book we hope you enjoy the ride.

-Jesse Emspak
eBook Editor

SECTION 1

Seeing the Unseen: Finding Black Holes

Evidence Points to Violent Past for the Milky Way's Black Hole

by John Matson

At the center of our galaxy is a sleeping giant, a black hole more than four million times as massive as the sun. The Milky Way’s supermassive black hole is mostly quiet, nibbling on small objects at the galactic center and giving off only faint belches of radiation as it digests its prey. But in the past the sleeping giant may have been wide awake—and wreaking havoc.

Astrophysicists at the Harvard–Smithsonian Center for Astrophysics uncovered evidence of two ghostly but enormous beams of gamma rays, each extending some 30,000 light-years from the galactic center, that seem to mark a violent episode of black hole consumption relatively recently in cosmic history.

Supermassive black holes in some other galaxies are much more active than Sagittarius A* (Sgr A*), the lurker at the center of the Milky Way. As black holes devour gas, dust or even fully formed stars, matter compresses and heats up in an accretion disk outside the event horizon, a black hole's point of no return. That messy accretion process can produce copious radiation or outflows of plasma. As a result, feeding black holes can glow brightly, even across billions of light-years, and some emit giant jets of particles that extend away from the black hole. But such jets had not been identified near our own, mostly dormant, supermassive black hole.

The jets appear as faint lines in maps of the galaxy made by NASA's Fermi Gamma-Ray Space Telescope. Harvard graduate student Meng Su and astrophysicist Douglas Finkbeiner identified the feature by subtracting known gamma-ray sources from Fermi's maps and teasing out the patterns in the diffuse emission left behind. Their finding appeared in a study published in The Astrophysical Journal in 2012. The jets had not been detected at other wavelengths or with other telescopes, but next-generation x-ray space telescopes may be able to confirm or refute their existence.

If the jets emanating from Sgr A* prove to be real features and not just a trick of the eye, they would indicate that, not so long ago, the black hole was much feistier than it is now. One explanation for the jets is that during a feeding-driven outburst, material was funneled outward from the black hole along narrow, oppositely directed jets constrained by a corkscrewing magnetic field. High-energy electrons rushing along those jets collided with background photons, boosting them up to gamma-ray energies, which Fermi's Large Area Telescope can detect.

But just when the jets would have been produced is not simple to pin down—the gamma rays now reaching telescopes could be relics of a whole series of outbursts. Indeed, the hypothesized jets are not the first piece of indirect evidence that Sgr A* gorges itself from time to time. Radiation bouncing off of molecular clouds in the galaxy has been interpreted as an echo of earlier outbursts just hundreds of years ago. "We know that even on a few-century timescale the black hole flickers on and off," Finkbeiner says.

The newfound jets may be related to another feature that Su and Finkbeiner, along with their colleague Tracy Slatyer, now at the Institute for Advanced Study, recently spotted extending from the galactic center. In 2010 the researchers, again using the Fermi telescope, identified huge bubbles above and below the plane of the Milky Way, glowing with gamma rays. The bubbles could also have been produced by energetic electrons, ejected during a black hole feeding frenzy, colliding with mundane photons and raising them up to gamma-ray energies. The so-called Fermi bubbles and the jets may stem from the same outburst a million or so years ago—the jets would be the high-velocity black-hole output, and the bubbles the decelerated material spreading outward in a series of shocks and eddies. But whereas the jets and bubbles overlap in location and scale, the jets are tilted by about 15 degrees, and the researchers note that they could result from separate episodes.

If the two features are indeed related, the jets must have been much more robust at some point in the past to provide the energy that carved the Fermi bubbles out of intergalactic space. And, in turn, the black hole must have been much more voracious as well. "It didn't get to be four [million] or five million solar masses by never doing anything," Finkbeiner says. "At some time in the past it was consuming large amounts of material and making a mess of things."

--Originally published: Scientific American online, June 1, 2012.

Ripples in Spacetime

by Clara Moskowitz

On February 11, 2016, scientists announced that the Laser Interferometer Gravitational-Wave Observatory (LIGO) had discovered ripples in spacetime created by merging black holes. About 1.3 billion years ago two black holes swirled closer and closer together until they crashed in a furious bang. Each black hole packed roughly 30 times the mass of our sun into a minute volume, and their head-on impact came as the two were approaching the speed of light. The staggering strength of the merger gave rise to a new black hole and created a gravitational field so strong that it distorted spacetime in waves that spread throughout space with a power about 50 times stronger than that of all the shining stars and galaxies in the observable universe.

Such events are, incredibly, thought to be common in space, but this collision was the first of its kind ever detected and its waves the first ever seen. At the much anticipated LIGO conference in Washington, D.C. (one of at least five simultaneous events held in the U.S. and Europe), the scientists announced that the more than half-century search for gravitational waves had finally succeeded.

“This was truly a scientific moonshot, and we did it, we landed on the moon,” LIGO executive director David Reitze said during the announcement.

“There are people who’ve put their entire life into this search, and there are people who died before having a chance to see anything,” says LIGO team member Szabolcs Márka, a physicist at Columbia University. “It’s really a wonderful feeling that you have validated the investment of the tremendous amount of work. And it’s not just that you found something, but you gave something to everybody, to the rest of the human race.”

Albert Einstein first predicted gravitational waves in 1916 based on his general theory of relativity, but even he waffled about whether or not they truly exist. Scientists began seeking these ripples in spacetime in the 1960s, but none succeeded in measuring their effects on Earth until now. LIGO’s first discovery, accepted for publication in Physical Review Letters, not only provided the first direct evidence for gravitational waves but also opens the door to using them to study the powerful cosmic events that create them. “It’s a huge deal,” says Luis Lehner, a physicist at the Perimeter Institute for Theoretical Physics in Ontario, who is not affiliated with the LIGO project. “It has pushed the fundamental theory of gravity forward in a very strong way and gives us an incredible tool to probe very deep questions of the universe.”

After the initial observation, the LIGO team announced two more discoveries of gravitational waves, including a signal presented later in 2016 created by two black holes that were smaller in mass than the original pair and another signal reported this past spring from two black holes about three times as distant as the first collision.

“One of the Most Complex Systems Ever Built”

More than 1,000 scientists work on the $1-billion LIGO experiment, which is funded by the National Science Foundation. The project uses two detectors, one located in Washington State and the other in Louisiana, to sense the distortions in space that occur when a gravitational wave passes through Earth. Each detector is shaped like a giant L, with legs four kilometers long. Laser light bounces back and forth through the legs, reflecting off mirrors, and amazingly precise atomic clocks measure how long it takes to make the journey. Typically the two legs are exactly the same length, and so the light takes exactly the same amount of time to traverse each. If a gravitational wave passes through, however, the detector and the ground underneath it will expand and contract infinitesimally in one direction, and the two perpendicular legs will no longer be the same size. One of the lasers will arrive a fraction of a second later than the other.

LIGO must be unbelievably sensitive to measure this change in the length of the legs, which is smaller than one ten-thousandth the diameter of a proton, or less than the size of a soccer ball compared with the span of the Milky Way. “It’s one of the most complex systems ever built by mankind,” Márka says. “There are so many knobs to turn, so many things to align, to achieve that [sensitivity].” In fact, the experiment is so delicate that unrelated events such as an airplane flying overhead, wind buffeting the building or tiny seismic shifts in the ground below the detector can disturb the lasers in ways that mimic gravitational signals. “If I clap in the control room, you will see a blip,” says Imre Bartos, another member of the LIGO team at Columbia. The researchers carefully weed out such contaminating signals and also take advantage of the fact that the detectors in Washington and Louisiana are highly unlikely to be affected by the same contamination at the same time. “By comparing the two detectors, we can be even more certain that what we are seeing is something that’s coming from outside Earth,” Bartos adds.

LIGO began its first run in 2002 and hunted through 2010 without finding any gravitational waves. The scientists then shut down the experiment and upgraded nearly every aspect of the detectors, including boosting the power of the lasers and replacing the mirrors, for a subsequent run, called Advanced LIGO, that began officially on September 18, 2015. Yet even before then, the experiment was up and running: the signal arrived on September 14 at 5:51 a.m. Eastern time, reaching the detector in Louisiana seven milliseconds before it got to the detector in Washington. Advanced LIGO is already about three times more sensitive than the initial LIGO and is designed to become about 10 times more sensitive than the first iteration in the next few years.

Long Time Coming

Before this discovery, the strongest evidence of gravitational waves came indirectly from observations of superdense, spinning neutron stars called pulsars. In 1974 Joseph Taylor, Jr., and Russell Hulse discovered a pulsar circling a neutron star, and later observations showed that the pulsar’s orbit was shrinking. Scientists concluded that the pulsar must be losing energy in the form of gravitational waves—a discovery that won Taylor and Hulse the 1993 Nobel Prize in Physics. Ever since this clue, astronomers have been hoping to detect the waves themselves. “I’ve certainly been looking forward to this event for a long time,” Taylor says. “There is a long history, and I think projects that take this long to bear fruit require an awful lot of patience. It’s about time.”

The discovery is not just proof of gravitational waves but the strongest confirmation yet for the existence of black holes. “We think black holes exist out there. We have very strong evidence they do, but we don’t have direct evidence,” Lehner says. “Everything is indirect. Given that black holes themselves cannot give any signal other than gravitational waves, this is the most direct way to prove that a black hole exists.”

LIGO’s ability to study the characteristics of gravitational waves will allow scientists to study black holes in a whole new way. Researchers would like to know the details of how two black holes collide and whether a new black hole arises as theory suggests. “We’re talking about two objects that do not emit light—they’re completely dark,” says Janna Levin, a theorist at Barnard College, who is outside the LIGO collaboration. “In the details of a collision and in terms of the gravitational waves, you could see the formation of a new black hole.” The observatory should also be able to detect gravitational waves created by other cataclysmic events, such as exploding supernovae and collisions of two neutron stars.

LIGO and future gravitational-wave experiments will also allow physicists to put general relativity to the test. The 100-year-old theory has stood the test of time, but it still conflicts with the theory of quantum mechanics that rules over the subatomic realm. “We know general relativity should show cracks at some point, and the way it shows them will guide our theory to one that is more complete,” Lehner says. “This is pushing the theory over six orders of magnitude compared with the previous strongest test,” which came from observations of pulsars.

Advanced LIGO is now on its second observing run and will shut down for upgrades over a period of about 15 months starting later this year. Scientists expect to increase LIGO’s sensitivity by a factor of four or five times, which should allow it to detect black hole binaries about 100 times as often as it currently does. LIGO is the first of many observatories that will join this new era of gravitational astronomy. A similar project, Italy’s Advanced Virgo experiment, has joined with LIGO, although it is running at a lower sensitivity. Later this decade the Kamioka Gravitational Wave Detector (KAGRA) in Japan will begin observations. And plans for a detector in India are in the works. Ground-based telescope projects called pulsar timing arrays aim to study gravitational waves by noting delays in light from pulsars arriving on Earth after traveling through wave-stretched space. And a spacecraft called Lisa Pathfinder concluded in June 2017 its 16-month mission to test technology for a proposed space-based observatory that will be sensitive to longer-wavelength gravitational waves from supermassive black hole collisions.

“Every time you open a new window to the universe we always discover new things,” Lehner says. “It’s like Galileo pointing the first telescope to the sky. Initially he saw some planets and moons, but then as we got radio, UV and x-ray telescopes, we discovered more and more about the universe. We are pretty much at the moment where Galileo was beginning to see the first objects around Earth. It will have such a huge impact on the field.”

--Originally published: Scientific American 26(4s); 26-29 (Fall 2017).

Here Come the Waves

by Davide Castelvecchi

In the mid-1980s Bernard Schutz came up with a new solution to one of astronomy’s oldest problems: how to measure the distance from the earth to other objects in the cosmos. For generations, researchers have relied on an object’s brightness as a rough gauge for its distance. But this approach carries endless complications. Dim, nearby stars, for example, can masquerade as bright ones that are farther away. Schutz, a physicist at Cardiff University in Wales, realized that gravitational waves could provide the answer. If detectors could measure these ripples in spacetime emanating from interacting pairs of distant objects, scientists would have all the information needed to calculate how strong the signal was to start with—and so how far the waves must have traveled to reach the earth. Thus, he predicted, gravitational waves could be unambiguous markers of how quickly the universe is expanding.

His idea was elegant but impractical: nobody at the time could detect gravitational waves. But in August 2017 Schutz finally got the opportunity to test this concept when the reverberations of a 130-million-year-old merger between two neutron stars passed through gravitational-wave detectors on the earth. As luck would have it, the event occurred in a relatively nearby galaxy, producing a much cleaner first measure than Schutz had dreamed. With that one data point, Schutz was able to show that his technique could become one of the most reliable for measuring distance. “It was hard to believe,” he says. “But there it was.”

More mergers like that one could help researchers to resolve an ongoing debate over how fast the universe currently is expanding. But cosmology is just one discipline that could make big gains through detections of gravitational waves in the coming years. With a healthy bunch of discoveries already under their belts, gravitational-wave scientists have a long list of what they expect more data to bring, including insight into the origins of the universe’s black holes; clues about the extreme conditions inside neutron stars; a chronicle of how the universe structured itself into galaxies; and the most stringent tests yet of Albert Einstein’s general theory of relativity. Gravitational waves might even provide a window into what happened in the first few moments after the big bang.

Researchers have started working down this list with the help of the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo observatory near Pisa, Italy, and a similar detector in Japan. They will get an extra boost from space-based interferometers and from terrestrial ones that are still on the drawing board—as well as from other methods that could soon start producing their own first detections of gravitational waves.

Like many scientists, Schutz hopes that the best discoveries will be ones that no theorist has even dreamed of. “Any time you start observing something so radically new, there’s always the possibility of seeing things you didn’t expect.”

Making Waves

When two black holes or neutron stars spiral into each other, they produce distinctive ripples in spacetime called gravitational waves. Teams with LIGO’s two detectors in the U.S. and with Virgo, the observatory’s counterpart in Italy, have announced the detection of 10 events so far.

Illustration by Nik Spencer/Nature

Spinning Clues

For a field of research that is not yet five years old, gravitational-wave astronomy has delivered discoveries at a staggering rate, outpacing even the rosiest expectations. In addition to the discovery in 2017 of the neutron-star merger, LIGO has recorded 10 pairs of black holes coalescing into larger ones since 2015. The discoveries are the most direct proof yet that black holes truly exist and have the properties predicted by general relativity. They have also revealed, for the first time, pairs of black holes orbiting each other.

Researchers now hope to find out how such pairings came to be. The individual black holes in each pair should form when massive stars run out of fuel in their cores and collapse, unleashing a supernova explosion and leaving behind a black hole with a mass ranging from a few to a few dozen suns.

There are two leading scenarios for how such black holes could come to circle each other. They might start as massive stars in each other’s orbit and stay together even after each goes supernova. Alternatively, the black holes might form independently but be driven together later by frequent gravitational interactions with other objects—something that could happen in the centers of dense star clusters.

Either way, the objects’ energy gradually disperses in the form of gravitational waves, a process that pulls the pair into an ever tighter and faster spiral, eventually fusing into one more massive black hole. Ilya Mandel, a LIGO theorist at the University of Birmingham in England, says that for LIGO and Virgo to see such pairs merge, typical black holes need to have started their mutual orbit separated by a distance of less than one quarter that between the earth and the sun. “If you start out with the two black holes any farther apart, it will take longer than the age of the universe” for them to merge, Mandel says.

The black hole mergers discovered so far are not sufficient to determine which formation scenario dominates. But in an analysis of the first three detections, a group including Mandel and Will M. Farr, a theoretical astrophysicist and LIGO member at the University of Birmingham, suggested that just five more observations could provide substantial evidence in favor of one scenario or the other. This would involve scrutinizing the gravitational waves for clues about how black holes rotate: those that pair up after forming independently should have randomly oriented spins, whereas those with a common origin should have spin axes that are parallel to each other and roughly perpendicular to the plane in which they orbit.

Further observations could also provide insight into some of the fundamental questions about black hole formation and stellar evolution. Collecting many measurements of masses should reveal gaps—ranges in which few or no black holes exist, says Vicky Kalogera, a LIGO astrophysicist at Northwestern University. In particular, “there should be a paucity of black holes at the low-mass end,” she says, because relatively small supernovae tend to leave behind neutron stars, not black holes, as remnants. And at the high end—around 50 times the mass of the sun—researchers expect to see another cutoff. In very large stars, pressures at the core are thought eventually to produce antimatter, causing an explosion so violent that the star simply disintegrates without leaving any remnants at all. These events, called pair-instability supernovae, have been theorized, but so far there has been scant observational evidence to back them up.

Eventually the black hole detections will delineate a map of the universe in the way galaxy surveys currently do, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology, who was the principal designer of LIGO. Once the numbers pile up, “we can actually begin to see the whole universe in black holes,” he says. “Every piece of astrophysics will get something out of that.”

To ramp up these observations, LIGO and Virgo have plans to improve their sensitivity, which will reveal not only more events but also more details about each merger. Among other things, physicists are eager to see the detailed “ringdown” waves that a postmerger black hole emanates as it settles into a spherical shape—an observation that could potentially reveal cracks in the general theory of relativity.

Having more observatories spread around the globe will also be crucial. One such detector is KAGRA, a detector under construction deep underground in Japan. Its location—and in particular its orientation with respect to incoming waves— will complement LIGO’s and Virgo’s and enable researchers to nail down the polarization of the gravitational waves, which encodes information about the orientation of the orbital plane and the spin of the spiraling objects. And India is planning to build another observatory in the next decade, made in part with spare components from LIGO.

An even bigger trove of discoveries could come from observing neutron star mergers. So far researchers have announced only one such detection, called GW170817. That signal, seen in August 2017, was almost certainly the most intensely studied event in astronomy’s history. And it solved a number of longstanding mysteries in one stroke, including the origin of gold and other heavy elements in the universe, as well as the cause of some gamma-ray bursts.

Further observations could allow scientists to explore the interiors of these objects. Neutron stars are thought to be as dense as matter can possibly be without collapsing into a black hole, but exactly how dense is anybody’s guess. No laboratory experiment can study those conditions, and there are dozens of proposals for what happens there. Some theories predict that quarks—the subatomic components that make up protons and neutrons—should break free from one another and roam about, perhaps in superconducting, superfluid states. Others posit that heavier “strange” quarks form and become part of exotic cousins of the neutron.

Pinning down the radii of neutron stars might allow physicists to evaluate the theories because they predict different “equations of state”—formulas that link pressure, temperature and density of matter. Such equations determine to what extent matter can be compressed and thus how wide or narrow a neutron star will be for a given mass and how massive such stars can get.

The 100-second-long signal in August 2017 eventually became too high in pitch for LIGO and Virgo to detect, which prevented the observatories from seeing the two neutron stars’ final moments, when they should have deformed each other in ways that would have revealed their size and hardness or resistance to compression. Still, says Bangalore S. Sathyaprakash, a LIGO theoretical physicist at the Pennsylvania State University, from that one event, “we can rule out equations of state that allow neutron star sizes larger than 15 kilometers in radius”—a figure that is consistent with other measurements and favors “softer” matter.

Future detections—and detectors—will give much more detail. Sathyaprakash says that the Einstein Telescope, a possible next-generation observatory dreamed up by a team in Europe, could take physicists far beyond an upper limit. “We want to be able to pin down the radius to the level of 100 meters,” he says—a precision that would be astounding, given that these objects are millions of light-years away.

The Gravitational-Wave Spectrum

Much like electromagnetic waves, gravitational waves are emitted by many different objects over a wide range of frequencies. Terrestrial interferometers such as the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo are sensitive to only a subset of those frequencies, which limits their ability to “see” certain cosmic phenomena. They will not detect collisions of supermassive black holes found in the hearts of galaxies, for example. But space-based interferometers and other approaches picking up gravitational waves could extend physicists’ reach.

Illustration by Nik Spencer, Nature

Siren Calls

Signals similar to GW170817, which was observed through both gravitational waves and light, could have dramatic implications for cosmology. Schutz calculated in 1985 that the frequency, or pitch, of waves from spiraling objects, together with the rate at which that pitch increases, reveals information about the objects’ collective mass. That determines how strong their waves should be at the source. By measuring the strength of the waves that reach the earth—the amplitude of the signal actually picked up by interferometers—one can then estimate the distance that the waves have traveled from the source. All other things being equal, a source that is twice as far, for example, will produce a signal half as strong. This type of signal has been dubbed a standard siren, in a nod to a common method of gauging distances in cosmology: stars called standard candles have a well-known brightness, which allows researchers to work out their distance from the earth.

By coupling the distance measurement of GW170817 with an estimate of how fast the galaxies in that region are receding from the earth, Schutz and his collaborators made a new and completely independent estimate of the Hubble constant—the universe’s current rate of expansion. The result, part of a crop of papers released by LIGO, Virgo and some 70 other astronomy teams in October 2017, “ushers in a new era for both cosmology and astrophysics,” says Wendy L. Freedman, an astronomer at the University of Chicago, who has made highly precise measurements of the Hubble constant using time honored but less direct techniques.

As a direct and independent measure of this constant, standard sirens could help to resolve a disagreement among cosmologists. State-of-the-art techniques, refined over nearly a century of work that started with Edwin Hubble himself, now give estimates that differ by a few percent. This first standard-siren measurement does not resolve the tension: the expansion rate it predicts falls somewhere in the middle of the range and, because it is based on just one merger event, has a large error bar. But in the future, researchers expect standard sirens to nail down the Hubble constant with an error of less than 1 percent. So far standard candles have done it with precisions of 2 to 3 percent.

Cosmic Signposts

Neutron star mergers are new tools for measuring the Hubble constant—the current expansion rate of the universe.

Illustration by Nik Spencer, Nature

Standard sirens could become even more powerful tools with space-based interferometers such as the Laser Interferometer Space Antenna (LISA), a trio of probes that the European Space Agency, which is leading the mission, plans to launch in the 2030s. LISA is designed to be sensitive to low-frequency waves that ground-based observatories cannot detect. This would give it access to more massive systems, which radiate stronger gravitational waves. In principle, LISA could pick up sirens from across the universe and, with the help of conventional telescopes, measure not just the current rate of cosmic expansion but also how that rate has evolved through the eons. Thus, LISA could help ad dress cosmology’s biggest puzzle: the nature of dark energy, the as yet unidentified cosmic component that is driving the universe’s expansion to accelerate.

Whereas ground-based interferometers detect events that are brief and far between, LISA is expected to hear a cacophony of signals as soon as it turns on, including a constant chorus of tight binary white dwarfs—the ubiquitous remnants of sun-sized stars—in our own galaxy. “It’s as if we lived in a noisy forest and we had to single out the sounds of individual birds,” says astrophysicist Monica Colpi of the University of Milan–Bicocca in Italy, who is part of a committee setting the mission’s science goals.

Occasionally, LISA should see black hole mergers such as the ones LIGO does, but on a much grander scale. Most galaxies are thought to harbor a central supermassive black hole that weighs millions or even billions of solar masses. Over a scale of billions of years, galaxies might merge several times; eventually their central black holes might merge, too. These events are not frequent for individual galaxies, but because there are trillions of galaxies in the observable universe, a detectable merger should occur somewhere at least a few times per year. Scientists are also pursuing a separate way of detecting gravitational waves from pairs of these behemoths at earlier stages of their orbits. Using radio telescopes, they monitor pulsars inside the Milky Way and look for small variations in their signals caused by the passage of gravitational waves through the galaxy. Today there are three “pulsar-timing arrays,” in Australia, Europe and North America and a fourth forming in China.

Thanks to LISA’s planned sensitivity and the strong signals produced by spiraling supermassive black holes, the observatory should be able to pick up gravitational waves from pairs of supermassive black holes months before they merge and see the merger in enough detail to test general relativity with high precision. After years of operation, LISA could accumulate enough distant events for researchers to reconstruct the hierarchical formation of galaxies—how small ones combined to form larger and larger ones—in the universe’s history.

Illustration by Nik Spencer

On the ground, too, physicists are beginning some “grand new ventures,” Weiss says. A U.S. team envisions a Cosmic Explorer with 40-kilometer detecting arms—10 times as long as LIGO’s—that would be sensitive to signals from events much farther away, perhaps across the entire observable universe.

The concept for the Einstein Telescope in Europe calls for a detector with 10-kilometer arms arranged in an equilateral triangle and placed in tunnels 100 meters or so underground. The quiet conditions there could help to broaden the observatory’s reach to frequencies one tenth those detectable by current machines. That might allow scientists to find black holes beyond the range thought to be prohibited by pair-instability supernovae; at high enough masses, stars should have a different collapse mechanism and be able to form black holes of 100 solar masses or more.

If scientists are lucky, gravitational waves might even let them access the physics of the big bang itself at epochs that are not observable by any other means. In the first instants of the universe, two fundamental forces—the electromagnetic force and the weak nuclear force—were indistinguishable. When these forces separated, they might have produced gravitational waves that, today, could show up as a “random hiss” detectable by LISA, Schutz says. This hypothetical signal is distinct from a much longer-wavelength one from even earlier on, which might appear in the universe’s oldest visible radiation: the cosmic microwave back ground. In 2014 a team reported that it had ob served this effect with the BICEP2 telescope at the South Pole, but the researchers later acknowledged problems with that interpretation.

With the reopening of both LIGO and Virgo in 2019, the next big discovery on Weiss’s wish list is the signal from a collapsing star—something that astronomers might also observe as a type of supernova. But he has high hopes for what else might be on the horizon. “If we don’t see something that we hadn’t thought of,” Weiss says. “I’d be disappointed.”

--Originally Published: Scientific American 28(2s); 46-53 (Spring 2019).

Finally, the Face of a Black Hole

by Seth Fletcher

It actually worked.

The Event Horizon Telescope (EHT) has always sounded like an unlikely proposition: create a virtual Earth-size telescope to take a picture of the shadow of a black hole. Every piece of that sentence is a little bit insane.

But it worked. After more than a decade of technical development and fundraising and astronomer-herding and telescope-wrangling, the astronomers of the EHT unveiled the first picture of a black hole—a bubble of pure gravity, a hole in spacetime, a prediction of general relativity so strange that Albert Einstein himself long refused to believe the concept was possible.

In six simultaneous press conferences on four continents, astronomers lit up projection screens with an image that, for experts at least, is unmistakable: a dark circle surrounded by a ring of light and lopsided fiery blobs, clumps of matter heated to 100 million degrees and circling the black hole at close to the speed of light.

Technically, this is a picture of the shadow of a black hole: specifically, a supermassive black hole—a 6.5-billion-solar-mass beast 55 million light years from Earth—at the center of the galaxy Messier 87. Black holes trap everything that falls in, including light, which is why they are black. In a sense, they are fundamentally unseeable, one-way cosmic escape hatches leading to … well, that part is unclear. But because of the way they warp spacetime, they impose a dark silhouette on the glowing, superheated matter that circles them. This is the barely imaginable scene the EHT captured during an observing run two years ago, and revealed to the world today.

Since the 1960s, when indirect astronomical evidence and breakthroughs in theoretical physics made the existence of black holes all but undeniable, these objects have been in a sort of epistemological limbo: they were the most likely explanation for all sorts of otherwise inexplicable phenomena, but no one held out much hope of ever seeing one.

That changed in the late 1990s, when astronomers around the world began building powerful high-frequency radio observatories on high ground in Hawaii, Mexico, Chile and the South Pole. Theoretical astrophysicists calculated that, in theory, uniting these telescopes using a technique called Very Long Baseline Interferometry would allow them to see the shadows of supermassive black holes that were sufficiently huge and close to Earth. The biggest prize has always been Sagittarius A*, the four-million solar-mass black hole at the center of the Milky Way. Sagittarius A* remains unseen—at least by the public. But M87, the only other known black hole within the EHT’s grasp, has now been captured.

Usually big scientific discoveries like this leak early, but this time the astronomers kept surprisingly tight control. France Córdova, the director of the National Science Foundation—which invested $28 million in the EHT over the years—saw the image for the first time at the morning press conference. It moved her to tears.

The NSF and other funding institutions promised that the result would be “groundbreaking,” and it certainly was—the first direct evidence for the existence of event horizons, the defining feature of a black hole. But anyone hoping that this first image would chip away at Einstein’s iron-clad theory of general relativity came away disappointed: the black hole looks pretty much exactly like the century-old theory says it should.

But this first picture is only a start. Since the 2017 observation, the EHT array has already grown in size and acuity, with a new telescope joining and another on the way, and with the installation of ever-more-powerful digital signal processing hardware at each site. Sera Markoff, an astrophysicist with the University of Amsterdam, said in the future all sorts of other telescopes will join the EHT to observe M87, Sagittarius A* and other black holes simultaneously in many different bands of light—radio, x-ray, gamma ray, infrared and optical.

It is also worth noting that in the two hours after the press conference, at least six scientific papers on the observation appeared online. They almost certainly contain clues and new questions that will take more time to process than a 24/7 news cycle can tolerate. For now, though, it is worth pausing for a moment to consider the strangeness of nature, and the remarkable fact that these sentient, tool-using bipeds on a small world in a backwater solar system somehow managed to turn their planet into a telescope and take a picture of an exit chute from the universe.

--Originally Published: Scientific American Online, April 10, 2019.

The Event Horizon Telescope, the Hawking Effect and the Foundations of Physics

by Steve Giddings

The Event Horizon Telescope’s spectacular success in taking pictures of the black hole in the galaxy M87 becomes even more exciting when one realizes its observations may possibly provide clues about one of the deepest puzzles in physics.

This is the “paradox” of black hole information, which points to a basic inconsistency between the existence of black holes and the quantum-mechanical laws believed to describe our universe. Resolving this inconsistency may require a conceptual revolution as profound as the overthrow of classical physics by quantum mechanics.

Black holes are apparently ubiquitous in the cosmos, and yet remain the most mysterious objects it contains; their very existence threatens our present foundations of physics. Observations like EHT’s, and the recent gravitational wave detections of LIGO, are providing increasingly good evidence for their existence. But, while the basic principles of quantum mechanics are thought to govern all the other laws of nature, when they are applied to black holes they lead to a contradiction, exposing a basic flaw the current form of these laws. So, the most profound role of black holes may be in teaching us something new and deep about the fundamental laws of nature.

This problem arises from one of the simplest questions asked when one first encounters black holes: what happens to stuff that fall into them? A little refinement is needed. First, in our present quantum-mechanical laws, matter can change into different forms; particles can for example change into different kinds of particles. But the one thing that is sacred and never destroyed is quantum information. If we know the complete quantum description of a system, we should always be able to exactly determine its earlier (or later) quantum description with no loss of information. So a more careful question is what happens to quantum information that falls into a black hole.

And what may be Stephen Hawking’s greatest discovery is his prediction that black holes destroy quantum information. This arises from his calculations showing that black holes evaporate; they emit particles and shrink until they are expected to completely disappear. The problem is that the emitted particles carry essentially no information about what went into the black hole. So, his calculations appear to show that quantum information that falls into a black hole is ultimately destroyed, contradicting quantum mechanics.

This creates a deep crisis in physics. Great advances have followed from previous such crises. One that seems very parallel to the black hole crisis was that of atomic stability, which led to the quantum revolution in physics. In that case, classical physics predicted the fundamental instability of atoms, in obvious contradiction with the existence of stable matter. But, by the time the stability of matter was understood, the laws of physics had been rewritten in a fundamentally different way. Increasingly it seems that the black hole crisis is similar in that it will lead to another paradigm shift in physics.

At first Hawking suggested that it was quantum mechanics that fails, and information destruction is allowed. However, it was soon realized that this implies a drastic breakdown of energy conservation, which would utterly destroy our present description of the universe. Apparently, the resolution needs to be sought elsewhere.

Another idea was that black holes don’t completely evaporate but instead stop shrinking at a tiny size, leaving behind microscopic remnants storing the original information. But, it was then realized, basic quantum principles would predict disastrous instabilities in which ordinary matter explodes into such remnants, also contradicting everyday experience.

Other ideas continue to be explored. One idea is that some novel physics prevents black holes from forming. This would be very strange, since very large black holes can form when the surface density of collapsing matter reaches that of, say, ordinary water, and we believe we understand physics at such densities. Another idea is that new physical processes cause black holes to transform into massive remnants of a new kind, containing the original information, long before they reach microscopic size. Versions of both these scenarios have been explored both within and outside string theory, which some believe to be the correct approach to reconciling quantum mechanics with gravity. For example, a proposal in string theory is that a black hole transforms into a massive remnant called a “fuzzball,” or that a fuzzball forms instead of a black hole forming. But, these proposals raise other important problems.

Specifically, in Einstein’s description, in which gravity corresponds to curvature of spacetime, information must never propagate superluminally—faster than the speed of light. The very definition of a black hole says that once one forms, the stuff inside its event horizon can’t escape because that would require faster-than-light travel. So, if some new process were to transmit internal information to the exterior, say as part of a process converting a black hole into a massive remnant, that information would have to travel superluminally. A similar need for superluminal signaling would apparently be needed in scenarios where a black hole is prevented from forming in the first place.

Superluminal signaling can lead to serious trouble. The prohibition on sending a superluminal signal is ordinarily referred to as locality. In empty flat space, violation of locality appears to create another paradox. Specifically, if you can send a signal faster than light, the laws of relativity say that other observers flying past you at a high speed will see this signal going backward in time. This leads to a paradox since it opens the door to our signaling into the past, for example asking someone to kill our grandmother before our mother is born.

But, despite this kind of answer appearing to contradict fundamental physical principles, it is worth a closer look. The severe nature of the black hole crisis strongly suggests a resolution via some subtle violation of this locality principle, one that doesn’t produce such paradoxes. Quantum mechanics implies information is never destroyed. So, information that falls into a black hole must ultimately escape the black hole, plausibly through some new and subtle delocalization related to the basic principles of quantum gravity.

If information does escape black holes, we can ask if it has to be as obvious and abrupt as the formation of a massive remnant or fuzzball. In fact, the growing evidence for black holes suggests that there are objects in the universe that look and act a lot like classical black holes, without large departures from Einstein’s description. So, we might investigate whether there can be some more innocuous new effects that delocalize information and allow it to “leak” from black holes, without a drastic failure of the usual spacetime picture.

My recent work has found two variants of such effects. In one, the geometry near a black hole appears to bend and ripple in a way that depends on the information in the black hole—but does so gently, so that it does not for example destroy an observer crossing through the region where the horizon would ordinarily be found. In this “strong, nonviolent” scenario, such “shimmering” of spacetime can transfer the information out. Interestingly, however, it has also been found that there is a more subtle, intrinsically quantum, scenario that allows information to escape the black hole. In this “weak, nonviolent” scenario, even tiny quantum fluctuations of the spacetime geometry near the black hole can transfer information to particles emanating from the black hole. The fact that the information transfer is still large enough is related to the huge amount of possible information that a black hole can contain.

These scenarios for information transfer, despite appearing superluminal with respect to the spacetime picture of a black hole, do not necessarily produce a paradox. That is because the signaling is connected to the existence of the black hole, the spacetime of which is different from flat space, in such a way that the earlier argument about signaling into the past no longer holds. This story is also distinct from another recent proposal by Hawking, together with Malcolm Perry and Andy Strominger, that information is preserved outside a black hole through its ordinary gravitational field.

Such rippling of a black hole “quantum halo” can also distort light passing near a black hole, and this is where the Event Horizon Telescope comes in. If the strong, nonviolent scenario is correct, the shimmering could cause distortions of EHTs images that change with time. These changes could happen over a time of about an hour for the black hole in the center of our galaxy, and since EHT averages over multi-hour observations, may be hard to see, once EHT is able to image this black hole.

But the relevant fluctuation time for M87, which is over a thousand times larger, is more like tens of days. This suggests looking for these distortions, using longer-duration observations at EHT than their initial seven-day span. If such distortions are found, that would provide a spectacular clue to the quantum physics of black holes, and if they are not, that begins to point to the more subtle quantum scenario, or to something more exotic.

If either scenario is correct, that is a profound clue to more basic laws of nature. The essential message is plausibly that when one takes gravity into account, information is subtly delocalized in a way that departs from its localization in the current laws of physics. In particular, this appears likely to be tied to the statement that spacetime itself is not a fundamental notion in physics, but instead may arise as an approximation to a more basic mathematical structure in quantum mechanics.

Whatever the resolution, black holes contain crucial clues to the basic quantum physics of gravity. So, like with the atom and quantum mechanics, understanding black holes plausibly will help guide the next conceptual revolution in fundamental physics, and EHT observations may begin to provide us with key information about their behavior.

--Originally Published: Scientific American Online April 11, 2019.

How to Swallow a Sun

by S. Bradley Cenko & Neil Gehrels

At the heart of our Milky Way and of virtually every other large galaxy lurks a deep cosmic mystery—a supermassive black hole. Squeezing millions to billions of times more mass than our sun into regions smaller in size than our solar system, these objects are so bizarre as to seem almost mystical. No one yet understands exactly how nature has managed to compress so much matter into such small spaces. What is clear is that the hidden gravitational hands of supermassive black holes reach out to shape their surrounding galaxies in profound yet subtle ways. By studying the growth and behavior of these ghostly black holes, scientists hope to unlock the secrets of how galaxies themselves are born and evolve.

The trouble is that because they emit no light, supermassive black holes spend most of their time dormant and invisible to us. They come to life only when they feed, but their meals are surprisingly rare—most of the gas, dust and stars swirling around them reside in stable orbits and will never be consumed. Yet they are always hungry, and whenever something sizable does happen to fall in, the resulting feeding frenzies can be seen from far, far away.

For most of the past half a century, scientists have primarily observed just one variety of feasting black holes: quasars. Discovered by astronomer Maarten Schmidt in 1963, quasars are the ultraluminous centers of active galaxies that can be seen all the way to the outskirts of the visible universe, each shining brighter than billions of suns. They are thought to occur when massive clouds of gas and dust plunge onto a supermassive black hole for hundreds of thousands or millions of years, compressing, heating and glowing as they circle the black hole’s maw. Quasars, however, are less than ideal objects of study. They are extreme events that are typically quite distant and relatively rare, and they constitute a small portion of any supermassive black hole’s lifetime. They thus provide a limited view, leaving astronomers blind to how supermassive black holes more routinely feed and grow in the local universe. Researchers have also studied supermassive black holes by clocking the speeds of stars whizzing around them, but these measures work solely for objects very close to home—in the Milky Way or one of our immediate galactic neighbors—where current telescopes can resolve individual stars.

In 1988 astronomer Martin Rees proposed a third way to study supermassive black holes—a method that has recently begun to bear fruit. Instead of watching for the steady glow of quasars or the speeds of orbiting stars, astronomers can look for brief, bright flares of light from a black hole’s vicinity. Called tidal disruption events (TDEs), these outbursts occur when a supermassive black hole consumes an unlucky star. Unfolding over months rather than millennia, they allow researchers to track the feeding process from beginning to end and are bright enough to be observed in galaxies both near and far.

How to Destroy a Star

A tidal disruption event is far more dramatic than the gentle tides that can wash your beach towels away at the seashore, but it is not all that different in principle. Earth’s tides are chiefly caused by the gravitational tug of the moon, which pulls more strongly on the side of Earth closest to it. This difference between the moon’s gravitational pull on the far and near sides of Earth is called a tidal force. That tidal force raises a bulge, or high tide, both on the moon-facing side of Earth and, somewhat paradoxically, on the opposite face as well, which also creates a corresponding low tide, oriented 90 degrees from the Earth-moon axis. When a star is in the vicinity of a supermassive black hole—perhaps pushed there by the gravitational nudging of another nearby star—the intense tidal forces can rip it to shreds.

The details of the star’s demise depend on the sizes of both the infalling star and the supermassive black hole. A small, dense object, such as a white dwarf star, will be far more resilient against tidal forces than a larger, puffier, sunlike star—similar to how a bowling ball would be harder to rip apart than a mass of cotton candy. The very largest supermassive black holes, those containing billions of solar masses, are too big to easily create TDEs—they swallow stars whole before the tidal force becomes large enough to tear the stars apart. The tidal forces around a black hole with millions of solar masses, in contrast, will tear apart most stars that approach within about 50 million kilometers—roughly the distance of Mercury from the sun.

As spectacular as the wholesale disassembly of a star may seem, this is just the beginning of the fireworks display. After the initial disruption, the stellar debris will spread out and gradually diverge from the star’s original orbital path. Basic orbital mechanics dictates that roughly half of the debris will be expelled as long filaments of material streaming from the vicinity of the black hole, whereas the other half will loop back around to form an accretion disk—a structure of spiraling rings slowly funneling into the black hole. When the disk’s material falls in, it accelerates to nearly the speed of light and glows as gravitational and frictional forces compress and heat it to temperatures approaching 250,000 degrees Celsius. Over a span of weeks or months a typical TDE will cause a previously dormant and invisible black hole to briefly outshine all the stars in its galaxy.

The First Discoveries

Although theorists first predicted TDEs decades ago, astronomers did not detect any until the 1990s and early 2000s. This delay was in part caused by the rarity of TDEs—they occur once every 100,000 years in a galaxy like the Milky Way, according to estimates. They can also be hard to see. Simple theoretical models suggested that the glow of a TDE’s accretion disk should peak in the so-called soft x-ray or far ultraviolet parts of the electromagnetic spectrum—wavelengths that are difficult to access from the ground because of interference from interstellar dust and Earth’s atmosphere. Those same models also suggested that astronomers could use a TDE to make relatively precise estimates for the mass of its accompanying black hole—a critical data point for learning exactly how a black hole’s size alters its behavior and effects on its galactic surroundings. To measure a black hole’s mass, astronomers can simply time how long an accompanying TDE takes to reach peak brightness (which reveals how fast an accretion disk forms and feeds the black hole). Because TDEs are so bright, they allow researchers to pinpoint masses for a wider range of supermassive black holes than any other known phenomenon.

The first TDE candidates were found in data from the ROSAT x-ray and Galaxy Evolution Explorer ultraviolet space telescopes. They appeared as weeks- to months-long flaring events sourced to the centers of previously dormant galaxies. As the first manifestations of this long-predicted phenomenon, these discoveries were especially important for establishing an entirely new field of study. Because they were predominantly found in old data, however, astronomers were unable to study them across multiple wavelengths in real time to unlock their deepest secrets. To catch TDEs as they happened, astronomers would either have to be very lucky or able to continually survey mammoth swaths of sky.

As luck would have it, in the past decade steady progress in data storage and sensors has made such ambitious surveys possible. A high-end optical camera can now image one square degree or more of sky in a single snapshot, a situation akin to suddenly seeing the heavens with a panoramic lens after years of studying them by looking through a straw. By repeatedly surveying large areas of the sky and digitally combining the resulting images to tease out faint, temporary features, astronomers are now able to more easily discover and study TDEs and a host of other transient astrophysical phenomena. These new widefield surveys, with names such as the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Palomar Transient Factory (PTF) and the All-Sky Automated Survey for Supernovae (ASAS-SN), were designed largely to identify supernovae and asteroids, but they can do much more. Because they can image millions of galaxies each night, they are also sensitive to more exotic transients such as TDEs.

New Questions for a New Era

In 2010, shortly after Pan-STARRS’s on-sky debut, a team led by astronomer Suvi Gezari discovered a TDE called PS1-10jh, which took place around a black hole of perhaps two million solar masses in a galaxy some 2.7 billion light-years from Earth. Because this TDE was noticed soon after the data were collected, Gezari and her colleagues were, for the first time, able to watch it unfold in subsequent optical and ultraviolet observations. What they found was shocking.

Based on careful measurements of its spectrum, this particular TDE appeared to be much too cold. At about 30,000 degrees C, it was more than eight times cooler than predicted by most basic theories of accretion disks. Moreover, rather than fading over weeks as its accretion disk cooled and dissipated, PS1-10jh maintained a constant temperature for many months after the initial discovery. Strangest of all, Pan-STARRS had detected signs of ionized helium in the afterglow—something that could be produced only by temperatures in excess of 100,000 degrees C. And although the TDE appeared to be helium-rich, it also seemed to be devoid of hydrogen, the most abundant element in the universe and the main ingredient of stars. Theorists got to work examining what could produce such confounding results.

To explain PS1-10jh’s lack of hydrogen, the Pan-STARRS team suggested that the disrupted star had lost its thick envelope of hydrogen at some point in the past, possibly in a previous interaction with the black hole, leaving only the star’s helium-rich core to feed the observed accretion disk. But this alone could not account for the TDE’s curious thermal discrepancies—its surprisingly low temperature and, in contradiction, its abundance of helium ionized by much higher temperatures. To solve that mystery, other theorists have postulated that PS1-10jh’s black hole–hugging accretion disk was not actually being directly observed. Instead astronomers must have seen a surrounding veil of gas much farther out from the black hole that absorbed the intense radiation produced by the accretion disk and reemitted it at lower temperatures. This veil would have the added bonus of clarifying the apparent absence of hydrogen without the need for an exotic, helium-rich core as the TDE’s progenitor. Given the right temperature and a rather high density, such a veil could potentially mask the presence of hydrogen, hiding it in plain sight.

The only trouble was that a thick veil of gas would not be stable at the required distance from the galaxy’s central black hole—over time the gas would either fall into the black hole or dissipate to invisibility. The murky origins of this material are still the subject of intense debate and study but, broadly speaking, fall into two possibilities, both related to the dynamics of a feeding black hole. As the remains of a disrupted star loop around a black hole to form a growing accretion disk, shock waves can ripple outward well away from the disk to forestall some of the outlying debris from immediately falling in, creating a temporary screen of material. Alternatively, a newborn TDE accretion disk might initially funnel so much material inward that it briefly exceeds a black hole’s capacity to feed, forming transient winds or outflows just outside the black hole that push stellar debris out past the accretion disk to much larger distances.

As astronomers sorted through these muddled possibilities for PS1-10jh and other newfound TDEs that soon followed, one thing became perfectly clear: TDEs were a much more complicated phenomenon than anyone had previously appreciated. Yet the biggest surprise was still to come.

A Shock from Swift

That surprise arrived in the predawn hours of March 28, 2011, with an automated alert sent to the pagers and cell phones of a dedicated team of astronomers around the world. The Swift satellite had just detected a pulse of high-energy radiation from the depths of space. Built by nasa in partnership with research institutions in Italy and the U.K., Swift is an agile space telescope, designed to study all types of exploding objects across the sky. But its main targets are gamma-ray bursts, or GRBs—cataclysmic stellar explosions that are the most luminous astrophysical events in the universe. Whenever a torrent of gamma rays trickles into Swift’s sensors, the telescope quickly reorients to observe the source in x-rays and optical light and phones home to trigger a complex chain of events on Earth. Within moments of an alert from Swift, astronomers scramble to commandeer the world’s largest and most powerful telescopes to look for any emberlike afterglow associated with a GRB before it fades from view forever. Since its launch in 2004, Swift has discovered more than 1,000 GRBs, but this particular event, later dubbed Swift J1644+57, would prove to be unlike anything the satellite had seen before.

As their name suggests, GRBs tend to be brief, typically lasting anywhere between a fraction of a second and a few minutes. When we turned our telescopes to Swift J1644+57 on that early March morning, we expected to see the standard, fading afterglow of a short-lived GRB. Instead we observed bright, erratic gamma-ray flaring that lasted for a day, followed by months of intense but fading x-ray emissions. Soon we had traced the blast to a galaxy some 3.8 billion light-years away in the constellation of Draco. One of our colleagues, Joshua S. Bloom of the University of California, Berkeley, suggested we had witnessed a TDE and correctly predicted that this particular gamma-ray source would be found at the galaxy’s center—the stomping ground of supermassive black holes. But whereas all previous TDEs had been detected at longer, lower-energy wavelengths where observers saw the thermal emission from the accretion disk of a shredded star, this was something wholly different.

How could a TDE produce gamma rays? The best answer we could come up with is that black holes are messy eaters. A black hole would devour most of a disrupted star’s gas, locking it away forever behind an event horizon (the boundary past which the black hole’s gravitational pull is so great that not even light can escape). But all black holes probably spin, and this spin can push a few percent of the disrupted star’s total gas toward the black hole’s poles, outside the event horizon, where the gas is accelerated and ejected as collimated beams of particles moving at nearly the speed of light. The fast-moving beams emit gamma rays and x-rays as they hurtle through the cosmos. Apparently Swift had happened to find itself in the path of Swift J1644+57’s beam. It was a lucky catch—not all TDEs appear to power such relativistic outflows, and most of those that do likely miss our line of sight.

With the detection of Swift J1644+57 as inspiration, the Swift team began a concerted search for more events. As of early 2017, two more TDEs emitting gamma-ray jets have been found. These rarest and most intense death cries of stars are a novel way to study one of the foremost research topics in modern high-energy astrophysics—the creation and behavior of relativistic particle jets.

The Death of Worlds

Whether through the thermal emissions from accretion disks of stellar debris or rather with the gamma rays pouring off a star-devouring black hole’s relativistic jets, TDEs offer a new window on the behavior and evolution of supermassive black holes and their surroundings. Most important, unlike the far larger and longer-duration jets and accretion disks of quasars, which are produced by immense clouds of gas chaotically plummeting onto a supermassive black hole across very long timescales, TDEs are brief, clean events that are more easily studied. No human being will ever live long enough to witness the complete life cycle of a single quasar, but already astronomers have found and studied more than 20 TDEs from beginning to end. And in the details of those stellar catastrophes, they have glimpsed tantalizing quirks begging for further study. By precisely measuring the fluctuating flares from TDEs, astronomers are learning not only about black holes but also about the detailed compositions and internal structures of stars being ripped apart billions of light-years away.

They may even eventually learn about a star’s companions, too—planets swallowed by black holes. Each flickering flash from a faraway galactic center could signal the deaths of entire worlds. Surveys of stars in our own galaxy have found that almost every one harbors planets; planets probably accompany most, if not all, of the stars in other galaxies as well, including those that suffer TDEs. Even if they were not directly consumed, planets could still find themselves in the path of the transient relativistic jets produced by some TDEs, which extend light-years beyond their black hole sources. Life in any planetary systems unlucky enough to be struck by such a beam would be rapidly extinguished. Someday astronomers may witness a TDE right in our cosmic backyard, when the four-million-solar-mass black hole quiescently lurking at the gas-impoverished center of our very own Milky Way flares to life as it consumes some errant star. It would be very bright but also very safe because we are too far from the galactic center for the TDE’s most dangerous effects to reach us here.

The advent of even more powerful surveys heralds a new era of TDE discoveries. The Large Synoptic Survey Telescope (LSST), an eight-meter telescope currently being constructed in Chile with a field of view covering 10 square degrees of sky, will by itself uncover thousands of these outbursts within a decade of its debut. In some ways, the most challenging aspect of LSST science will be sifting through the overwhelming number of transient discoveries. Planned radio observatories such as the Square Kilometer Array being built in Australia and South Africa are particularly well suited to identify relativistic jets, even if such jets are “off axis,” meaning they are not beamed directly along our line of sight. In the not too distant future, astronomers may have assembled a TDE catalog with thousands on thousands of entries, more than any individual could study in a lifetime, shedding new light on the masses and behaviors of those elusive hungry ghosts, the otherwise inaccessible supermassive black holes that reside in the hearts of galaxies all across the universe. From that rich, ever growing corpus of knowledge, we can only dream of what further revolutionary discoveries may come.

--Originally Published: Scientific American 316(4), 38-45 (April, 2017).

Meet “Spikey,” a Possible Pair of Merging Supermassive Black Holes

by Nola Taylor Redd

A strangely flaring object at the center of a distant galaxy may be the key to unlocking the mystery of how the universe’s most monstrous black holes merge.

Weighing in at millions to billions of times the mass of our sun, supermassive black holes are the ultimate heavyweights—and they lurk at the centers of almost every large galaxy. Although they emit no light, these objects can nonetheless create spectacular celestial fireworks as they feed on gas and dust, creating jets of high-energy particles and whirling disks of debris that can be seen clear across the cosmos as active galactic nuclei (AGNs). Now, scientists have identified a flare in faraway AGN that they suspect is created by a supermassive black hole amplifying the emissions of another one nearby, suggesting that the pair may merge in the next 100,000 years. If the two are, in fact, primed to merge, they would offer astronomers an unprecedented view into the poorly understood process of how giant black holes manage to get together at all.

In 2017 astrophysicists Daniel D'Orazio and Rosanne Di Stefano detailed how a pair of soon to merge supermassive black holes should gravitationally lens one another and how the resulting signal could be seen if the imminent merger’s orbital plane aligned with Earth. Material surrounding the black holes should glow in the x-ray wavelength as it accelerates toward either member of the pair If one black hole passes in front of the other, the immense, spacetime-warping gravitational field of the “foreground” black hole will act much like a lens, magnifying the background light source. “It’s a very distinctive signature,” says Di Stefano, a researcher at the Center for Astrophysics at Harvard University and the Smithsonian Institution.

In October 2019, she and D’Orazio, working with several collaborators, reported on the discovery of an object emitting a signal that matches their theoretical prediction. Data gathered in 2011 by NASA’s planet-hunting Kepler space telescope revealed an unusual AGN with a strange spike. If the object, nicknamed Spikey, repeats its flare again this spring, as predicted by D’Orazio and his colleagues, it will be what he calls the “smoking gun” confirming Spikey is a pair of supermassive black holes on the cusp of merging. D’Orazio, an astronomer at Harvard, presented the new analysis last month at a meeting of the American Astronomical Society in Honolulu.

When galaxies collide, the supermassive black holes at their centers eventually find their way to the heart of the newly created galaxy and are ultimately drawn together. Observations of the cores of merging galaxies have revealed either a single supermassive black hole (presumably where two or more have already merged) or black holes that are orbiting within a few parsecs of one another (a parsec is roughly 3.26 light-years).

“We are very confident that when two galaxies merge, the black holes they host will get within a parsec of each other,” says Scott Hughes, an astrophysicist at the Massachusetts Institute of Technology, who did not take part in the study.

The problem comes in the final parsec, where gravity is not strong enough to overcome the centrifugal force of each black hole’s orbit to pull the pair closer together. Without a steady influx of material to shake things up, the two may stop just shy of merging and remain in a holding pattern over the lifetime of the universe. This “final parsec problem” does not affect pairs of smaller, stellar-mass black holes, which can more easily merge by bleeding off excess orbital energy via their copious emission of gravitational waves. But larger black holes need something to push them over that final hump before their own gravitational-wave emission can kick in, at which point an eventual merger becomes inevitable.

“We don’t have a good understanding of what goes on in that final parsec,” says Matthew Graham, a cosmologist at the California Institute of Technology, who was not involved in the new study. “We have a theoretical understanding, but we don’t have good observational evidence to match against theory.” At least, researchers do not have such evidence quite yet.

In addition to revealing thousands of exoplanets, Kepler also discovered a few dozen AGNs. A 2018 study of these objects revealed unusual flaring activity in one called KIC 11606854. A closer look revealed that the flare’s waxing and waning light mirrored predictions of how a pair of merging black holes might gravitationally lens one another. Hello, Spikey.

“It ended up being very fortuitous,” says Betty Hu, a graduate student at Harvard University and first author of a paper in Monthly Notices of the Royal Astronomical Society reporting Spikey’s discovery. The researchers studying the Kepler AGNs passed the information on to D’Orazio and his colleagues, who found the signal matched up “very well” to the lensing model, Di Stefano says.

According to Di Stefano, the merging black holes might each be ringed by a “mini disk” embedded in a larger shared disk that orbits both objects. The mini disks could dissipate as the black holes gobble them down, only to be occasionally replenished with material from the larger outlying disk. Each black hole munching on a minidisk has a beneficial side-effect, shedding additional orbital energy and allowing the two to spiral closer together, potentially overcoming the final parsec problem. According to the researchers’ models, Spikey should merge in the next 100,000 years or so—an eyeblink on astronomical timescales.

A single flare alone, however, is not enough to confirm Spikey is a pair of merging black holes. D’Orazio and his colleagues are already planning to study Spikey this spring in search of more evidence. Based on their best estimates of the pair’s orbits, they have tentatively identified the next gravitational-lensing event as most likely to occur in April 2020. But, Hu says, lingering uncertainties mean the flare could take place anywhere between February and July.

The team has already secured time on NASA’s Chandra X-ray Observatory to watch for April’s predicted flare, which should span about 10 days. In the meantime, the researchers are continuing to monitor the system using ground-based instruments. If Spikey starts acting up before April, they hope to catch a glimpse in order to shift their observations with Chandra and other facilities to compensate. “I think [D'Orazio] has done a fantastic job of trying to figure out all the ways possible to follow up on this system because it is the best candidate [of merging black holes],” Di Stefano says.

If Spikey shows the predicted flare this spring, it would be a big deal. “If it holds up and is, in fact, a binary, I think it will give us a case of what to look for if we’re trying to find cases of close binaries not yet merged,” Hughes says. Such an example should make hunting merging supermassive black holes easier in the future.

And that result would be good news for the European Space Agency's Laser Interferometer Space Antenna (LISA) mission, set to launch sometime in the 2030s to hunt for gravitational waves emitted by supermassive black holes. Although Spikey probably will not merge on LISA’s watch, it can give mission planners a better idea of how many merging giants are out there for the spacecraft to see.

Then again, Spikey could fail to flare again; perhaps it is not a pair of supermassive black holes at all. According to Graham, the last few years have seen a rising number of claims of potentially merging supermassive black holes that wound up being something else.

If July passes with no sign of the unique signature, then it could be that the original event was just a never before seen flare type from a relatively normal AGN. Although there are still a handful of other candidates for near-merging supermassive black holes waiting to be confirmed, a nondetection would set merging-black-hole hunters almost back to square one.

But a nondetection would not necessarily mean Di Stefano and D’Orazio’s model is wrong. “This is a process that has to happen” somewhere in the universe, Di Stefano says. As long as two black holes are orbiting one another, gravitational lensing should occur; it is just a matter of the pair being in a suitable orientation for the effect to be seen from Earth. In their original paper, she and D'Orazio predicted roughly 10 percent of binaries would be properly angled to give astronomers a glimpse of their gravitational-lens flares.

“Should Spikey not work out, we know that this process happens,” Di Stefano says. “Ultimately, we should be able to detect it, but we may have to look at other systems to see it.” Graham agrees. “It’s a conceptually neat idea,” he says. “These things should be lensing.”

--Originally published: Scientific American online, February 10, 2020.

Astronomers Spy a Black Hole Devouring a Neutron Star

by Charlie Wood

Some 870 million years ago, two dead stars became one. Their merger shook the fabric of space with a gravitational wave that swept through Earth on August 14, 2019, rippling through three pairs of carefully calibrated lasers designed to detect their passage. An automated system sent out a preliminary alert 21 seconds later, vibrating smartphones and pinging laptops around the world.

Since the Nobel Prize–winning first gravitational-wave detection in 2015, which stemmed from a pair of colliding black holes, such alerts have become commonplace. This time, however, astrophysicists instantly knew that the observed event was special. “My jaw dropped when I saw the data,” says Geoffrey Lovelace of California State University, Fullerton (C.S.U.F.), a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration.

The wave was detected by LIGO in the U.S. and the Virgo Observatory in Italy at 21:11:18 UTC on August 14, 2019. An automatic first pass pegged it as resulting from an unprecedented merger between a pair of bodies too light to classify, sending astronomers scrambling to look for additional electromagnetic emissions from the event. Subsequent analysis recategorized the signal as a collision between a black hole and a neutron star, a stellar remnant in which gravity squeezes an entire sun’s mass into a ball the size of a city. This would be the first such event detected with confidence and, after black hole–black hole mash-ups and mergers between two neutron stars, the third variety of collision detected by gravitational waves. If the current analysis stands, this event, dubbed S190814bv, will mark the beginning of a new era of astrophysical studies, with implications for how researchers understand Einstein’s general theory of relativity, the deaths of stars and the behavior of extreme matter.

An “Off-the-Charts” Signal

Chad Hanna, a LIGO collaborator and astrophysicist at Pennsylvania State University, was celebrating his wedding anniversary with his wife when his phone went off. His group specializes in rapid classification of LIGO events, so he immediately logged in to check the wave’s details. “The first thing I knew was that it was extremely significant,” Hanna says, “kind of off-the-charts loud.”

The LIGO-Virgo collaboration’s algorithmic pipeline spits out a basic classification based on the shape of a wave, its duration and other factors almost instantly—Hanna’s team aims for under 20 seconds—so astronomers can immediately slew their telescopes in the celestial direction the wave came from.

On August 14, the automatic system confidently declared that at least one of the objects that produced S190814bv fell into the “mass gap,” a wasteland, spanning three to five solar masses, seemingly bereft of black holes and neutron stars. All known black holes weigh more than five suns, while all known neutron stars—born from lighter stars that stopped short of becoming black holes—weigh less than three suns. A mass gap detection would have been a first for LIGO-Virgo—one that would have sharpened the theoretical line separating the heaviest neutron stars from the lightest black holes—but the preliminary label would not last. “There was a handoff around the globe,” says Jocelyn Read, an astrophysicist at C.S.U.F. and a LIGO member, beginning with researchers in the U.S. on the afternoon of August 14 and with calculations continuing in Europe well into the following morning.

American scientists woke up on August 14 to a new classification. Human analysis had pegged the event as a neutron star–black hole merger with greater than 99 percent confidence. LIGO-Virgo has heard the collisions of more than a dozen black hole pairs, as well as two pairs of neutron stars, but it has never conclusively heard the rumbles from a black hole swallowing a neutron star.

“This is something I’ve waited for a long time,” says James Lattimer, an astronomy professor at Stony Brook University and a pioneering nuclear astrophysicist, who showed that neutron star–black hole mergers can spray heavy elements such as gold and uranium into space in his 1976 thesis.

Researchers detected a similar wave in April 2019, but they were not able to confirm it came from deep space—the signal associated with that potential event, models suggest, had a one-in-seven chance of being a false alarm produced by terrestrial sources, meaning a spurious detection would be expected about once every 20 months. August's signal, however, was so clear that a false alarm would be a once-in-trillions-of-years event. “When it’s more than the age of the universe,” Lovelace says, “you know it’s the real deal.”

S190814bv’s deafening signal, however, does not guarantee that astrophysicists have definitely bagged their first neutron star–black hole collision. While the current label clearly puts the heavier object in black hole territory (more than five suns), it leaves the lighter partner in the murky zone below three solar masses. If further analysis places that partner between one and two solar masses, it must be a neutron star. But a measurement closer to three suns could break either way—toward the universe’s heaviest known neutron star or its lightest known black hole.

Future mass estimates will give a clearer picture, but first LIGO-Virgo will have to check the wave against our best models, which are too complicated to run overnight. Theoretical tools get shaky as the masses skew away from two evenly balanced partners, so researchers caution they must tread lightly in this uncharted territory. “We’re still analyzing and checking things,” Lovelace says. “But it’s the most promising case like this that’s come up so far.

Looking for Light

Virgo’s detector in Italy—along with only one of LIGO’s two detectors—recognized the wave initially, but the collaboration was able to manually incorporate data from the second LIGO detector overnight. Triangulating from that third detection allowed researchers to pinpoint the source’s location in the sky more precisely than any previous wave so soon after detection. “I opened up [the new] sky map, and I was like, ‘Oh, they accidentally updated a blank sky map,’” Read recalls thinking before she noticed the tiny dot marking the wave’s origin.

The narrowed location, which amounted to 0.06 percent of the sky’s total area, came as a boon to astronomical teams hunting for a flash of gamma rays or visible light that could accompany the death of a neutron star. “In principle, it’s a manner of minutes to cover that area,” says Marcelle Soares-Santos, a cosmologist at Brandeis University, who coordinated follow-up observations using the Dark Energy Camera on a four-meter telescope in Chile.

The black hole may have shredded the neutron star, leaving behind a ring of glittering wreckage that faded as it fell into the hole’s waiting maw. Alternatively, the black hole could have swallowed the neutron star in one clean gulp, with little left to see. LIGO-Virgo simulations for S190814bv predict the latter scenario, but no one knows for sure what actually transpired. For a first-time observation, even seeing nothing can be informative. “We’re going in with an open mind,” Soares-Santos says. “If there’s no electromagnetic counterpart, we will be able to establish with enough significance that it will have a big impact on the theories.”

Probing Neutronium

And neutron star theories abound. Nuclear physicists seek a glimpse inside the objects, where matter exists at densities that challenge the present best models. If the pressure dissolves neutrons into a plasma of fundamental particles, for instance, neutron stars of a certain mass should appear smaller than they otherwise would be. Fine features of the detected gravitational wave produced as the star spiraled into the black hole may reveal the star’s size and, accordingly, the consistency of the matter that fills it. Similarly, whether astronomers see a flash or not will also set limits on the star’s size. Such precise measurements of a neutron star’s dimensions are “sort of the holy grail of nuclear physics,” says Ben Margalit, a postdoctoral researcher at the University of California, Berkeley, who is not part of the collaboration that observed the event.

A black hole obliterating a neutron star also represents a new arena for testing general relativity. Applying Einstein’s theory of gravity to the smooth fabric of spacetime around black holes is tough enough, Lovelace says. Adding in hot, turbulent magnetized neutron star matter—an exotic substance sometimes called “neutronium”—elevates the challenge to a messy new level.

Even if August's ripple in spacetime doesn’t divulge any of nature’s secrets, researchers feel confident it is just the first of many to come. “I hope it tells us something about black hole–neutron star [mergers],” Lovelace says. “But if not, it still makes me really optimistic that the gravitational sky is bright.”

--Originally published: Scientific American online, August 19, 2019.

SECTION 2

Black Hole Origins

Zeroing In on How Supermassive Black Holes Formed

by Yasemin Saplakoglu

Supermassive black holes—objects containing hundreds of millions to billions of times the mass of a star—are one of the deepest mysteries of modern astrophysics. They lurk at the hearts of most large galaxies, including our own Milky Way. Given their ubiquity, these black holes may play a vital part in the formation and evolution of the universe. But how they grew so massive has long puzzled theorists worldwide.

The most sensible suggestion—that these monstrosities could only have grown so great by swallowing enormous quantities of gas over billions of years—is now known to be wrong. Recent observations have revealed the existence of black holes billions of times more massive than the sun just 800 million years after the big bang. And so, the riddle goes: How did they get there so quickly? Most astrophysicists agree supermassive black holes must stem from smaller “seed” black holes. They just don’t agree on how humble such a seed must be. One school of thought holds that the seed black holes should be big—thousands to several tens of thousands of times the sun’s mass; the other posits the seeds could be small—no heavier than a hundred solar masses.

Both camps must grapple with the fact black holes are messy eaters: Gravity can only cram so much gas down one’s maw before the material begins to pile up around it, forming white-hot disks that emit intense radiation and push additional incoming gas away, effectively cutting off the food supply. This is called the Eddington limit, and it is thought to severely hinder the rate at which any black hole can swallow matter and grow. The advantage of models using small seeds is that such welterweight black holes are relatively straightforward to make; the disadvantage is that to rapidly grow into supermassive black holes they must treat the Eddington “limit” as more of a suggestion, and rely on various potential exceptions to circumvent its constraints. Big-seed models, by contrast, respect the limit by giving growing supermassive black holes a huge head start to gobble up more gas before hitting it—but their bigger seeds are correspondingly more difficult to make. The giant clouds of gas that could collapse to form big seeds can also fragment into smaller clumps, forming clusters of stars rather than large black holes.

Whether advocating for big or little seeds, “there have been many theories that try to explain the existence or assembly of supermassive black holes, but none of them can give a natural solution,” says Naoki Yoshida, an astrophysicist at the University of Tokyo. Yoshida is a proponent of big seeds, and a co-author of a study published in Science suggesting how they formed and gave rise to the early universe’s surprising population of supermassive black holes. His “natural solution” invokes high-speed streams of gas that flowed through the universe in the aftermath of the big bang as the critical catalyst. Specifically, it rests on the putative interaction between the zooming gas and dark matter—the mysterious invisible substance that seems to act as gravitational glue for galaxies.

Growing a Black Hole

Along with his collaborators at The University of Texas at Austin and the University of Tübingen in Germany, Yoshida used computer simulations to re-create conditions in the early universe by feeding the program cosmological parameters such as the density of dark matter, which astronomers have calculated from measurements of the early universe’s makeup. “We tried to reproduce this initial state as close to the real observations [as we could],” Yoshida says, “and we let that universe evolve over time.”

According to the group’s simulations, in some parts of the universe the gravity from dark matter would have ensnared fast-moving streams of primordial hydrogen and helium left behind by the big bang. In its aftermath, researchers recently discovered, these early gases accelerated in some areas to incredible speeds—or “really fast winds,” as Yoshida calls them. “You can just imagine it’s very hard to trap a gas that is moving very fast,” Yoshida says. Picture placing your hand against the jet from a fire hose, he says—your arm would just swing back with the force. “The only way to stop this strong wind is really to cause strong enough gravity,” he says. And the researchers calculate that in every three-billion-light-year expanse of the early universe, there was a large enough clump of dark matter with enough gravity to suck in and trap this wind—like having sufficient strength to push the jet of water back in the opposite direction. This attraction between the gas and the dark matter created a large gas cloud and prevented small stars from forming along the way.

This simulated gas cloud then collapsed into a massive star, which continued to swallow more gas until it reached 34,000 times the sun’s mass. This unusually hypothetical massive star could only reach such magnitudes if it was made purely of hydrogen and helium—the two elemental gases that swirled around the early universe before any stars underwent supernova explosions that created heavier elements such as carbon, nitrogen and oxygen. The idea for a massive star had been proposed before, but this is the first time a group simulated it. “Our computer simulation really showed this kind of phenomena actually happens and that this kind of monstrous star can actually be formed,” Yoshida says. After reaching this gigantic mass the star finally collapsed and the seed for a supermassive black hole was born. “We didn’t really look for a nice solution; [it] came as a natural result,” he says. “That’s why I think this is really the final solution, at least to the question of the origin of [supermassive black holes].” Not everyone agrees, however.

Good Answer, but…

Other scientists who favor the big-seed hypothesis have different ideas about how those seeds form in the first place. A study published in Nature Astronomy, for example, proposes such seeds form not through the murky motions of dark matter but rather via the behavior of ordinary stars in galaxies. In this scenario intense bursts of ultraviolet light from vigorous star formation in a nearby young galaxy could prevent stars from forming in a giant gas cloud, allowing it to remain whole long enough to directly collapse into a black hole with a mass of up to 100,000 suns.

John Wise, an astrophysicist at Georgia Institute of Technology and co-author of the Nature Astronomy study, thinks this new work is an important step forward in the field because Yoshida and his colleagues were the first to simulate the effects of these early gas motions on the formation of supermassive black holes. But he says it does not rule out his own theory. “I think there are multiple pathways [for] how to form these supermassive black holes,” he says. “This [pathway] is just another, and I think it’s totally possible.” Yet he does note that it is rare to find such fast-moving gases in the early universe. “These velocities do fluctuate [depending on] where you are in the universe, so there is still some low probability of this actually happening.” The chances of coming across an area of the early universe with such a fast-moving wind are only 0.3 percent, according to Yoshida. Conversely, he and his colleagues note, giant gas clouds directly adjacent to young star-manufacturing–galaxies also seem like rare occurrences. “The net probability of this event happening is really uncertain,” Yoshida says.

Greg Bryan, an astrophysicist at Columbia University and senior author of the Nature Astronomy paper praises the new findings. “This is not a definitive answer, but it’s the best by far looking at this particular mode for black hole formation,” he says. He is a bit worried, however, about how close their simulations were to forming a smaller set of stars. For a black hole to form, a bunch of these early gases needs to gather in a very small region, which wouldn’t happen if they dispersed to form a bunch of stars. If the conditions in the simulation changed a tiny bit, he says, it wouldn’t form the massive seed. “On the other hand, as much as I trust any of the models, I trust theirs,” Bryan adds.

Fulvio Melia, an astrophysicist at the University of Arizona, is not as ecstatic about this theory. “There are a lot of unknown physics that [the authors] have to rely on, just like all the other proposals [for] forming massive seeds or having these objects grow at a very high rate,” he says. “They have to make specific assumptions about how the dark matter behaves, but we don’t [even] know what it is.”

Ending the Seeds of Doubt

To definitively answer this question of how these massive beasts came to be, these scientists all point to the future possibility of observing the “seeds” in the early universe using advanced next-generation telescopes. This possibility may not be that far off. There are several initiatives such as the proposed ATHENA mission from the European Space Agency set to launch in 2028, which aims to detect x-ray emissions from these supermassive giants. NASA’s upcoming James Webb Space Telescope, slated to fly next year, could also provide insights with its studies of the universe’s first stars and galaxies.

“The exciting thing is that there is a way of testing these ideas over the next few years because people will go out and do a thorough search over the whole sky for these objects,” Melia says. As to why there’s so much discussion, he adds: “What people are proposing is something different than what we know about in [parts of] the universe closer to where we are.”

--Originally published: Scientific American Online, September 29, 2017.

Did Astronomers Just Discover Black Holes from the Big Bang?

by Nola Taylor Redd

Since their first direct detection in 2015, gravitational waves have become one of the hottest topics in astronomy. With facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), researchers have mostly used these ripples in spacetime to study the inner workings of merging black holes, but LIGO has also detected gravitational waves from other sorts of celestial crashes, such as the collisions of ultradense stellar remnants called neutron stars. Sometimes, however, LIGO serves up gravitational waves that leave astronomers scratching their heads—as was the case for GW190425, an event detected in April 2019 that was attributed to a neutron star merger.

The trouble is that LIGO’s data suggest this neutron star pair was substantially overweight—collectively, some 3.4 times the mass of the sun, which is half a solar mass heavier than the most massive neutron star binaries ever seen. “It is the heaviest known by a pretty wide margin,” says Chad Hanna, an astrophysicist at Pennsylvania State University who hunts gravitational waves.

That extra weight has some theorists suspecting that GW190425 did not arise from colliding neutron stars at all but rather something much more exotic: A merger of two primordial black holes (PBHs), never before seen objects that are considered a dark horse candidate for dark matter—the invisible, unidentified something that makes up most of the matter in the universe. Theorized to have formed from density fluctuations in the very early universe, these ancient black holes could still exist today and could explain the mass discrepancy identified in the recent LIGO observations.

Almost a half-century ago, cosmologist Stephen Hawking proposed that PBHs could have sprung fully formed from regions of the infant universe that were especially dense with matter. Since then, the idea’s popularity among astrophysicists and cosmologists has wildly waxed and waned. Today, in the absence of direct evidence for their existence, PBHs are seen by many researchers as a hypothesis of last resort, only to be considered when no other scenario readily fits observations. The possibility that PBHs are real and widespread throughout the universe cannot yet be dismissed, however—especially as searches for other dark matter candidates come up empty.

PBHs make an appealing candidate for dark matter for several reasons, but the most important one is that, being black holes, they are quite dark yet still pack a hefty gravitational pull. Despite that fact, Hanna says that if PBHs were abundant enough to account for all of the universe’s dark matter, astronomical surveys that hunted for them should not have come up empty. Consequently, he adds, PBHs can only make up a small fraction of dark matter—if they exist at all.

Not everyone agrees. “Primordial black holes can comprise the whole of dark matter,” says Juan García-Bellido, a theoretical cosmologist at the Autonomous University of Madrid. The trick, he adds, is for the ancient objects to exhibit an array of masses rather than a single definitive size. If PBHs run the gamut from a thousand times less massive than the sun to a billion times larger, they could make up all of the universe’s dark matter. “All published constraints that claim to rule out primordial black holes as dark matter assume they exist in a monochromatic, or single-mass, spectrum and are uniformly distributed in space,” García-Bellido says. For such large mass ranges to manifest, the PBHs would have to cluster in compact groups in which they could occasionally collide, merge and grow larger.

Because PBHs would have been created shortly after the big bang, they initially could have easily connected with one another. The early universe was a much smaller place than it is today after dramatically expanding for nearly 14 billion years, making it easier for the objects to find other PBHs and pair up with them. As the universe continued to expand, and the first stars and galaxies emerged, however, those connections would have become increasingly rare. So while it is possible that LIGO has observed merging PBHs, it is unlikely, according to astronomer Katerina Chatziioannou, a LIGO team member at the Flatiron Institute in New York City and co-author of a study in the Astrophysical Journal Letters that pegs GW190425 as the product of colliding neutron stars.

In April 2019, alerted to LIGO’s detection of GW190425, telescopes around the world hunted for a corresponding electromagnetic signal that would typically be expected from the explosive collision of two neutron stars. But the skies remained dark, as they would if a pair of primordial black holes had slammed together. “We would not have expected any light to have come out of the merger of two primordial black holes,” Chatziioannou says. Even so, she adds, the lack of light does not rule out neutron stars. Massive neutron stars could have had a relatively placid merger, collapsing directly into a black hole before sparking any celestial fireworks. It is also possible that the event’s location in the sky could be from somewhere that Earth-bound telescopes could not probe, such as in a region behind the sun. “There are good reasons for them to have missed it if there was any light,” she says.

The newest observations provide only tantalizing hints that PBHs might be out there, occasionally coming together in the cosmic dark. A clearer signature would come from a pair of black holes in which each weighed less than the sun. “If you find a black hole below a solar mass, that, at the very least, comes from a mechanism that no one has predicted, astrophysically, outside of a primordial black hole,” Hanna says. García-Bellido concurs. “The smoking gun would be the discovery of a less than one-solar-mass black hole—or a black hole with a mass larger than 50 [times the sun],” he says.

Although LIGO’s observations could mark the first detection of PBHs, both Chatziioannou and Hanna agree that it is more likely that the gravitational waves merely came from overweight neutron stars. Theories for the formation of such bulky neutron stars already exist, and they require no speculative scenarios from the dawn of the universe. “It’s definitely a lot less likely that [the sources of these events] are primordial black holes than just neutron stars that are heavier than what we see in the galaxy,” Chatziioannou says. “It's not impossible; it’s just less likely.”

Although Hanna calls the case for GW190425 as a primordial black hole binary “weak,” García-Bellido remains more optimistic. “All LIGO events could be due to primordial black holes,” he says. Only time—and more data—will tell.

--Originally published: Scientific American online, January 13, 2020.

Black Hole Factories May Hide at Cores of Giant Galaxies

by Charlie Wood

In 2015, when scientists, for the first time ever, directly detected gravitational waves—ripples in spacetime—from colliding black holes, the result came as a shock to some astronomers. Based on previous studies of black holes using x-rays, many experts expected each member of a merging pair would typically weigh about 10 times the sun’s mass, but the 2015 merger featured twin giants three times that heavy. And the Laser Interferometer Gravitational-Wave Observatory (LIGO), which enabled the discovery, has been spotting bizarrely big pairs ever since.

Black holes can certainly get supermassive—leviathans weighing billions of suns lurk at the hearts of most large galaxies. The question is: How could they grow so huge? Much of their bulk must be acquired after birth as they feast on gas and stars, but some theories suggest that mergers or chains of mergers may form a supermassive black hole’s initial seed. Black hole matchmaking in the loneliness of space is not easy, though, so astrophysicists still puzzle over what circumstances could bring the objects together.

An emerging theory holds that LIGO’s heavyweights arise near the cores of colossal galaxies, where violently incandescent disks of gas whirl around central supermassive black holes. Thanks to that omnipresent gas, these so-called active galactic nuclei (AGNs) could be factories for building big black holes out of smaller ones. If so, gravitational-wave detectors such as LIGO should be able to tease out signs of the hierarchical assembly of these swollen giants. Although the overall number of detections remains too low for conclusive classifications, some researchers have recently pointed to two hefty mergers as tantalizing hints of what AGN-facilitated black hole fusions might look like—a step toward using gravitational waves to study not only black holes but also the stars and galaxies that birthed them.

Figuring out how black hole pairs form “tells us a lot about stars,” says Maya Fishbach, a LIGO member, who was not involved in the recent research. “Stars are the building blocks of galaxies. They’re the atoms of astronomy.”

Run-of-the-mill black holes, born from the remnants of an exploded star, would typically start off in orbits skewed against the plane of an AGN’s disk of gas. Each time they would dip into that disk, however, friction would slow them and tip their paths in line with the disk. Once embedded, uneven pressures may shepherd those black holes from their initially scattered locations into special rings around the galaxy’s central black hole—a trapping process analogous to the one that forms the seeds of planets in dusty disks around a single star but with black holes instead of microscopic piles of dust.

Imre Bartos, a LIGO collaboration member at the University of Florida, estimates that these galactic “migration traps” can quickly collect tens of thousands of black holes, many of which will get close enough to pair off. Then friction from the lingering gas would drive them to collide 1,000 times earlier than they otherwise would in empty space. “They will be forced to merge together,” he says. It’s like “a black hole assembly line, where we are adding black holes one after another.”

Most of the universe’s black hole mergers are thought to be one-off finales between stellar binaries—star couplets that were born, lived and died together—of moderate mass. But if AGN disks really are cranking out large black holes made from small ones, that population should eventually stand out in two ways from gravitational-wave observations.

Now in its third observing run, LIGO announced dozens of preliminary gravitational-wave candidates for astronomical observation in 2019. But only 10 black hole mergers appear in the published catalogue from its first two observing runs, and nine of them seem to have come from pairs that spun slowly or not at all. A twirling crash—as would happen in an AGN disk—would, however, spin merging black holes up, typically causing subsequent generational mergers to spin even faster. Specifically, two black holes of even mass should spin at 70 percent of a theoretical top speed after colliding, so Bartos and his colleagues are on the lookout for collisions between already whirling dervishes.

They are also watching for mammoth mergers. Stars above a certain size are thought to undergo supernovae so savage that they blow their core to smithereens, preventing them from collapsing to form black holes. Theorists are unsure where the limit lies, but many expect the mass of stellar black holes to top out around that of 50 suns. “If you see a single event with 80 solar masses,” says Davide Gerosa, an astrophysicist at the University of Birmingham in England, “that’s a strong signature of some exotic formation channel.” Black hole nurseries would be one explanation for heavy outliers.

Recent data, however, may complicate that simple picture. In November 2019 astronomers announced the discovery of what seems to be a huge black hole born together with a partner star. If the current controversial estimate of the former’s mass—roughly 70 suns—stands, then the 50-sun limit may be a less clear-cut line for generational mergers. “I don’t think we’ve been hitting this problem hard enough,” Gerosa says.

Globular clusters, small clumps of stars within a galaxy, are another possible cosmic construction site for abnormally heavy, quickly spinning black holes. In these star-rich regions, black holes presumably could form dense crowds in which they would occasionally bump into one another. But recent research by Gerosa and one of his colleagues found that the recoil from such collisions would likely eject most pairs from the globular cluster, preventing them from finding future companions to merge with. Larger groups of stars, such as those within AGN disks, are more likely locations for strings of mergers, Gerosa says, because black holes there require much more recoil to escape.

After years of theoretical speculation, some researchers are starting to see hints of what may be an extra heavy population beginning to reveal itself. LIGO’s heaviest catalogued merger, GW170729, is exactly the kind an AGN disk would produce, Bartos and his colleagues proposed in Physical Review Letters in November. In that event, one of the black holes weighed roughly 50 suns, and a measure of the pair’s collective spin clocked them turning at about 40 percent of top speed before the merger—a hint that that an earlier collision could have spun them up.

Another candidate AGN-driven event appeared in October, when researchers at the Institute for Advanced Study posted a preprint paper announcing two possible mergers from LIGO’s data that, while not meeting LIGO’s criteria for publication, may well be genuine. Called GW170817A, and with a mass of roughly 56 suns and a combined premerger spin of 50 percent the maximal value, this candidate merger matches predictions for an AGN collision even more closely than GW170729, according to a study published in Astrophysical Journal Letters in November 2019. “This is exactly the same kind of event,” Bartos says.

Neither candidate is a smoking gun for an AGN black hole assembly line, however. GW170817A only registered in one of LIGO’s two detectors—a potential sign that it was a false alarm arising from contaminating noise on Earth rather than some far-off celestial cataclysm. Moreover because only a small fraction of the universe’s stars reside in AGNs, Bartos’s group concluded that the suggestive properties of these two mergers are just as likely to reflect normal binary black holes that just happened to be extra heavy and to spin extra fast as they are AGN black holes.

Other researchers agree that AGN disks could occasionally smack black holes together but stress that the community will need more data, as well as better predictions, to conclusively prove the reality of this rare collision type or others. “I don’t think there’s anyone who would be able to pick one side, because they’ll know that if they’re proven wrong, it will be in, like, a year,” Fishbach says.

Regardless the ability to distinguish one-off stellar binaries from AGN assembly lines and other putative production mechanisms for black hole mergers is coming. As LIGO’s catalogue swells, categories based on spin and mass should become much clearer. Bartos suggests that traditional astronomy based on light rather than gravitational waves could help, too. Gravitational crashes that align in the sky with known AGNs will supply further hints. And if astronomers can use their telescopes to rapidly observe gravitational-wave sources with AGN signatures as they are detected, a December 2019 publication in the Astrophysical Journal Letters proposes, they may glimpse flashes of light hypothesized to come from postcollision shock waves in the gas.

In the midst of building this new black hole taxonomy, astrophysicists are already brainstorming what they will be able to do with it. Light reveals what a galaxy is made of, says Katelyn Breivik, an astrophysicist at the Canadian Institute for Theoretical Astrophysics, but gravitational waves may unmask its more subtle dynamics. “If you have black holes that are embedded in these disks,” she says, “they are like literal gravitational probes into the shape of these disks,” revealing mass and motion.

While those probes have yet to materialize, LIGO’s big, spinning black holes encourage Bartos that they are not far off. “I was used to predicting the far future,” he says. “Coming from there to having these black holes that basically reproduce what you’re predicting is super exciting.”

--Originally published: Scientific American Online, December 4, 2019.

The First Monster Black Holes

by Priyamvada Natarajan

Imagine the universe in its infancy. Most scientists think space and time originated with the big bang. From that hot and dense start the cosmos expanded and cooled, but it took a while for stars and galaxies to start dotting the sky. It was not until about 380,000 years after the big bang that atoms could hold together and fill the universe with mostly hydrogen gas. When the cosmos was a few hundred million years old, this gas coalesced into the earliest stars, which formed in clusters that clumped together into galaxies, the oldest of which appears 400 million years after the universe was born. To their surprise, scientists have found that another class of astronomical objects begins to appear at this point, too: quasars.

Quasars are extremely bright objects powered by gas falling onto supermassive black holes. They are some of the most luminous things in the universe, visible out to the farthest reaches of space. The most distant quasars are also the most ancient, and the oldest among them pose a mystery.

To be visible at such incredible distances, these quasars must be fueled by black holes containing about a billion times the mass of the sun. Yet conventional theories of black hole formation and growth suggest that a black hole big enough to power these quasars could not have formed in less than a billion years. In 2001, however, with the Sloan Digital Sky Survey, astronomers began finding quasars that dated back earlier. The oldest and most distant quasar known, which was reported in December 2017, existed just 690 million years after the big bang. In other words, it does not seem that there had been enough time in the history of the universe for quasars like this one to form.

Many astronomers think that the first black holes—seed black holes—are the remnants of the first stars, corpses left behind after the stars exploded into supernovae. Yet these stellar remnants should contain no more than a few hundred solar masses. It is difficult to imagine a scenario in which the black holes powering the first quasars grew from seeds this small.

To solve this quandary, a decade ago some colleagues and I proposed a way that seed black holes massive enough to explain the first quasars could have formed without the birth and death of stars. Instead these black hole seeds would have formed directly from gas. We call them direct-collapse black holes (DCBHs). In the right environments, direct-collapse black holes could have been born at 10⁴ or 10⁵ solar masses within a few hundred million years after the big bang. With this head start, they could have easily grown to 10⁹ or 10¹⁰ solar masses, thereby producing the ancient quasars that have puzzled astronomers for nearly two decades.

The question is whether this scenario actually happened. Luckily, when the James Webb Space Telescope (JWST) launches in 2021, we should be able to find out.

The First Seeds

Black holes are enigmatic astronomical objects, areas where the gravity is so immense that it has warped spacetime so that not even light can escape. It was not until the detection of quasars, which allow astronomers to see the light emitted by matter falling into black holes, that we had evidence that they were real objects and not just mathematical curiosities predicted by Einstein’s general theory of relativity.

Most black holes are thought to form when very massive stars—those with more than about 10 times the mass of sun—exhaust their nuclear fuel and begin to cool and therefore contract. Eventually gravity wins, and the star collapses, igniting a cataclysmic supernova explosion and leaving behind a black hole. Astronomers have traditionally assumed that most of the black holes powering the first quasars formed this way, too. They could have been born from the demise of the universe’s first stars (Population III stars), which we think formed when primordial gas cooled and fragmented about 200 million years after the big bang. Population III stars were probably more massive than stars born in the later universe, which means they could have left behind black holes as hefty as several hundred solar masses. These stars also probably formed in dense clusters, so it is likely that the black holes created on their deaths would have merged, giving rise to black holes of several thousand solar masses. Even black holes this large, however, are far smaller than the masses needed to power the ancient quasars.

Theories also suggest that so-called primordial black holes could have arisen even earlier in cosmic history, when spacetime may have been expanding exponentially in a process called inflation. Primordial black holes could have coalesced from tiny fluctuations in the density of the universe and then grown as the universe expanded. Yet these seeds would weigh only between 10 and 100 solar masses, presenting the same problem as Population III remnants.

As an explanation for the first quasars, each of these pathways for the formation of black hole seeds has the same problem: the seeds would have to grow extraordinarily quickly within the first billion years of cosmic history to create the earliest quasars. And what we know about the growth of black holes tells us that this scenario is highly unlikely.

Feeding a Black Hole

Our current understanding of physics suggests that there is an optimal feeding rate, known as the Eddington rate, at which black holes gain mass most efficiently. A black hole feeding at the Eddington rate would grow exponentially, doubling in mass every 10⁷ years or so. To grow to 10⁹ solar masses, a black hole seed of 10 solar masses would have to gobble stars and gas unimpeded at the Eddington rate for a billion years. It is hard to explain how an entire population of black holes could continuously feed so efficiently.

In effect, if the first quasars grew from Population III black hole seeds, they would have had to eat faster than the Eddington rate. Surpassing that rate is theoretically possible under special circumstances in dense, gas-rich environments, and these conditions may have been available in the early universe, but they would not have been common, and they would have been shortlived. Furthermore, exceptionally fast growth can actually cause “choking,” where the radiation emitted during these super-Eddington episodes could disrupt and even stop the flow of mass onto the black hole, halting its growth. Given these restrictions, it seems that extreme feasting could account for a few freak quasars, but it cannot explain the existence of the entire detected population unless our current understanding of the Eddington rate and black hole feeding process is wrong.

Thus, we must wonder whether the first black hole seeds could have formed through other channels. Building on the work of several other research groups, my collaborator Giuseppe Lodato and I published a set of papers in 2006 and 2007 in which we proposed a novel mechanism that could have produced more massive black hole seeds from the get-go. We started with large, pristine gas disks that might otherwise have cooled and fragmented to give rise to stars and become galaxies. We showed that it is possible for these disks to circumvent this conventional process and instead collapse into dense clumps that form seed black holes weighing 10⁴ to 10⁶ solar masses. This outcome can occur if something interferes with the normal cooling process that leads to star formation and instead drives the entire disk to become unstable, rapidly funneling matter to the center, much like water flowing down a bathtub drain when you pull the plug.

Disks cool down more efficiently if their gas includes some molecular hydrogen—two hydrogen atoms bonded together—rather than atomic hydrogen, which consists of only one atom. But if radiation from stars in a neighboring galaxy strikes the disk, it can destroy molecular hydrogen and turn it into atomic hydrogen, which suppresses cooling, keeping the gas too hot to form stars. Without stars, this massive irradiated disk could become dynamically unstable, and matter would quickly drain into its center, rapidly driving the production of a massive, direct-collapse black hole. Because this scenario depends on the presence of nearby stars, we expect DCBHs to typically form in satellite galaxies that orbit around larger parent galaxies where Population III stars have already formed.

Simulations of gas flows on large scales, as well as the physics of small-scale processes, support this model for DCBH formation. Thus, the idea of very large initial seeds appears feasible in the early universe. And starting with seeds in this range alleviates the timing problem for the production of the supermassive black holes that power the brightest, most distant quasars.

Looking for Proof

But just because DCBH seeds are feasible does not mean they actually exist. To find out, we must search for observational evidence. These objects would appear as bright, miniature quasars shining through the early universe. They should be detectable during a special phase when the seed merges with the parent galaxy—and this process should be common, given that DCBHs probably form in satellites orbiting larger galaxies. A merger would give the black hole seed a copious new source of gas to eat, so the black hole should start growing rapidly. In fact, it would briefly turn into a special kind of quasar that outshines all the stars in the galaxy.

These black holes will not only be brighter than their surrounding stars, they will also be heavier—a reversal of the usual order of things. In general, the stars in a galaxy outweigh the central black holes by about a factor of 1,000. After the galaxy hosting the DCBH merges with its parent galaxy, however, the mass of the growing black hole will briefly exceed that of the stars. Such an object, called an obese black hole galaxy (OBG), should have a very special spectral signature, particularly in the infrared wavelengths between one and 30 microns where the JWST’s Mid-Infrared Instrument (MIRI) and Near-Infrared Camera (NIRCam) cameras will operate. This telescope will be the most powerful tool astronomers have ever had for peering into the earliest stages of cosmic history. If the telescope detects these obese black hole galaxies, it will provide strong evidence for our DCBH theory. Traditional black hole seeds, on the other hand, which derive from dead stars, are likely to be too faint for the JWST or other telescopes to see.

It is also possible that we might find other evidence for our theory. In the rare case that the parent galaxy that merges with the DCBH also hosts a central black hole, the two holes will collide and release powerful gravitational waves. These waves could be detectable by the Laser Interferometer Space Antenna (LISA), a European Space Agency/NASA mission expected to fly in the 2030s.

A Fuller Picture

It is entirely possible that both the DCBH scenario and small seeds feeding at super-Eddington rates both occurred in the early universe. In fact, the initial black hole seeds probably formed via both these pathways. The question is, Which channel created the bulk of the bright ancient quasars that astronomers see? Solving this mystery could do more than just clear up the timeline of the early cosmos. Astronomers also want to understand more broadly how supermassive black holes affect the larger galaxies around them.

Data suggest that central black holes might play an important role in adjusting how many stars form in the galaxies they inhabit. For one thing, the energy produced when matter falls into the black hole may heat up the surrounding gas at the center of the galaxy, thus preventing cooling and halting star formation. This energy may even have far-reaching effects outside the galactic center by driving energetic jets of radiation outward. These jets, which astronomers can detect in radio wavelengths, could also heat up gas in outer regions and shut down star formation there. These effects are complex, however, and astronomers want to understand the details more clearly. Finding the first seed black holes could help reveal how the relation between black holes and their host galaxies evolved over time.

These insights fit into a larger revolution in our ability to study and understand all masses of black holes. When the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first detection of gravitational waves in 2015, for instance, scientists were able to trace them back to two colliding black holes weighing 36 and 29 solar masses, the lightweight cousins of the supermassive black holes that power quasars. The project continues to detect waves from similar events, offering new and incredible details about what happens when these black holes crash and warp the spacetime around them. Meanwhile a project called the Event Horizon Telescope aims to use radio observatories scattered around Earth to image the supermassive black hole at the center of the Milky Way. Scientists hope to spot a ringlike shadow around the black hole’s boundary that general relativity predicts will occur as the hole’s strong gravity deflects light. Any deviations the Event Horizon Telescope measures from the predictions of general relativity have the potential to challenge our understanding of black hole physics. In addition, experiments looking at pulsing stars called pulsar timing arrays could also detect tremors in spacetime caused by an accumulated signal of many collisions of black holes. And very soon the JWST will open up an entirely new window on the very first black holes to light up the universe.

Many revelations are in store in the very near future, and our understanding of black holes stands to be transformed.

--Originally published: Scientific American 318(2); 24-29 (February 2018).

SECTION 3

Can Black Holes Rewrite Physics?

Escape from a Black Hole

by Steven B. Giddings

Humankind caught its first glimpse of a black hole on April 10, 2019. The Event Horizon Telescope (EHT) team, which uses an Earth-spanning network of radio observatories acting in concert, shared images it had captured of an apparent black hole with 6.5 billion times the mass of our sun in the center of the nearby M87 galaxy. This was a breathtaking achievement—our first views of one of the most mysterious objects in the universe, long predicted but never directly “seen.” Even more exciting, the images, and the observations that should follow, are beginning to provide new clues about one of the deepest puzzles in physics.

This enigma is the “paradox” of what happens to information in a black hole. By investigating this question, physicists have discovered that the mere existence of black holes is inconsistent with the quantum-mechanical laws that so far describe everything else in our universe. Resolving this inconsistency may require a conceptual revolution as profound as the overthrow of classical physics by quantum mechanics.

Theorists have explored many ideas, but there has been little direct evidence to help resolve this problem. The first image of a black hole, however, begins to offer actual data to inform our theories. Future EHT observations—especially those that can show how black holes evolve over time—and recent detections of colliding black holes by gravitational-wave observatories could provide important new insights and help to usher in a whole new era of physics.

The Information Problem

Though deeply mysterious, black holes seem to be ubiquitous in the cosmos. The EHT observations and the gravitational-wave measurements are just the latest and most robust evidence that black holes, despite sounding fantastical, do indeed appear to be real—and remarkably common. Yet their very existence threatens the present foundations of physics. The basic principles of quantum mechanics are thought to govern all the other laws of nature, but when they are applied to black holes they lead to a contradiction, exposing a flaw in the current form of these laws.

The problem arises from one of the simplest questions we can ask about black holes: What happens to stuff that falls into them? We need a little refinement here to fully explain. First, according to our present quantum-mechanical laws, matter and energy can shift between different forms: particles can, for example, change into different kinds of particles. But the one thing that is sacred and never destroyed is quantum information. If we know the complete quantum description of a system, we should always be able to exactly determine its earlier or later quantum description with no loss of information. So a more precise question is, What happens to quantum information that falls into a black hole?

Our understanding of black holes comes from Albert Einstein’s general theory of relativity, which describes gravity as arising from the curvature of space and time; a common visualization of this idea is a heavy ball deforming the surface of a trampoline. This warping of spacetime causes the trajectories of massive bodies and light to bend, and we call that gravity. If mass is sufficiently concentrated in a small-enough vicinity, the nearby spacetime deformation is so strong that light itself cannot escape a region inside what we call the event horizon: we have a black hole. And if nothing can travel faster than light—including information—everything must get stuck inside this boundary. Black holes become cosmic sinkholes trapping information along with light and matter.

But the story becomes stranger. What may be Stephen Hawking’s greatest discovery is his 1974 prediction that black holes evaporate. This finding also led to the startling idea that black holes destroy quantum information. According to quantum mechanics, pairs of “virtual particles” pop into existence all the time, everywhere. Typically such a pair, consisting of a particle and its antimatter counterpart, quickly annihilates, but if it forms near the horizon of a black hole, one particle might pop up inside this boundary and the other outside. The outside particle can escape, carrying away energy. The law of energy conservation tells us that the black hole has thus lost energy, so the emission of such particles causes the black hole to shrink over time until it completely disappears. The problem is that the escaping particles, known as Hawking radiation, carry essentially no in formation about what went into the black hole. Therefore, Hawking’s calculations appear to show that quantum information that falls into a black hole is ultimately destroyed—contradicting quantum mechanics.

This revelation initiated a deep crisis in physics. Great advances have followed from previous such crises. For instance, at the beginning of the 20th century, classical physics seemed to predict the inevitable instability of atoms, in obvious contradiction to the existence of stable matter. That problem played a key role in the quantum revolution. Classical physics implied that because orbiting electrons within atoms are constantly changing direction, they continually emit light, causing them to lose energy and spiral into the nucleus. But in 1913 Niels Bohr proposed that electrons actually travel only within quantized orbits and cannot spiral in. This radical idea helped to establish the basis of quantum mechanics, which fundamentally rewrote the laws of nature. Increasingly it seems that the black hole crisis will similarly lead to another paradigm shift in physics.

The Information Problem

Black holes were predicted by general relativity, and mounting astrophysical evidence supports their existence. But in 1974 Stephen Hawking argued that black holes eventually evaporate. If so, everything that falls into them is ultimately destroyed, including the information contained in the matter that fell in. The problem is that quantum mechanics and energy conservation forbid such destruction of information. In response, physicists have come up with several suggestions for how to modify our picture of black holes to make them compatible with quantum physics:

Quantum Alternatives

When Hawking first predicted black hole evaporation, he suggested that quantum mechanics must be wrong and that information destruction is allowed. Yet physicists soon realized this change would require a drastic breakdown of the law of energy conservation, which would disastrously invalidate our present description of the universe. Apparently the resolution must be sought elsewhere.

Another early idea was that black holes do not completely evaporate but instead stop shrinking at a tiny size, leaving behind microscopic remnants containing the original information. But, scientists realized, if this were true, basic properties of quantum physics would predict catastrophic instabilities causing ordinary matter to explode into such remnants, also contradicting everyday experience.

Obviously something is deeply wrong. It is tempting to conclude that the flaw is in Hawking’s original analysis and that somehow information does escape a black hole emitting Hawking radiation. The challenge here is that this scenario would conflict with a foundational concept of present-day physics, the principle of locality, which states that information cannot move from one place to another superluminally—that is, faster than the speed of light. But according to our definition of black holes, the only way to escape one is to travel faster than light, so if information does escape, it must be doing so superluminally, in conflict with locality. In the four decades since Hawking’s discovery, physicists have tried to find a loophole to this argument that stays within conventional physics, but none has emerged.

The closest attempt was a 2016 proposal by Hawking, Malcolm Perry and Andrew Strominger, who suggested that a mistake in the original analysis implies information never fully enters a black hole but instead leaves a kind of imprint in the form of what they called “soft hair” outside it. Closer examination seems to be closing this loophole, however, and most experts do not believe this can be the answer. In short, more radical steps appear to be needed.

An obvious idea is that there is some unknown physics that prevents true black holes from existing at all. The conventional picture of black hole formation says that when very large stars burn out and die, their mass collapses under the force of gravity into a black hole. But what if they never reach that stage and actually transform into objects with “better” behavior? In fact, we know that when lower-mass stars such as our sun burn out and collapse, they do not form black holes and instead form dense remnants—for example, white dwarfs or neutron stars. Perhaps some unknown laws of physics also prevent larger stars from forming black holes and instead lead them to become a kind of “massive remnant”—something more like a neutron star than a black hole.

The problem with this suggestion is that we cannot explain what would stabilize such objects—no known physics should prevent their continued collapse under gravity, and any imagined physics that did would apparently require superluminal signaling from one side of the collapsing matter to the other. In fact, conventional large black holes can form from very low-density matter. To illustrate, if the 6.5-billion-solar-mass black hole in M87 arose from the collapse of a dust cloud (which is theoretically possible, although the actual process was apparently more complex), it would have happened when the dust reached the density of air at the top of Mount Everest. (Air on top of Everest does not form a black hole, because there is not enough of it; one would require an accumulated 6.5 billion solar masses.) Some drastic and superluminal new physical process would need to take over in such a low-density regime to instantly convert the collapsing cloud into a massive remnant instead of allowing a black hole to form.

A related idea is that something could cause black holes to change into massive remnants containing the original information after they form but long before they evaporate. But once again, this story requires nonlocal transfer of information from the interior of the initial black hole to the final remnant.

Despite their problems, physicists have explored versions of both these scenarios. For example, in 2003 Samir Mathur put forward a proposal based on string theory, which posits that fundamental particles are tiny strings. His idea is that a black hole transforms into a “fuzzball,” a kind of massive remnant, or that a fuzzball forms instead of a black hole in the first place. Thanks to the complicated physics of string theory and its allowance for more than the traditional four dimensions of spacetime, fuzzballs might have a complex higher-dimensional geometry; instead of the sharp traditional boundary of a black hole at the event horizon, a fuzzball would have a fuzzier and larger boundary where one encounters strings and higher-dimensional geometry.

Alternatively a more recent version of a remnant scenario is the proposal that instead of a black hole with an event horizon, a massive remnant forms with a surface “firewall” of high-energy particles where the horizon would be. This firewall would incinerate anything that encountered it, turning it into pure energy that added to the firewall. Both the firewall and the fuzzball, though, share the problem of needing locality violation, and the resulting objects would have other properties that are very hard to explain.

Modifying Locality

A common thread in massive-remnant proposals is that saving quantum mechanics appears to require violation of the locality principle. But doing so carelessly is expected to be as disastrous as modifying quantum mechanics and, in fact, typically leads to another paradox. Specifically, the laws of relativity say that if you send a faster-than-light signal in empty, flat space, observers traveling past you at a high-enough speed will see the signal going backward in time. The paradox arises because this superluminal signaling then allows you to send a message into your past, for example, asking someone to kill your grandmother before your mother is born.

Even though this kind of answer appears to contradict fundamental physical principles, it is worth a closer look. Modifying locality seems crazy, but we have not found an alternative that does not. The severe nature of the black hole crisis strongly suggests a resolution via some subtle violation of the locality principle, one that does not produce such paradoxes. Put differently, quantum mechanics implies information is never destroyed, so information that falls into a black hole must ultimately escape, possibly through some new, subtle “delocalization” of information that might become clear when we can finally find a way to unify quantum mechanics and gravity—one of the most profound problems of present-day physics. In fact, we have other reasons to think such a subtlety could be present. The very idea of localized information—that it can exist in one place and not in another—is more delicate in theories that include gravity than in those that do not, because gravitational fields extend to infinity, complicating the concept of localization.

If information does escape black holes, it might not require a change as obvious and abrupt as the formation of a massive remnant, whether fuzzball, firewall or another variant. The growing evidence for black holes suggests there are objects in the universe that look and act a lot like classical black holes, without large departures from Einstein’s predictions. Is Einstein’s general relativity so drastically wrong in its description of black holes, or might there be some more innocuous, currently unknown effects that delocalize information and allow it to leak from black holes, avoiding such a dramatic failure of the entire spacetime picture?

In my recent theoretical work, I have found two versions of such effects. In one, the geometry of spacetime near a black hole is altered, making it bend and ripple in a way that depends on the information in the black hole—but gently, so that it does not, for example, destroy an astronaut falling through the region where the horizon would ordinarily be found. In this “strong, nonviolent” scenario, such shimmering of spacetime can transfer the information out. Interestingly, I have also found that there is a subtler, intrinsically quantum way for information to escape the black hole. In this “weak, nonviolent” scenario, even tiny quantum fluctuations of the spacetime geometry near the black hole can transfer information to particles emanating from the hole. The fact that the information transfer is still large enough to save quantum mechanics is related to the huge amount of possible information a black hole can contain. In either picture, a black hole effectively has a “quantum halo” surrounding it, where interactions pass information back to its surroundings.

Notably, these scenarios, despite appearing to require superluminal travel of information, do not necessarily produce a grandmother paradox. The information signaling here is tied to the existence of the black hole, which has a spacetime geometry that is different from that of flat space, so that the earlier argument about communicating with the past no longer holds. These possibilities are tantalizing from another perspective: the locality principle is also what prohibits our own faster-than-light travel; the quantum mechanics of black holes seems to be telling us there is something wrong with the present formulation of this principle.

Rewriting the Laws of Physics

So far such a quantum-halo scenario has not been predicted by a more complete theory of physics that reconciles quantum mechanics with gravity, but it is strongly indicated by the need to resolve the problem and by assumptions based on what we see. If such a scenario is correct, it probably represents an approximate description of a deeper reality. Our very notions of space and time, which underlie the rest of science, appear to require significant revision. The present work to understand black holes may be akin to the first attempts to model the physics of the atom by Bohr and others. Those early atomic descriptions were also approximate and only later led to the profound theoretical structure of quantum mechanics. Although modifying locality seems crazy, we might find solace by noting that the laws of quantum mechanics also seemed very crazy to the classical physicists grappling with their discovery.

Given the immense challenge in sorting out the story of quantum black holes and the more complete theory describing them, physicists are eager for experimental and observational evidence to help guide us. The exciting recent advances have given humankind two direct observational windows on black hole behavior. In addition to the EHT’s images of black holes, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its companion facilities have begun to detect gravitational waves from collisions between apparent black holes. These waves carry valuable information with them about the properties and behavior of the objects that created them.

From a naive viewpoint, it seems preposterous that the EHT or LIGO could detect any departure from Einstein’s description of black holes. Traditionally his theory has been expected to need modification only when spacetime curvatures become extremely large, near the center of a black hole; in contrast, curvatures are very weak near the horizon of a large black hole. But the information crisis I have described suggests otherwise. A large part of the theoretical community has now reached the consensus that some changes to the current laws of physics are needed to describe phenomena not just deep inside a black hole but all the way out past the horizon. We appear to have crossed the Rubicon. For the case of the black hole in M87, the distance at which we expect to find deviations from classical predictions is several times the size of our solar system.

Already LIGO and the EHT have ruled out wilder possibilities that could be considered in an attempt to give a logically consistent description of black holes. Specifically, if black holes were replaced by massive remnants more than about twice the diameter of the supposed black hole, we would have seen signs in the data from both experiments. In the case of the EHT, much of the light that produced the now famous image comes from a region around one and a half times the diameter of the event horizon. And for LIGO, part of the gravitational-wave signal that we detect is likewise produced from the region where the colliding objects reach similarly small separations. Although study of these signals is still in early phases, the EHT and LIGO have revealed very dark and very compact objects that produce signals just like those predicted for unmodified black holes.

Still, it is important to investigate these signals more closely. Sufficiently careful analysis might in fact uncover more clues about the quantum physics of black holes. Even if no new effects are observed, we then have information that constrains possible descriptions of their quantum behavior.

Sufficiently large-diameter remnants are now ruled out, but what about remnant scenarios that modify the black hole description only very near the horizon? Although a complete discussion would require a fuller theory of these remnants—such as fuzzballs or firewalls—we have some initial indicators. Specifically, if these objects had radii barely larger than the radius of the corresponding black hole horizon, then it is likely that neither EHT nor LIGO observations would be able to reveal such a structure because very little light or gravitational radiation escapes from the region very near the horizon.

One possible exception is the possibility of gravitational “echoes.” As first suggested in 2016 by Vitor Cardoso of the University of Lisbon, Edgardo Franzin of the University of Barcelona and Paolo Pani of Rome University, if two such remnants combine to form a final remnant that has similar properties, gravitational waves can reflect off the merged remnant’s surface and might be observed. Whereas most near-horizon scenarios are hard to rule out through observation, however, it is difficult to explain how such structures could be stable, instead of collapsing under their own weight to form black holes. Of course, this is a general problem for all massive-remnant scenarios, but it becomes even more challenging in the presence of the extreme forces in such a collision.

Prospects are better for testing some of the scenarios where new interactions behave like subtle modifications of spacetime geometry but extend well outside the horizon. For example, in the strong nonviolent scenario, the rippling of a black hole’s quantum halo can distort light passing near the black hole. If this scenario is correct, the shimmering could cause distortions of the EHT’s images that change with time.

In my work with EHT scientist Dimitrios Psaltis, we found these changes could happen over roughly an hour for the black hole in the center of our galaxy. Because the EHT combines multihour observations into an average, such effects may be hard to see. But the relevant fluctuation time for the black hole in M87, which is more than 1,000 times larger, is more like tens of days. This work suggests we should look for these distortions by using longer-duration EHT observations than the project’s initial seven-day span. If the experiment found such distortions, they would be a spectacular clue to the quantum physics of black holes. If they do not appear, that will begin to point to the subtler weak quantum scenario or to something even more exotic.

The weak nonviolent scenario is harder to test because of the relative smallness of the expected changes to the geometry. Yet preliminary investigation shows that this scenario can alter how gravitational waves are absorbed or reflected, possibly yielding an observable modification to gravitational-wave signals.

If either scenario is correct, we will learn more not only about what quantum black holes are but also about the deeper laws of nature. Right now we do not fully understand how to think about information localization when gravitational fields are present. Quantum physics suggests that spacetime itself is not a fundamental part of physics but instead arises only as an approximation of a more basic mathematical structure. Evidence for quantum black hole effects could help make this concept more concrete.

To learn more, it is important to extend and improve both EHT and gravitational-wave measurements. For the EHT, it would be useful to have significantly longer-duration observations, as well as images of other targets such as our galaxy’s central black hole, both of which are anticipated. For gravitational waves, more observations with increased sensitivity would be helpful and will be assisted when additional detectors come online in Japan and India, adding to the existing facilities in the U.S. and Europe. Furthermore, a strong complementary theoretical effort is needed to refine scenarios, to better clarify their origins and explanations, and to assess more thoroughly the question of how significantly they can affect EHT or gravitational-wave signals.

Whatever the resolution to the crisis, black holes contain crucial clues to the basic quantum physics of gravity, as well as to the very nature of space and time. Just as with the atom and quantum mechanics, a better understanding of black holes is likely to help guide the next conceptual revolution in physics. EHT and gravitational-wave observations have the potential to provide us with key information, either by ruling out quantum black hole scenarios or by discovering new phenomena associated with them.

--Originally published: Scientific American 321(6); 50-57 (December 2019).

Black Holes, Wormholes and the Secrets of Quantum Spacetime

by Juan Maldacena

Theoretical physics is full of mind-boggling ideas, but two of the weirdest are quantum entanglement and wormholes. The first, predicted by the theory of quantum mechanics, describes a surprising type of correlation between objects (typically atoms or subatomic particles) having no apparent physical link. Wormholes, predicted by the general theory of relativity, are shortcuts that connect distant regions of space and time. Work done in recent years by several theorists, including myself, has suggested a connection between these two seemingly dissimilar concepts. Based on calculations involving black holes, we realized that quantum mechanics’ entanglement and general relativity’s wormholes may actually be equivalent—the same phenomena described differently—and we believe the likeness applies to situations beyond black holes.

This equivalence could have profound consequences. It suggests that spacetime itself could emerge from the entanglement of more fundamental microscopic constituents of the universe. It also suggests that entangled objects—despite having long been viewed as having no physical connection to one another—may in fact be connected in ways that are far less fantastical than we thought.

Furthermore, the relation between entanglement and wormholes may help in developing a unified theory of quantum mechanics and spacetime—what physicists call a theory of quantum gravity—that derives the physics of the macroscopic cosmos from the laws that govern the interactions of the atomic and subatomic realms. Such a theory is necessary for understanding the big bang and the interior of black holes.

Interestingly, both quantum entanglement and wormholes date back to two articles written by Albert Einstein and his collaborators in 1935. On the surface, the papers seem to deal with very different phenomena, and Einstein probably never suspected that there could be a connection between them. In fact, entanglement was a property of quantum mechanics that greatly bothered the German physicist, who called it “spooky action at a distance.” How ironic that it now may offer a bridge to extend his relativity theory to the quantum realm.

Black Holes and  Wormholes

To explain why I think quantum entanglement and wormholes could be related, we must first describe several properties of black holes, which are intimately involved in this idea. Black holes are regions of curved spacetime that differ drastically from the relatively nondistorted space we are used to. The distinctive feature of a black hole is that we can separate its geometry into two regions: the exterior, where space is curved but objects and messages can still escape, and the interior, lying beyond the point of no return. The interior and exterior are separated by a surface called the event horizon. General relativity tells us that the horizon is just an imaginary surface; an astronaut crossing it would not feel anything special at that location. But having crossed it, a space traveler would be doomed to being squeezed into a region with huge curvature and with no possibility of escape. (In fact, the interior is actually in the future compared with the exterior, so the traveler cannot escape, because he or she cannot travel to the past.)

Just a year after Einstein introduced general relativity, German physicist Karl Schwarzschild found the simplest solution to Einstein’s equations describing what would later be called black holes. The geometry that Schwarzschild came up with was so unexpected that it took until the 1960s for scientists to fully understand that it describes a wormhole joining two black holes. From the outside the black holes appear to be separate entities sitting at distant locations, yet they share an interior.

In a 1935 paper, Einstein and his colleague Nathan Rosen, then at the Institute for Advanced Study in Princeton, N.J., anticipated that this shared interior was a kind of wormhole (although they did not understand the full geometry it predicted), and for this reason wormholes are also called Einstein-Rosen (ER) bridges.

The wormhole in Schwarzschild’s solution differs from black holes that form naturally in the cosmos in that it contains no matter—merely curved spacetime. Because of the presence of matter, naturally formed black holes have only one exterior. Most researchers view the full Schwarzschild solution, with its two exteriors, as a mathematical curiosity irrelevant to the black holes in the universe. Nevertheless, it is an interesting solution and physicists have wondered about its physical interpretation.

The Schwarzschild solution tells us that the wormhole connecting the two black hole exteriors varies with time. It elongates and becomes thinner as time progresses, like stretching out a piece of elastic dough. Meanwhile the two black hole horizons, which at one point touch, separate rapidly. In fact, they pull apart so quickly that we cannot use such a wormhole to travel from one exterior to the other. Alternatively, we can say that the bridge collapses before we can cross it. In the dough-stretching analogy, the collapse of the bridge corresponds to the dough becoming infinitesimally thin as it gets stretched more and more.

It is important to note that the wormholes we are discussing are consistent with the laws of general relativity, which do not allow faster than light travel. In that way they differ from science-fiction wormholes that allow instantaneous transport between distant regions of space, as in the movie Interstellar. Sci-fi versions often violate the known laws of physics.

A science-fiction story involving our type of wormhole might look like the following. Imagine two young lovers, Romeo and Juliet. Their families do not like each other and so put Romeo and Juliet on different galaxies, forbidding them to travel. These lovebirds are very smart, however, and manage to construct a wormhole. From the outside the wormhole looks like a pair of black holes, one in Romeo’s galaxy and one in Juliet’s galaxy. The lovers decide to jump into the interior of their respective black holes. Now, according to their families, they simply committed suicide by jumping in and are never heard from again. Unbeknownst to the outside world, though, the geometry of the wormhole is such that Romeo and Juliet actually meet in the shared interior! And they can live together happily for a while before the bridge collapses, destroying the interior and killing them both.

Quantum Entanglement

The 1935 paper discussing the other phenomenon of interest to us—entanglement—was written by Einstein, Rosen and Boris Podolsky (also then at the Institute for Advanced Study). The three authors came to be known as EPR. In this famous work, the physicists argued that quantum mechanics allows for the existence of certain strange correlations between distant physical objects, a property that would only later be called entanglement.

Correlations between distant objects can also happen in classical physics. Imagine, for example, that you leave home with a single glove because you forgot the other one at home. Before searching your pocket, you do not know whether you have the left or right glove. Once you see that you have the right-hand glove, though, you will immediately know that the one at home is the lefty. But entanglement involves a different kind of correlation, one that exists between quantities governed by quantum mechanics, which are subject to Heisenberg’s uncertainty principle. This principle says that there are pairs of physical variables that are impossible to know accurately at the same time. The best-known example involves the position and velocity of a particle: if we measure its position accurately, its velocity becomes uncertain, and vice versa. EPR wondered what would happen if we decided to measure either the positions or the velocities of the individual particles in a pair separated by a wide distance.

The example that EPR analyzed involves two particles with the same mass moving in a single dimension. Let us call these particles R and J because they are the particles that we will imagine being measured by Romeo and Juliet. We can prepare them in such a way that their center of mass has a well-defined position, which we will call xcm , equal to xR (the position of R) plus xJ (the position of J). We can require the center of mass to equal zero—in other words, we can say that the two particles are always equidistant from the origin. We can also make the particles’ relative velocity, vrel , equal to the velocity of R (vR) minus the velocity of J (vJ), take a precise value; for example, vrel equals some number we can call v₀. In other words, the difference between the two velocities must stay the same. We are here specifying a position and a velocity accurately but not for the same single object, so we do not violate Heisenberg’s uncertainty principle. If we have two different particles, nothing prevents us from knowing the position of the first and the velocity of the second. Similarly, once we fix the position of the center of mass, we cannot say anything about the velocity of the center of mass, but we are free to fix the relative velocity.

Here we get to the most amazing part and the thing that makes quantum entanglement seem so strange. Suppose that our particles are far away from each other, and two distant observers, Romeo and Juliet, decide to measure the particles’ positions. Now, because of how the particles have been prepared, if Juliet determines any specific value for xJ , then Romeo will find that his particle’s position is the negative of Juliet’s (xR = −xJ). Note that Juliet’s result is random: the position of her particle will vary from measurement to measurement. Romeo’s result, however, is completely fixed by Juliet’s. Now suppose they both measure their own particle’s velocity. If Juliet gets a specific result for vJ, then Romeo will surely find that his velocity is the value of Juliet’s plus the relative velocity (vR = vJ + v₀). Again Romeo’s result is completely determined by Juliet’s. Of course, Romeo and Juliet are free to choose which variable they will measure. In particular, if Juliet measures the position and Romeo measures the velocity, their results will be random and will not display any correlation.

The strange thing is that even though Romeo’s measurements of the position and velocity of his particle are constrained by Heisenberg’s uncertainty principle, if Juliet decides to measure the position of her particle, Romeo’s particle will have a completely certain position once he knows the result of Juliet’s measurement. And the same thing will happen with the velocity. It appears as if, when Juliet measured the position, Romeo’s particle immediately “knew” that it must have a well-defined position and an uncertain velocity, whereas the opposite should be the case if Juliet measured the velocity. At first glance this situation appears to allow an instantaneous transmission of information: Juliet can measure the position, and then Romeo would see a definite position for his particle, thus inferring that Juliet measured the position. Romeo would not be able to realize, however, that his particle has a definite position without knowing the actual value of the position that Juliet measured. So in fact, correlations caused by quantum entanglement cannot be used to send signals faster than the speed of light.

Although it has been experimentally confirmed, entanglement may still seem just an esoteric property of quantum systems. Yet during the past two decades these quantum correlations have led to a number of practical applications and breakthroughs in fields such as cryptography and quantum computing.

Entanglement Meets Wormholes

Entanglement is a concept from the theory of quantum mechanics describing a special type of correlation between two distant objects. Wormholes, which were predicted by general relativity, are theoretical bridges in spacetime joining distant black holes. Physicists now think that these two phenomena, seemingly disconnected, may be fundamentally related.

Credit: Illustrated by Jillian Ditner

Equivalence

How might our two very different, bizarre phenomena—wormholes and entanglement—be related? A further look at black holes points the way to the answer. In 1974 Stephen Hawking showed that quantum effects will cause black holes to emit radiation in the same way a hot object does—proving that the traditional wisdom that nothing can escape a black hole is too simplistic. The fact that black holes radiate implies that they have a temperature—a notion with important ramifications.

Since the 19th century physicists have known that temperature stems from the movement of the microscopic constituents of a system. In a gas, for example, temperature arises from the agitation of molecules. Therefore, if black holes have temperatures, one can expect that they also have some kind of microscopic constituents that collectively are capable of adopting various possible configurations, or so-called microstates. We also believe that, at least as seen from the outside, black holes should behave as quantum systems; that is, they should be subject to all the laws of quantum mechanics. In summary, when we look at a black hole from the exterior we should find a system that can have many microstates, with the probability of its being in any of these configurations essentially equal for each microstate.

Because black holes look like ordinary quantum systems from the outside, nothing prevents us from considering an entangled pair of them. Imagine a couple of very distant black holes. Each has a large number of possible microscopic quantum states. Now imagine an entangled pair of black holes in which each quantum state in the first black hole is correlated with the corresponding quantum state of the second. In particular, if we measure a certain state for the first hole, the other hole must be in exactly the same state.

The interesting thing is that, based on certain considerations inspired by string theory (one approach toward a theory of quantum gravity), we can argue that a pair of black holes with their microstates entangled in this way (that is, in what might be called an EPR entangled state) would produce a spacetime in which a wormhole (an ER bridge) links the interior of both black holes. In other words, quantum entanglement creates a geometric connection between the two black holes. This result is surprising because entanglement, we thought, involves correlations without a physical connection. But the two distant black holes in this case are physically connected through their interior and brought close via the wormhole.

Leonard Susskind of Stanford University and I have called the equivalence of wormholes and entanglement “ER = EPR,” because it relates the two articles that Einstein and his colleagues wrote in 1935. From EPR’s point of view, the observations near the horizons of each black hole are correlated because the black holes are in a state of quantum entanglement. From ER’s vantage point, the observations are correlated because the two systems are linked through the wormhole.

Now, going back to our Romeo and Juliet sci-fi story, we can see what the lovers should do to form an entangled pair of black holes to produce the wormhole. First they need to create many entangled particle pairs, similar to the ones discussed earlier, with Romeo possessing one member of each entangled pair and Juliet the other. They then need to build very complex quantum computers that will manipulate their respective quantum particles and combine them in a controlled way to create a pair of entangled black holes. Such a feat would be terribly hard to achieve in practice, but it seems possible according to the laws of physics. Besides, we did say Romeo and Juliet were very smart!

A Universal Principle?

The ideas that led us here have been developed over the years by many researchers, beginning with a 1976 article by Werner Israel, then at the University of Alberta. There was also interesting work in 2006 on the connection between entanglement and the geometry of spacetime by Shinsei Ryu and Tadashi Takayanagi, both then at the University of California, Santa Barbara. Susskind and I were motivated by research published in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully, all then at U.C. Santa Barbara. They discovered a paradox related to the nature of an entangled black hole’s interior. The ER = EPR idea, which says that the interior is part of a wormhole connecting the black hole to another system, alleviates some aspects of this paradox.

Although we identified the connection between wormholes and entangled states using black holes, it is tempting to speculate that the link is more general—that whenever we have entanglement we have a kind of geometric connection. This expectation should hold true even in the simplest case, in which we have only two entangled particles. In such situations, however, the spatial connection could involve tiny quantum structures that would not follow our usual notion of geometry. We still do not know how to describe these microscopic geometries, but the entanglement of these structures might somehow give rise to spacetime itself. It is as if entanglement can be viewed as a thread connecting two systems. When the amount of entanglement becomes larger, we have lots of threads, and these threads could weave together to form the fabric of spacetime. In this picture, Einstein’s relativity equations are governing the connections and reconnections of these threads; quantum mechanics is not just an add-on to gravity—it is the essence of the construction of spacetime.

For now, this picture is still wild speculation, but several clues point toward it, and many of us physicists are pursuing its implications. We believe that the seemingly unrelated phenomena of entanglement and wormholes might in fact be equivalent and that this equivalence provides an important clue for developing a description of quantum spacetime—and a long-awaited unification of general relativity and quantum mechanics.

--Originally published: Scientific American 315(5); 26-31 (November 2016).

Burning Rings of Fire

by Joseph Polchinski

Falling into a black hole was never going to be fun. As soon as physicists realized that black holes exist, we knew that getting too close to one spelled certain death. But we used to think that an astronaut falling past the point of no return—the so-called event horizon—would not feel anything special. According to Einstein's general theory of relativity, no signposts would mark the spot where the chance of escape dropped to zero. Anyone journeying past the horizon would just seem to fall down, down, down into a pit of blackness.

Recently, however, my colleagues and I have recast that picture in light of some new information about the effects of quantum mechanics on black holes. It now seems that our astronaut would have an experience very different from Albert Einstein's prediction. Rather than falling seamlessly into the interior, the astronaut would encounter a “firewall” of high-energy particles at the horizon that would be instantly lethal. The wall might even mark the end of space.

In 2012, four of us, all then at the University of California, Santa Barbara—my colleague Donald Marolf, then graduate students Ahmed Almheiri and James Sully, and I (now known by the acronym AMPS)—arrived at this conclusion after using ideas from string theory to take a closer look at the physics of black holes, particularly at an interesting argument put forward in the 1970s by Stephen Hawking. Hawking had identified a deep conflict between the predictions of quantum theory and relativity in these extreme environments. According to his reasoning, either quantum mechanics or Einstein's depiction of spacetime is flawed. The battle over which view is correct has swung back and forth ever since.

As with Hawking's original claim, our recent firewall proposal has raised a storm of disbelief, and no satisfactory alternative has yet emerged. If quantum mechanics is to be trusted, firewalls are the consequence. Yet their existence raises theoretical puzzles as well. It seems that physicists must give up one of our widely cherished beliefs, but we cannot agree on which one. We hope, however, that out of this confusion will come a more complete understanding of quantum mechanics and relativity—and, ideally, a way to finally resolve the apparent contradictions between these two reigning theories of physics.

The Singularity

General relativity, which gave birth to the very concept of black holes, derives its picture of these mysterious entities and their event horizons from an understanding of gravity's effect on space and time. According to the theory, if enough mass comes together, gravity's pull will cause it to start collapsing. Nothing can stop this process until all the mass is compressed into a single point where spacetime is infinitely dense and infinitely curved, called the singularity—in other words, a black hole.

Any space travelers who pass the black hole's event horizon boundary will be unable to escape the gravitational pull and will soon be drawn into the singularity. Even light, once it is past the horizon, cannot escape. The singularity is a very dramatic place, but the horizon itself is supposed to be unremarkable, according to what is called the equivalence principle of general relativity; individuals falling freely into a black hole will see the same physical laws as anywhere else as they cross the horizon. Theorists are fond of saying that the entire solar system could be falling into a giant black hole right now, and we would not experience anything out of the ordinary.

Black Hole Radiation

The challenge Hawking posed to the traditional picture of black holes began in 1974, when he considered a strange prediction of quantum mechanics. According to this theory, pairs of particles and their antimatter counterparts constantly pop into existence and then disappear almost at once. If such fluctuations happen just outside the horizon of the black hole, Hawking showed, the pair could separate. One would fall into the singularity, and the other would escape from the black hole and carry away some of its mass. Eventually the black hole's entire mass could be depleted through this process, termed Hawking evaporation. For black holes found in nature, evaporation is unimportant: these black holes add mass at a much more rapid rate from gas and dust falling in than they lose to radiation. But for theoretical purposes, we can investigate what would happen if a black hole were completely isolated and we had enough time to watch the full process of evaporation. By pursuing such a thought experiment, Hawking revealed two apparent contradictions between general relativity and quantum mechanics.

The Entropy Problem

In pondering the isolated black hole, Hawking noted that the light spectrum of the eponymous radiation streaming away from it would look the same as that of a radiating hot body, meaning that the black hole has a temperature. In general, temperature arises from the motion of atoms inside objects. The thermal nature of Hawking radiation, then, suggested that the black hole should have a microscopic structure made of some kind of discrete building blocks or bits. Physicist Jacob D. Bekenstein, now at the Hebrew University of Jerusalem, had also reached this conclusion two years earlier by engaging in thought experiments involving throwing things into black holes. The work of Bekenstein and Hawking gives a formula for the number of bits, a measure known as the black hole entropy. Entropy is a gauge of disorder, which becomes greater as the number of states that an object can have grows. The larger the number of bits in a black hole, the more possible arrangements they can have and the greater the entropy.

In contrast, general relativity describes a black hole as having a smooth geometry and indicates that every black hole of given mass, spin and charge should be exactly the same: in the words of the late physicist John Wheeler of Princeton University, “Black holes have no hair.” So here is a contradiction: relativity says no hair, whereas quantum mechanics says black holes have a large amount of entropy, meaning some microscopic structure, or hair.

The Information Paradox

Hawking evaporation also gives rise to a challenge to quantum theory. According to Hawking's calculation, the particles that escape from a black hole do not depend at all on the properties of the material that went into the hole—usually a massive star that collapsed. For example, we could send a note with a message into the black hole, and there would then be no way to reconstruct the message from the final particles that would emerge. Once the note passed through the horizon, it could not influence anything that came out later, because no information can escape from the interior. In quantum mechanics, every system is described by a formula called the wave function, which encodes the chances that the system will be in any particular state.

In Hawking's thought experiment, the loss of information means that we have no way to predict the wave function of Hawking radiation based on the properties of the mass that went into the black hole. Information loss is forbidden by quantum mechanics, so Hawking concluded that the laws of quantum physics had to be modified to allow for such loss in black holes.

You might be saying to yourself, “Of course, black holes destroy information—they destroy everything that enters them.” But compare what happens if we simply burned the note. The message would certainly be scrambled, and it would be impractical to reconstruct it from the smoke. But the process of burning is described by ordinary quantum mechanics, applied to the atoms in the note, and the quantum description of the smoke would be a definite wave function that would depend on the original message. In theory, then, the message could be reconstructed through the wave function. In the case of black holes, however, there would be no definite wave function for the resulting radiation.

Based on this analogy, many theorists concluded that Hawking was wrong, that he had mistaken the scrambling of information for actual information loss. Further, some argued, if information can be lost, then it will not just happen in the exotic situation of black hole evaporation but everywhere and all the time—in quantum physics, anything that can happen will happen. If Hawking were right, we would see the signs in everyday physics, probably including severe violations of the law of conservation of energy.

Hawking's argument, though, stands up to simple objections. Unlike burning paper, black holes have horizons beyond which information cannot escape. Thus, we seem to have a sharp paradox: either modify quantum mechanics to allow information loss or modify relativity to allow information to escape from the black hole interior.

A third possibility also exists—that the black hole does not evaporate completely but ends up as a microscopic remnant containing all the information of the original star that created it. This “solution” has its own difficulties, however. For example, such a small object containing so much information would violate the Bekenstein-Hawking idea of entropy.

Black Holes and Branes

String theory is one attempt to rectify some of the problems that arise when relativity and quantum mechanics collide, as in the case of black holes. This theory replaces the pointlike particles of previous theories with tiny loops or strands of string; these strings manage to eliminate some of the mathematical difficulties that arise when quantum mechanics and relativity are combined. Replacing points with strings does not, however, immediately change the black hole story.

A break came in 1995, when I was looking at another kind of thought experiment, studying strings in small spaces. Building on work that I and several others had done a few years earlier, I showed that string theory, as it was then understood, was not complete. Rather it required the existence of objects with more dimensions than the three of space and one of time we are familiar with. In black holes these higher-dimensional objects, called D-branes, would be tiny—wrapped up in hidden dimensions too small for us to detect. The next year Andrew Strominger and Cumrun Vafa, both now at Harvard University, showed that strings and D-branes together provide the precise number of bits to account for black hole entropy, at least for certain very symmetrical black holes. The entropy puzzle was partly solved.

The next question was, What about information loss? Then, in 1997, Juan Maldacena, now at the Institute for Advanced Study in Princeton, N.J., came up with a way around the information loss problem—a solution sometimes called the Maldacena duality. A duality is a surprising equivalence between two things that seem very different. Maldacena's duality shows that the mathematics of a theory combining quantum mechanics and gravity—a quantum theory of gravity—based on string theory are equivalent to the mathematics of an ordinary quantum theory under a special set of circumstances. In particular, the quantum physics of a black hole is equivalent to that of an ordinary gas of hot nuclear particles. It also means that spacetime is fundamentally different from what we perceive, more like a three-dimensional hologram projected from a more fundamental two-dimensional surface of a sphere.

Using Maldacena's duality, physicists also get a way to describe the quantum mechanics of black holes in the bargain. If Maldacena's assumptions are true, then ordinary quantum laws would apply to gravity as well, and information cannot be lost. By a less direct argument, evaporating black holes cannot leave behind any remnants, so it must be that the information gets out with the Hawking radiation.

Maldacena's duality is arguably the closest we have come to unifying general relativity and quantum mechanics, and Maldacena discovered it by chasing down the black hole puzzles of entropy and information loss. It is not yet proved to be true, but it is supported by much evidence—enough that in 2004 Hawking announced that he had changed his mind about the need for black holes to lose information and publicly paid off a bet with physicist John Preskill at the International Conference on General Relativity and Gravitation in Dublin.

Physicists widely believed that no single observer would see any violation of relativity or any other laws near a black hole that lived by Maldacena's rules, although his duality falls short in not giving a clear explanation for how information gets from the inside of a black hole to the outside.

About 25 years ago Leonard Susskind of Stanford University and Gerard 't Hooft of Utrecht University in the Netherlands proposed a solution to the original information problem that involves a kind of relativity principle called black hole complementarity. In essence, the argument holds that an observer who jumps into a black hole sees the information inside, whereas one who stays outside sees it come out. There is no contradiction because these two observers cannot communicate.

The Firewall

Maldacena's duality and black hole complementarity seemed to dispel all the paradoxes, but many of the details had yet to be filled in. Three years ago my own AMPS collaboration tried to make a model of how the combined picture would work, building on ideas of physicists Samir D. Mathur of Ohio State University and Steven Giddings of U.C. Santa Barbara (and extending, unbeknownst to us, an earlier argument of Samuel Braunstein of the University of York in England). After failing repeatedly to make a successful model, we realized that the problem ran deeper than our mathematical shortcomings and that a contradiction remained.

This contradiction pops up when considering the phenomenon of quantum entanglement—the most unintuitive part of quantum theory and the one furthest from our experience. If particles were like dice, entangled particles would be two dice that always added to seven: if you roll the dice, and the first comes up as two, then the second will always come up as five, and so on. Similarly, when scientists measure the properties of one entangled particle, the measurement also determines the characteristics of its partner. It is a further consequence of quantum theory that a particle can be fully entangled only with one other: if particle B is entangled with particle A, then it cannot also be entangled with particle C. Entanglement is monogamous.

In the case of the black hole, think about a Hawking photon; call it “B,” emitted after the black hole is at least halfway evaporated. The Hawking process implies that B is part of a pair; call its partner that falls into the black hole “A.” A and B are entangled. Furthermore, the information that originally fell into the black hole has been encoded into all the Hawking radiation particles. Now, if information is not lost, and the outgoing Hawking photon B ends up in a definite quantum state, then B must be entangled with some combination, “C,” of the other Hawking particles that already escaped (otherwise, the output would not preserve the information). But then we have a contradiction: polygamy!

The price of saving quantum mechanics, keeping the entanglement between B and C and not having anything else out of the ordinary on the outside of the black hole, is the loss of entanglement between A and B. The Hawking photons A and B began just inside and outside the horizon when they arose as an ephemeral particle-antiparticle pair. In quantum theory, the cost of breaking this entanglement, like the cost of breaking a chemical bond, is energy. Breaking the entanglement for all the Hawking pairs implies that the horizon is a wall of high-energy particles, which we termed a firewall. An infalling astronaut, rather than moving freely through the horizon, encounters something dramatic.

Finding such a large departure from general relativity—a wall of energy in a place where nothing unusual should be happening—was disturbing, but the argument was simple, and we could not find a flaw. In a sense, we had just run Hawking's original argument backward, assuming that information is not lost and seeing where that assumption would lead. We concluded that, rather than the subtle effects of complementarity, there was a drastic breakdown of general relativity. As we began to describe the argument to others, the common reaction was first skepticism and then the same puzzlement that we experienced.

Either these strange firewalls actually exist, or it seems we must again consider letting go of some of the deeply held doctrines of quantum theory. Information may not be destroyed, but perhaps some rewriting of quantum mechanics is in store. Unfortunately, observing real black holes will not decide the issue—any radiation from a firewall would be weakened by the gravitational pull of the black hole, making the firewall very hard to see.

The End of Space

Furthermore, if the firewall exists, what is it? One idea is that the firewall is simply the end of space. Perhaps the conditions for spacetime to form do not exist inside the black hole. As Marolf once remarked, maybe the interior cannot form, because “the black hole's quantum memory is full.” If spacetime cannot occur inside, then space ends at the horizon, and an infalling astronaut who hits it dissolves into quantum bits residing on this boundary.

To avoid such bizarre scenarios, physicists have attempted to circumvent the firewall conclusion. One idea is that because Hawking radiation particle B must be entangled with both A and C, then A must be part of C: the photon behind the horizon is somehow the same bit that is encoded in the earlier Hawking radiation, even though they are in very different places. This notion is something like the original idea of black hole complementarity, but to make a concrete model of this scenario, it seems, one ends up modifying quantum mechanics again. The most radical idea, from Maldacena and Susskind, is that every pair of entangled particles is connected by a microscopic spacetime wormhole, so that large regions of spacetime, such as the black hole interior, can be built up from large amounts of entanglement.

Hawking had proposed that general relativity works for black holes but that quantum mechanics breaks down. Maldacena concluded that quantum mechanics is unmodified but that spacetime is holographic. Perhaps the truth is somewhere in the middle.

Many other ideas have been proposed, most of which give up one long-standing principle or another, and there is no consensus as to the right direction to resolve the problems. A common question is, What do firewalls imply for real-life black holes, such as the one in the center of our Milky Way galaxy? It is too early to say.

For now investigators are excited that we have discovered a new contradiction between two of the central theories of physics. Our inability to say definitively whether or not the firewall is real exposes a limitation in our current formulations of quantum gravity, and theoretical physicists are rethinking their basic assumptions about the workings of the universe. Out of this may come a deeper understanding of the nature of space and time and of the principles underlying all the laws of physics. Ultimately, by unraveling the quandaries at the heart of black hole firewalls, we may finally get the break we need to unify quantum mechanics and general relativity into a single working theory.

--Originally published: Scientific American 312(4); 36-41 (April 2015).

Have We Solved the Black Hole Information Paradox?

by Yasunori Nomura

Black holes, some of the most peculiar objects in the universe, pose a paradox for physicists. Two of our best theories give us two different—and seemingly contradictory—pictures of how these objects work. Many scientists, including myself, have been trying to reconcile these visions, not just to understand black holes themselves, but also to answer deeper questions, such as “What is spacetime?” While I and other researchers made some partial progress over the years, the problem persisted. In the past year or so, however, I have developed a framework that I believe elegantly addresses the problem and gives us a glimpse of the mystery of how spacetime emerges at the most fundamental level.

Here is the problem: From the perspective of general relativity, black holes arise if the density of matter becomes too large and gravity collapses the material all the way toward its central point. When this happens, gravity is so strong in this region that nothing—even light—can escape. The inside of the black hole, therefore, cannot be seen from the outside, even in principle, and the boundary, called the event horizon, acts as a one-way membrane: nothing can go from the interior to the exterior, but there is no problem in falling through it from the exterior to the interior.

But when we consider the effect of quantum mechanics, the theory governing elementary particles, we get another picture. In 1974, Stephen Hawking presented a calculation that made him famous. He discovered that, if we include quantum mechanical effects, a black hole in fact radiates, although very slowly. As a result, it gradually loses its mass and eventually evaporates. This conclusion has been checked by multiple methods now, and its basic validity is beyond doubt. The odd thing, however, is that in Hawking’s calculation, the radiation emitted from a black hole does not depend on how the object was created. This means that two black holes created from different initial states can end up with the identical final radiation.

Is this a problem? Yes, it is. Modern physics is built on the assumption that if we have perfect knowledge about a system, then we can predict its future and infer its past by solving the equation of motion. Hawking’s result would mean that this basic tenet is incorrect. Many of us thought that this problem was solved in 1997 when Juan Maldacena discovered a new way to view the situation, which seemed to prove no information was lost.

Case closed? Not quite. In 2012, Ahmed Almheiri and collaborators at the University of California, Santa Barbara, presented in their influential paper a strong argument that if the information is preserved in the Hawking emission process, then it is inconsistent with the “smoothness” of the horizon—the notion that an object can pass through the event horizon without being affected. Given that the option of information loss is out of the question, they argued that the black hole horizon is in fact not a one-way membrane but something like an unbreakable wall, which they called a firewall.

This confused theorists tremendously. As much as they disliked information loss, they abhorred firewalls too. Among other things, the firewall idea implies that Einstein’s general relativity is completely wrong, at least at the horizon of a black hole. In fact, this is utterly counterintuitive. For a large black hole, gravity at the horizon is actually very weak because it lies far away from the central point, where all the matter is located. A region near the horizon thus looks pretty much like empty space, and yet the firewall argument says that space must abruptly “end” at the location of the horizon.

The main thrust of my new work is to realize that there are multiple layers of descriptions of a black hole, and the preservation of information and the smoothness of the horizon refer to theories at different layers. At one level, we can describe a black hole as viewed from a distance: the black hole is formed by collapse of matter, which eventually evaporates leaving the quanta of Hawking radiation in space. From this perspective, Maldacena’s insight holds and there is no information loss in the process. That is because, in this picture, an object falling toward the black hole never enters the horizon, not because of a firewall but because of time delay between the clock of the falling object and that of a distant observer. The object seems to be slowly “absorbed” into the horizon, and its information is later sent back to space in the form of subtle correlations between particles of Hawking radiation.

On the other hand, the picture of the black hole interior emerges when looking at the system from the perspective of someone falling into it. Here we must “ignore” the fine details of the system that an infalling observer could not see because he or she has only finite time until they hit the singular point at the center of the black hole. This limits the amount of information they can access, even in principle. The world the infalling observer perceives, therefore, is the “coarse-grained” one. And in this picture, information need not be preserved because we already threw away some information even to arrive at this perspective. This is the way the existence of interior spacetime can be compatible with the preservation of information: they are the properties of the descriptions of nature at different levels!

To understand this concept better, the following analogy might help. Imagine water in a tank and consider a theory describing waves on the surface. At a fundamental level, water consists of a bunch of water molecules, which move, vibrate and collide with each other. With perfect knowledge of their properties, we can describe them deterministically without information loss. This description would be complete, and there would be no need to even introduce the concept of waves. On the other hand, we could focus on the waves by overlooking molecular level details and describing the water as a liquid. The atomic-level information, however, is not preserved in this description. For example, a wave can simply “disappear,” although the truth is that the coherent motion of water molecules that created the wave was transformed into a more random motion of each molecule without anything disappearing.

This framework tells us that the picture of spacetime offered by general relativity is not as fundamental as we might have thought—it is merely a picture that emerges at a higher level in the hierarchical descriptions of nature, at least concerning the interior of a black hole. Similar ideas have been discussed earlier in varying forms, but the new framework allows us to explicitly identify the relevant microscopic degrees of freedom—in other words, nature's fundamental building blocks—participating in the emergence of spacetime, which surprisingly involves elements that we normally think to be located far away from the region of interest.

This new way of thinking about the paradox can also be applied to a recent setup devised by Geoff Penington, Stephen H. Shenker, Douglas Stanford and Zhenbin Yang in which Maldacena’s scenario is applied more rigorously but in simplified systems. This allows us to identify which features of a realistic black hole are or are not captured by such analyses.

Beginning with the era of Descartes and Galilei, revolutions in physics have often been associated with new understandings of the concept of spacetime, and it seems that we are now in the middle of another such revolution. I strongly suspect that we may soon witness the emergence of a new understanding of nature at a qualitatively different and deeper level.

--Originally published: Scientific American online, January 30, 2020.

LIGO’s Latest Black-Hole Merger Confirms Einstein, Challenges Astrophysics

by Lee Billings

Some three billion years ago, when Earth was a sprightly ocean world dotted with protocontinents and inhabited solely by single-celled organisms, a pair of black holes spiraled together and collided in a far-off region of the universe, leaving behind a single black hole some 50 times heavier than our sun. Emitting no light, the entire affair should have remained forever lost to the void.

Instead, the invisible violence of the pair’s final moments and ultimate merging was so great that it shook the fabric of reality itself, sending gravitational waves—ripples in spacetime—propagating outward at the speed of light. In the early morning hours of January 4, 2017, those waves washed over our modern Earth and into the most precise scientific instrument ever built, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). There the waves shifted the positions of vacuum-insulated, laser-bathed mirrors by less than the radius of a single subatomic particle. Traveling at light-speed, the waves first perturbed LIGO mirrors set up in Hanford, Wash., before passing through a second set of mirrors in Livingston, La., some three milliseconds later. Synced together from each station’s moving mirrors and converted to audible frequencies, the cosmos-quaking gravitational waves sounded like a single, soft “chirp.” Analyzing it, researchers are teasing out remarkable and otherwise-inaccessible details about the hidden lives of black holes. The findings were described in the journal Physical Review Letters.

As inconceivable as it may seem, tuning in to such chirps is now becoming routine. First predicted by Einstein more than a century ago as a consequence of his theory of general relativity, gravitational waves were long thought to be beyond observational reach—if not entirely nonexistent. But the chirp from January 4, dubbed “GW170104,” was actually LIGO’s third and farthest-reaching detection of gravitational waves, coming from somewhere about 3 billion light-years away. It follows earlier chirps from two other events detected separately in late 2015 that each occurred closer by, yet still more than a billion light-years distant.

Other cosmic phenomena such as supernovae in the Milky Way and colliding neutron stars in our galactic neighborhood should also produce detectable gravitational waves, each with their own accompanying revolutionary insights, but so far all three of LIGO’s detections have been death-rattles from merging pairs of black holes in remote stretches of the universe.

Gravity Rainbows?

For the time being, thousands of scientists around the world are making the most of LIGO’s limited view and the project’s three confirmed detections. Whereas the “loudness” of each chirp has clearly conveyed each event’s distance from us, LIGO’s twin stations can at present only vaguely constrain their celestial sources, which may lie anywhere within huge swaths of the heavens containing thousands upon thousands of large galaxies. So thirsty are theorists for new insights into black holes and relativistic processes that, with each LIGO detection, observational astronomers have leapt into action to target those enormous patches of sky, hoping to see some afterglow or other emission of electromagnetic radiation—even though by definition the resulting larger black hole should emit no light.

Fortunately, even without light the merger’s gravitational waves reveal much. LIGO team members have already used the billion–light-year intergalactic traverses of the first two chirps to look for signs of “dispersion” in the propagation of gravitational waves—a phenomenon analogous to how rays of light traveling through a prism disperse based on their wavelength to form rainbows. According to Einstein’s theory of general relativity, gravitational waves should experience no dispersion at all—and any deviation from that prediction would suggest Einstein’s relativistic reckoning of the universe is somehow incorrect, potentially pointing the way to new breakthroughs in physics. Signs of any dispersion should have been obvious in LIGO’s third event, GW170104, because its gravitational waves traveled across three billion light-years, rather than the one billion of LIGO’s previous two events. But when researchers looked, they saw no gravitational rainbows. “We made very careful measurement of that effect,” said LIGO team member Bangalore Sathyaprakash of The Pennsylvania State University and Cardiff University. “But we did not discover any dispersion, once again failing to prove that Einstein was wrong.”

Using that same measurement, researchers also honed in on the mass of the graviton, the hypothetical particle that mediates the force of gravity. “Basically we are testing general relativity in a new regime,” says Laura Cadonati, a physicist at Georgia Institute of Technology and LIGO’s deputy spokesperson. “The fact that this event is twice as far as the previous two gives us a longer baseline to test the dispersion relation, and as a result we now have a limit on the mass of the graviton that is 30 percent tighter than the one we previously set. One could say we are putting general relativity to a tighter and tighter test—it is still holding, but with more signals we may find something that does not quite agree.”

Mysterious Middleweight Mergers

Although LIGO’s latest event may be a brick in the towering edifice of Einstein’s general relativity, it is also restructuring the foundations of our understanding of black holes. Before LIGO’s detections, astronomers only had definitive observations of two varieties of black holes: ones that form from stars that were thought to top out around 20 solar masses; and, at the cores of large galaxies, supermassive black holes of still-uncertain provenance containing millions or billions of times the mass of the sun. Both are thought to be important for understanding the formation and evolution of galaxies, and thus to some degree important for the formation and evolution of everything galaxies contain—including stars, planets and people. Most of the black holes in LIGO’s mergers have been middleweights, being heavier than that 20–solar mass limit but much lighter than the supermassive variety, raising questions about their origins and relationship to the two well-studied populations of black holes.

The prevailing explanation for LIGO’s bulky black holes is that they form from very massive stars that are also quite pristine, composed almost entirely of hydrogen and helium with scarcely any heavier elements at all. Most stars of such immensity would have more heavy elements, causing them to lose much of their mass via high-speed winds whereas “low metallicity” stars would have weaker winds and keep more of their star stuff, ultimately ending their lives by collapsing to become overlarge stellar black holes.

Making LIGO’s merging black hole pairs, one conventional theory goes, would then require the “binary evolution” of two massive, low-metallicity stars that form as a pair. If, for instance, the two stars are very close, over the courses of their lives they can swap gas from their atmospheres back and forth in a cyclic process that pulls their orbits even closer and eventually produces two tightly orbiting, supersize black holes. At the end of this process, the spins and orbits of both black holes would have become inextricably linked, so each black hole’s equator would be aligned with the plane of their shared orbit.

“Think of black holes as being like tornadoes that drag stars and matter around them,” Cadonati explains. “Now think of two going around each other, and each one spinning clockwise or counterclockwise,” aligned with the orbital motion. Two black holes with such an alignment would possess more rotational energy than an unaligned pair, and thus require ever-so-slightly more time to coalesce together in the final moments of their merger. The deepest mystery of GW170104, LIGO’s latest discovery, is that the merger happened too quickly for both of its progenitor black holes to be so aligned; in terms of Cadonati’s analogy, at least one of the orbiting “tornadoes” must have been paradoxically tilted near or on its side.

The most common explanation for black hole pairs with such “spin misalignment” is that they did not form from the binary evolution of isolated twin stars. Instead, each black hole must have formed independently, and somehow found its partner after millions or billions of years of wandering through the universe. Any eventual union through this “dynamical formation” channel would most likely take place in thick swarms of stars called globular clusters, says Fred Rasio, a physicist at Northwestern University who is not a member of the LIGO collaboration. “Imagine throwing a thousand black holes into a mosh pit where they kick each other around like crazy,” Rasio says. “Their spins will be randomized. The dynamics don’t care which way the holes are spinning, so when they are bound into a pair that merges, their spins have no correlation with how they orbit.”

Black Holes from the Big Bang?

According to some theorists, the best explanation for GW170104’s curious misalignment is that its black holes did not start out as stars at all. “Even in dense globular clusters, these black holes would not form in sufficient density to find each other in the age of the universe,” says Juan García-Bellido, a professor at the Autonomous University of Madrid who is not a member of the LIGO collaboration. García-Bellido is a leading proponent of the unorthodox idea that LIGO’s abnormally heavy, oddly misaligned merging black holes are actually part of a putative population of “primordial black holes.” Rather than arising from stars, such exotic objects could have emerged in the first moments after the big bang, coalescing from particularly dense regions of the fiery plasmatic fog that then suffused the universe. If grouped in clusters, primordial black holes could also form merging pairs with misaligned spins.

There is, however, an additional wrinkle to ascribing primordial origins to some or all of LIGO’s observed black holes—something that could be seen as either the theory’s most alluring feature, or a nasty bug. Clusters of primordial black holes dense enough to produce LIGO’s newfound population of merging ones, García-Bellido and others say, could also be a natural solution to the mystery of dark matter—the elusive and invisible 80 percent of the universe’s matter that astronomers see solely through its gravitational effects on glowing stars and gas in galaxies.

“The idea would be that [the primordial black holes] would be concentrated in halos around the matter we can see,” said Michael Landry, the head of LIGO’s Hanford Observatory, summarizing the speculative concept in response to a question at a recent press conference. “It’s not impossible that what we’re seeing are primordial black holes that form the dark matter.” On the other hand, Landry added, some teams of astronomers occasionally looking for halos of primordial black holes around the Milky Way have yet to find evidence they exist in sufficient numbers to account for the effects of dark matter. Whether black holes from the big bang explain dark matter—not to mention LIGO’s results—is an “open question,” Landry said.

Hearing the Black Hole Symphony

Whether born from binary evolution, dynamical pairing, the big bang or something else entirely, the true origins of LIGO’s mysterious black hole mergers could soon be revealed. The collaboration’s current best guess is that somewhere between 12 and 213 such mergers occur each year in a cubic volume of space a bit over three billion light-years on a side. This suggests LIGO—which is in the midst of upgrades to boost its sensitivity and planning for a new station in India—could eventually be detecting the chirps from black hole mergers at a rate of anywhere between once per day to once per week. Upgrades have been made to Virgo, a companion gravitational-wave observatory approaching LIGO’s sensitivity. Both projects simultaneously monitor the sky to better localize the origins of any new celestial gravitational grumbles. Beyond LIGO and Virgo, additional observatories are likely to debut in coming years around the world, creating a globe-girdling network for finer-grained gravitational-wave searches. By the 2020s, the chirps will come so fast and furious, from so many merging pairs of black holes, their sounds could form a symphony.

“It’s not a single one or two black hole binaries by which we can distinguish between different models,” Sathyaprakash said. “It’s only from a population of detections, which will give us distributions for spins and for masses. That’s where the differences between formation mechanisms will become clear.” Very heavy, misaligned black hole pairs could prove to be very rare, strengthening the case that most mergers come from isolated systems of binary stars—or they could prove common, suggesting denser, more dynamical origins. And if, García-Bellido says, any black hole in a LIGO merger proves to weigh less than our sun, this would be a “smoking gun” for primordial black holes, as such relatively minuscule black holes are thought impossible to form from stars.

“Before our discovery, we didn’t even know for sure that these [middleweight] black holes existed,” Cadonati said at the press conference announcing GW170104. “What we do know now is, first of all, they do exist, they may have played an important role in the early universe and we’re now starting to get a glimpse into how they behaved…. This has really opened a new window on the universe, and we’re learning more about where we’re coming from. That’s the big excitement.”

--Originally published: Scientific American online, June 1, 2017.

SECTION 4

Event Horizons: What We Don't Know

Erupting Black Hole Shows Intriguing "Light Echoes"

by Clara Moskowitz

We tend to think black holes gobble up all the matter around them—but they can actually spew out as much as they suck in. And sometimes they seem to go downright crazy.

Astronomers spotted one black hole, nearly 10,000 light-years from Earth, belching out an enormous explosion of x-ray light. Measurements of this tantrum have given scientists one of the clearest pictures yet of what happens when black holes erupt with energy. “One of our big questions is how do we go from this process of material flowing into the black hole to this process of flowing out?” says astronomer Erin Kara of the University of Maryland, College Park, lead author of a paper on the findings, published in Nature. “We know this is happening but we don’t understand how it works in detail.” Kara presented the discovery at the American Astronomical Society’s 2019 annual meeting in Seattle.

The outburst began on March 11, 2018, and quickly transformed a black hole that had been totally invisible to telescopes into one of the brightest objects (in terms of x-ray light) in the entire sky. The object, called MAXI J1820+070, was first spotted by the Monitor of All-sky X-ray Image (MAXI) experiment on the International Space Station. Another observatory on the station, the Neutron star Interior Composition Explorer (NICER), monitored the flare with near-daily observations over the next few months.

Not only did astronomers measure the black hole brightening extremely over this time, they also observed what they called “light echoes”—time lags between the x-ray light coming from two different areas around the black hole. Some light travels straight from a region called the corona, made of electrons and other charged particles close in to the black hole. Farther out and perpendicular to the corona is the “accretion disk”—a wider pancake of gas swirling around the hole and falling into it. Other light comes out of the corona and bounces off this disk, arriving at the NICER detectors later. As NICER watched the eruption, the time between echoes became shorter and shorter, indicating the distance between the disk and corona was shrinking. The scientists had evidence the boundaries of the disk were not changing, so they concluded the corona itself must be getting shorter and thus light did not have to travel so far to reach the disk. “This is the clearest detection to date of these light echoes off of the gas falling into a stellar-mass black hole in our own galaxy,” says Dan Wilkins, an astrophysicist at Stanford University who was not involved in the study. “Being able to detect a change in the time delays between the echoes over the course of the outburst means we can start to learn about what is happening around the black hole.”

Zeroing in on the corona is especially helpful because scientists think this region is likely the base from which powerful beams of particles and light, called relativistic jets, are launched. These jets travel close to light-speed and can be spotted coming from black holes across the universe. “The big fun of this paper, from my point of view, is that we really can ‘see’ the corona shrinking during the evolution of the outburst,” says Stephen Eikenberry of the University of Florida, a co-author on the new paper. “I don’t know of any real theoretical prediction for this ‘shrinkage’ nor of any prior observation of it, so this result will already require overhauling of the theories we have for jet formation.”

The black hole in this study holds about 10 times the mass of the sun. The new observations should help astronomers understand not just star-size black holes like this one but also the gargantuan “supermassive” black holes that are located at the centers of galaxies and contain millions of times more mass. “These stellar-mass systems are a convenient analogue for supermassive black holes,” Kara says. “They have similar components, but we see outbursts over several weeks and months whereas for supermassive black holes it’s years.” The new findings support one theory for how supermassive black hole coronas are structured—called the “lamppost model”—which posits coronas are lightbulb-shaped blobs above and below the black hole, as opposed to diffuse clouds. “These new observations are exactly in line with the lamppost model,” Wilkins says. “Indeed, we observed very similar behavior during flares from supermassive black holes, seeing a change in the size of the corona.”

Astronomers hope NICER, which launched in June 2017, and other new observatories will observe many more outbursts in the future and help fill in the missing details of erupting black holes. “It’s a really exciting time to be doing this,” says Joey Neilsen, a physicist at Villanova University and a co-author of the new paper. “We’re getting to a point where the observations are actually ahead of the theory. New missions are allowing us to see things that we hadn’t necessarily thought of before.”

--Originally published: Scientific American online, January 11, 2020.

Black Hole Pretenders Could Really Be Bizarre Quantum Stars

by Charles Q. Choi

When giant stars die, they don’t just fade away. Instead they collapse in on themselves, leaving behind a compressed stellar remnant, usually a city-sized, superdense ball of neutrons appropriately called a neutron star. In extreme cases, however, most theorists believe an expiring giant star will form a black hole—a pointlike “singularity” with effectively infinite density and a gravitational field so powerful that not even light, the fastest thing in the universe, can escape once falling in. A recent study is reinvigorating an alternative idea that objects with names such as “black stars” or “gravastars” might exist midway between neutron stars and black holes. If real, these exotic stellar corpses should appear nearly identical to black holes save in one key way—they could not irretrievably swallow light.

There are good reasons to seek such alternatives, because black holes raise a host of theoretical problems. For instance, their singularities are supposedly hidden by invisible boundaries known as event horizons. Throw something into a black hole and once it passes the event horizon it should be gone—forever—with no hope whatsoever of return. But such profound annihilation clashes with other long-cherished laws of physics that suggest the destruction of information is impossible, including information encoded within anything falling into black holes.

Conceived and developed across the past two decades in part to sidestep such conundrums, models of black stars and gravastars postulate these objects would lack singularities and event horizons. But questions have lingered as to whether such objects could actually form—and remain stable after they did. Research from theoretical physicist Raúl Carballo-Rubio of the International School for Advanced Studies in Italy provides a novel mechanism that might allow black stars and gravastars to exist.

Carballo-Rubio investigated a strange phenomenon known as quantum vacuum polarization. Quantum physics, the best description yet of how all known subatomic particles behave, suggests reality is fuzzy, limiting how precisely one can know the properties of the most basic units of matter—for instance, one can never absolutely know a particle's position and momentum at the same time. One strange consequence of this uncertainty is that a vacuum is never completely empty but instead foams with so-called virtual particles that continuously fluctuate into and out of existence.

In the presence of gigantic amounts of energy of the sort produced by the collapse of a giant star, previous research found these virtual particles can polarize, or arrange themselves depending on their properties, much as magnets are divided into north and south poles. Carballo-Rubio calculated that the polarization of these particles can produce a surprising effect inside the powerful gravitational fields of dying giant stars—a field that repels instead of attracts.

Matter and energy curve the fabric of spacetime, resulting in gravitational fields, according to Einstein's theory of general relativity. Planets and stars have a positive amount of energy on average, and the resulting gravitational fields are attractive in nature. When virtual particles polarize, however, the vacuum they occupy can on average possess negative energy, and “this curves spacetime in a way that the associated gravitational field is repulsive,” Carballo-Rubio says—which, of course, could prevent the formation of a black hole. (A similar phenomenon causes relatively light stellar remnants to form neutron stars instead of black holes; their gravitational fields are not strong enough to crush neutrons into a singularity.)

Two prior models suggested repulsive gravity might keep stellar remnants from collapsing to form black holes. One proposed stellar remnants instead formed gravastars, objects filled with quantum vacuum overlaid by a thin shell of matter. The other model suggested the result of these collapses were black stars, where “matter and the quantum vacuum are interlaced throughout the structure in a meticulous balance,” Carballo-Rubio says. Both objects still have powerful gravitational fields that profoundly warp light, so they look dark, like black holes.

Carballo-Rubio says there was previously great uncertainty regarding the properties of black stars and gravastars. His new work tackled this issue by creating a mathematical framework that incorporated the effects of repulsive gravity into equations describing the expansion and contraction of stars, a problem “that was thought to be tractable only with the help of computers,” he notes. His new model suggests a hybrid of a black star and a gravastar could exist—one where matter and quantum vacuum are spread throughout the structure, but with matter in higher concentrations in the shell than in the core. Carballo-Rubio detailed his study in February 2018 in Physical Review Letters.

“This work is interesting and worthwhile, showing that new kinds of solutions can exist to Einstein's equations which are not black holes,” says physicist Emil Mottola of Los Alamos National Laboratory, who was not involved in the study.

Some researchers would claim the quantum effects that Carballo-Rubio bases his argument on are negligible, however. As such, they may be too weak to support the existence of black stars and gravastars, says theoretical physicist Paolo Pani of Sapienza University of Rome, who did not participate in this work.

In addition, whereas Carballo-Rubio's work argues black stars and gravastars are mathematically possible, that “does not imply that they exist in nature,” says physicist Cecilia Chirenti of the Federal University of ABC in Brazil, who did not take part in the research. For example, Pani notes it is still unclear whether stellar remnants can naturally evolve to form these structures. Furthermore, Mottola says, “Caballo-Rubio does not explain why his solution is stable and what keeps it from collapsing to a black hole.”

One way to find out whether black stars, gravastars or black holes actually exist is by analyzing gravitational waves unleashed by what scientists currently interpret as merging black holes. When any mass moves, it generates gravitational waves that travel at the speed of light, stretching and squeezing spacetime along the way.

As black holes spiral toward each other, they should each give off gravitational waves, but their event horizons should absorb those directly falling onto them. Because black stars and gravastars lack event horizons, however, they can reflect gravitational waves, and the LIGO and Virgo observatories could detect these “echoes,” Pani says. If such signals are discovered, they could yield insights into both general relativity and quantum physics that could help lead to a model of “quantum gravity,” marrying both long-disparate theories.

--Originally published: Scientific American online, March 16, 2018.

Black Holes from the Beginning of Time

by Juan García-Bellido & Sébastien Clesse

More than a billion years ago two black holes in the distant universe spiraled around each other in a deathly dance until they merged. This spiraling collision was so violent that it shook the fabric of spacetime, sending perturbations— gravitational waves—rippling outward through the cosmos at the speed of light. In September 2015, after traveling more than a billion light-years, those ripples washed over our planet, registering as a “chirp” in the sensors of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO).

This was the first direct detection of gravitational waves, and the observation confirmed Albert Einstein’s century-old prediction of their existence. Yet the chirp revealed that each of the merger’s progenitor black holes was 30 times heavier than the sun. That is, their masses were two to three times larger than ordinary black holes born from supernova explosions of massive stars. These black holes were so heavy, it is hard to explain how they formed from stars at all. Furthermore, even if two such black holes did independently form from the deaths of very massive stars, they would have then had to find each other and merge—an event with an exceedingly low probability of occurring within the current age of the universe. It is thus reasonable to suspect that these massive black holes formed via some other, more exotic pathway that might not involve stars at all. Beyond its detection of gravitational waves, it may be that LIGO has unveiled something even more extraordinary: black holes that predate the formation of the stars themselves.

Although such “primordial” black holes have never before been seen, some theoretical models suggest they could have formed in astronomical numbers from the hot, dense plasma that filled the cosmos less than one second after the big bang. This hidden population could solve several outstanding mysteries in modern cosmology. In particular, primordial black holes could constitute some, if not all, of dark matter—the invisible 85 percent of the matter in the universe that acts as gravitational glue to hold galaxies and galaxy clusters together. Further studies with LIGO and other facilities will soon test these ideas, potentially unleashing a new revolution in our understanding of the cosmos.

The Fall of MACHOs, The Rise of WIMPs

Black holes would initially seem to be ideal candidates for dark matter because they emit no light. Indeed, along with other dark objects such as planets and brown dwarfs, they make up one long-proposed solution to the dark matter problem: MACHOs, short for massive compact halo objects. Found both in spherical halos surrounding each galaxy and near each galaxy’s luminous center, MACHOs would create the gravitational pull responsible for the otherwise anomalous motions of stars and gas that astronomers observe in the outskirts of galaxies. Simply put, galaxies seem to be rotating too fast to be held together by the visible mass in stars that we observe. Dark matter provides the extra pull to prevent spinning galaxies from flinging off their stars.

If MACHOs make up most of the universe’s dark matter, they must also account for other observations. Whatever dark matter is, it shapes the universe’s largest structures, determining the origin and growth of galaxies as well as clusters and superclusters of galaxies. These objects coalesce from the gravitational collapse of clumps of gas inside dark matter halos. Cosmologists have precisely mapped the spatial distribution of these clumps through deep and wide galaxy surveys and correlated them with tiny temperature fluctuations present in the cosmic microwave background (CMB), the big bang’s all-sky afterglow. The diffuse mass of dark matter in large galaxies and clusters also bends space to distort the light from far distant background objects—a phenomenon known as gravitational lensing.

The MACHO hypothesis, however, fell from favor a decade ago when MACHOs did not turn up in tentative, indirect searches for their existence. Most notably, astronomers looked for them via microlensing, a variety of gravitational lensing in which a black hole, a brown dwarf or even a planet passes in front of a background star and temporarily magnifies the star’s light. Several multiyear microlensing surveys of millions of stars in the Large and Small Magellanic Clouds, the main satellite galaxies of the Milky Way, found no evidence that MACHOs made up the entirety of our galactic halo. These results were conclusive enough to rule out MACHOs up to around 10 solar masses as the primary constituent of dark matter. As these surveys took place, theorists built the case for an alternative hypothesis—WIMPs, or weakly interacting massive particles.

WIMPs are predicted by certain extensions of the Standard Model of particle physics, but they remain at least as elusive as MACHOs. To date, no evidence of their existence has been found despite decades of searches using particle accelerators, underground detectors and space telescopes. As null results piled up in the search for WIMPs, some researchers began reconsidering the MACHO hypothesis, focusing particularly on primordial black holes. But what process could have seeded these strange objects throughout the observable universe, and how could they have eluded detection for so long?

Black Holes from the Big Bang

Physicists Bernard Carr and the late Stephen Hawking proposed the idea of primordial black holes in the 1970s. They considered black holes with masses smaller than that of a mountain. Such minuscule black holes would have already evaporated and vanished within the age of our nearly 14-billion-year-old universe, via a quantum-mechanical process discovered by Hawking and appropriately called Hawking radiation. But they also investigated the possibility that more massive, nonevaporating black holes could constitute the missing matter in galaxy clusters. The possibility that massive primordial black holes could actually be most or even all of the dark matter hinges on an idea known as cosmic inflation, first proposed by physicist Alan Guth in the early 1980s.

Inflation is a hypothetical phase of prodigious expansion immediately after the big bang. In 10–35 second, two points separated by less than an atomic radius would have become separated by four light-years, a distance comparable to that of the closest stars. Moreover, during inflation tiny quantum fluctuations are magnified to macroscopic scales by the rapid expansion, seeding the growing universe with underdense and overdense regions of matter and energy from which all cosmic structures subsequently emerge. As bizarre as it may seem, the theory of inflation is strongly supported by observations of such density fluctuations in the CMB.

In 1996 one of us (García-Bellido), together with Andrei Linde of Stanford University and David Wands of the University of Portsmouth in England, discovered a way for inflation to form sharp peaks in the spectrum of density fluctuations in the early universe. That is, we showed how quantum fluctuations enormously magnified by inflation would naturally produce particularly dense regions that would collapse to form a population of black holes less than one second after inflation ends. Such black holes would then behave as dark matter and would dominate the matter content of the present-day universe. This model generated a population of black holes with the same mass, determined by the amount of energy within the collapsing region. Many other groups then started exploring these ideas within different models of inflation.

In 2015 the two of us (Clesse and García-Bellido) proposed a scenario, similar to that of 1996, in which these primordial fluctuations exhibit a broad peak in their energy densities and spatial sizes, giving rise to primordial black holes with a wide range of masses. A key consequence of this scenario is the fact that large density fluctuations collapse in close spatial proximity to one another, generating clusters of black holes of different masses—from one 100th to 10,000 times the mass of our sun. Within half a million years of the big bang, each growing, evolving cluster could contain millions of primordial black holes in a volume just hundreds of light-years across.

Such clusters of primordial black holes would be sufficiently dense to explain LIGO’s mysterious black hole mergers, which one would not otherwise ex pect to occur with regularity. From time to time, the trajectories of two primordial black holes within a cluster can cross, so that both objects become gravitationally bound to each other. They would then spiral closer together for up to millions of years, radiating gravitational waves until they merged. In January 2015 we actually predicted that LIGO would detect gravitational waves from such massive mergers—waves identical to those LIGO then detected later that year. Our estimates for the rate of merger events within primordial black hole clusters fit perfectly within the limits set by LIGO. If LIGO and other similar facilities detect many more mergers within the next few years, it may be possible to determine the range of masses and spins for all the progenitor black holes. Such a statistical analysis of black hole mergers would provide crucial information for testing their potentially primordial origins.

A key aspect of this scenario is that it evades the constraints on MACHOs previously set by gravitational microlensing experiments—constraints that ruled out black holes of up to about 10 solar masses as the main constituent of dark matter. If primordial black holes exist and possess a wide range of masses, only a small fraction would be visible to these microlensing experiments, with the bulk remaining invisible. And if primordial black holes are grouped in clusters, this arrangement suggests a probability of less than one in 1,000 that a cluster would happen to be along the line of sight of the stars in the nearby satellite galaxies monitored for microlensing events.

To avoid this effect, one could search for microlensing events elsewhere in the sky, looking for the magnified light from stars in the Milky Way’s neighboring Andromeda galaxy or even from quasars in far distant galaxies. In this way, one could probe a much larger volume of galactic halos for signs of MACHOs—that is, for primordial black holes. Recent observations suggest that whereas MACHOs of up to 10 solar masses may not make up the entirety of an average galaxy’s halo, MACHOs between one tenth and a few solar masses could easily ac count for about 20 percent of the mass in a typical galactic halo. This value is consistent with our broad-mass primordial black hole scenario.

Simply put, we cannot yet rule out the possibility that dark matter is mostly made up of primordial black holes. Indeed, this proposed scenario could decipher several other cosmic mysteries related to dark matter and galaxy formation.

Black Holes Birthed by the Big Bang

The universe’s first black holes may have been born in the earliest moments of cosmic time, when all was a seething, thick fog of fundamental particles. In the 1970s theorists realized that dense regions of that fog could collapse under their own gravity just a second after the big bang, forming so-called primordial black holes (PBHs) that would then shape the structure of the evolving, expanding universe. Emitting no light, PBHs would be a natural—albeit difficult to detect—candidate for dark matter.

Credit: Illustration by Jen Christiansen and George Retseck

Many Problems, One Solution

Clusters of primordial black holes could clear up the so-called missing satellite problem—the apparent lack of “dwarf ” satellite galaxies that should form around massive galaxies such as our Milky Way. Current simulations modeling the cosmic distribution of dark matter accurately replicate the universe’s observed large-scale structure, in which halos of dark matter pull galaxy clusters into giant filaments and sheets surrounding great voids of lower density. On smaller scales, however, these simulations predict the existence of numerous subhalos of dark matter orbiting around massive galaxies. Each of these subhalos should host a dwarf galaxy, and hundreds should surround the Milky Way. Yet astronomers have found far fewer dwarf galaxies than predicted.

Many potential explanations for the missing satellite problem exist, mainly the notion that simulations fail to fully account for the influence of ordinary matter (hydrogen and helium in stars) on the formation and behavior of the predicted dwarf galaxies. Our scenario suggests that if clustered primordial black holes made up most dark matter, they would dominate the subhalos surrounding the Milky Way, absorbing a fraction of ordinary matter and reducing the rate of star formation in the subhalos. Moreover, even if these subhalos vigorously formed stars, these stars could easily be ejected by close encounters with massive primordial black holes. Both effects would greatly reduce the brightness of the satellites, making them very hard to detect without wide-field cameras of exquisite sensitivity. Fortunately, such cameras now exist, and astronomers have already used them to discover dozens of ultrafaint dwarf galaxies surrounding the Milky Way. These objects appear to host up to hundreds of times more dark matter than luminous stars, and our model predicts that thousands more should orbit our galaxy.

Simulations also predict a population of galaxies intermediate in size between dwarf galaxies and massive galaxies. Such objects are said to be too big to fail because they would be sufficiently large to readily form stars and be easily seen. Still, they have not turned up in astronomers’ searches of the Milky Way’s vicinity. This too-big-to-fail problem has a solution similar to that of the missing satellite problem: massive primordial black holes in the cores of intermediate-sized galaxies could eject stars and star-forming gas from these objects, rendering them effectively invisible to most surveys.

Primordial black holes could also resolve the origin of supermassive black holes (SMBHs). These monsters weigh from millions to billions of solar masses and are observed at the centers of quasars and massive galaxies very early in the universe’s history. Yet if these SMBHs formed and grew from the gravitational collapse of the universe’s first stars, they should not have acquired such gigantic masses in such a relatively short time—less than a billion years after the big bang.

In our scenario, although most primordial black holes have just tens of solar masses, a very small fraction will be far heavier, ranging from hundreds to tens of thousands of solar masses. Born less than a second after the big bang, these monstrous objects would then act as giant seeds for the formation of the first galaxies and quasars, which would rapidly develop SMBHs at their centers. Such seeds could also account for the existence of intermediate-mass black holes possessing 1,000 to a million solar masses observed orbiting SMBHs and at the centers of globular clusters of stars. In short, primordial black holes may be the missing link between conventional stellar-mass black holes and SMBHs. The observational case for this scenario is building rapidly: recent detections of unexpectedly abundant x-ray sources in the early universe are most easily explained by large numbers of primordial black holes producing x-rays as they gorge on gas less than one billion years after the big bang.

Seeing in the Dark

Even though massive primordial black holes could solve the mystery of dark matter, as well as many other long-standing problems of cosmology, the game is not yet over. Other models and explanations are still possible, and future observations should allow us to distinguish among the alternatives. Indeed, there already exist seven kinds of observations to test the primordial black hole scenario. They include the detection of ultrafaint dwarf galaxies, the influence of massive primordial black holes on the positions of stars in the Milky Way, the mapping of neutral hydrogen during the first epoch of star formation and future observations of distortions in the cosmic microwave background.

Beyond these experiments, we also now possess a completely new tool to unravel the mysteries of the universe in the form of Advanced LIGO and other gravitational-wave detectors. If indeed LIGO has detected merging members of a hidden population of massive primordial black holes, we should expect many more to be detected in coming years. Scientists from the Advanced LIGO and VIRGO collaborations have presented so far 10 confirmed detections of gravitational waves emitted during the merging of black hole binaries, and many new detections are expected in the next run, starting in the spring of 2019. These detections suggest that binary black holes are much more frequent than expected and that they are broadly distributed in mass, in agreement with our scenario of massive primordial black holes.

Taken together these new experiments and observations could confirm the existence of primordial black holes and their possible linkage to the universe’s missing matter. Soon we may no longer be in the dark about dark matter.

Is Dark Matter Made of Primordial Black Holes?

Seven observations point toward the hypothesis that primordial black holes constitute dark matter in the universe:

1. Mass, spin and merging rates of black holes. Gravitational-wave detectors are finding black hole mergers at a rate suggesting an abundance of black holes similar to the dark matter. In June 2017 LIGO announced the detection of black hole merger GW170104, for which the axis of rotation of one of the black hole progenitors was very likely not aligned with the axis of the orbital plane. This would be a sign that the black hole binary probably formed by a capture process, as expected for primordial black holes rather than from a preexisting star binary. Ultimately if we found a black hole with a mass lower than the so-called Chandrasekhar limit (1.4 solar masses, below which stars cannot produce a black hole), it would undeniably have a primordial origin. Fortunately, LIGO may very soon reach the sensitivity to detect such a black hole if its companion is more massive (greater than 10 solar masses). Finally, on cosmological scales, abundant black hole binaries should induce a diffuse background of gravitational waves, which could be detected by the future space-based Laser Interferometer Space Antenna (LISA) and by ground-based pulsar timing arrays.

2. Microlensing of stars in M31 and distant quasars. A broad mass distribution of primordial black holes, centered on a few solar masses, would perfectly explain not only the LIGO/VIRGO gravitational-wave detections but also the microlensing of stars in the Andromeda galaxy (M31) and of distant quasars if they constitute dark matter. Such a model passes all the other cosmological and astrophysical constraints.

3. Ultrafaint dwarf galaxies. In 2015 astronomers using data from the Dark Energy Survey discovered dozens of ultrafaint dwarf galaxies in the galactic halo, suggesting that hundreds of such dark matter–dominated dwarf galaxies could orbit around the Milky Way. Strangely, none of them had a radius smaller than 50 light-years. Regular close encounters of primordial black holes with stars would make such dwarf galaxies unstable. On the other hand, a rapid accretion episode in the densest halos provides a new mechanism to explicate why some dwarf galaxies are strongly dominated by dark matter and others are not. Future space-based facilities such as the European Space Agency’s Euclid mission and NASA’s Wide-field Infrared Survey Telescope (WFIRST) should detect many more ultrafaint dwarf galaxies and so will test this hypothesis.

4. Galactic dark matter profiles. New observations suggest the dark matter distribution is more homogeneous near the center of galaxies of all sizes, which would hardly be explained by any known astrophysical processes such as supernovae feedback. Primordial black holes of several solar masses would naturally ex plain the existence of core dark matter profiles, thanks to gravitational scattering that homogenizes their distribution in the densest regions.

5. Correlations between x-ray and infrared backgrounds. Alexander Kashlinsky of NASA’s Goddard Space Flight Center suggested that correlations between x-ray and infrared cosmic background fluctuations that cannot be explained by any known population of galaxies would naturally arise if dark matter is made of massive primordial black holes. They would emit x-rays when they accrete matter within black hole clusters where a first generation of stars could also ignite and shine in infrared.

6. Billions of supermassive black holes. Recent observations of Chandra suggest that billions of supermassive black holes already existed one billion years after the big bang, shortly after the formation of the first stars. The required black hole seeds could have a primordial origin.

7. Unexpectedly cool universe at the time of star formation. In February 2018 a modest radio telescope built by a small team of astron omers at the Massachusetts Institute of Technology and at Arizona State University, called EDGES, might have detected the light from the epoch of formation of the first star generation, 200 million years after the big bang. Yet the amplitude of the signal was totally unexpected and suggests that the universe was half as warm—or colder—as in any realistic scenario. We are exploring if primordial black hole dark matter could explain this observation, for instance, as an indirect effect of accretion of the neutral hydrogen gas.

--Originally published: Scientific American 28(2s); 40-45 (Spring 2019).

Maybe We Could "See" a Singularity After All

by Abraham Loeb

The singularities at the centers of astrophysical black holes mark the breakdown of Einstein’s theory of gravity, general relativity. They represent the only breakdown sites accessible to experimentalists, since the only other known singularity, the big bang, is believed to be invisible due to the vast expansion that occurred afterwards during cosmic inflation.

Every physicist knows these facts but very few discuss black hole singularities, as if the topic is taboo. The reason is simple: to explore the true nature of singularities we need a theory that unifies general relativity with quantum mechanics, and we do not have a unique, well-defined formalism for doing that. Even in the context of specific proposals for a unified model, such as string theory, the nature of black hole singularities is rarely discussed because of its mathematical complexity.

But perhaps the time is ripe now to open up this discussion, given that the 2017 Nobel Prize was awarded to the LIGO team for discovering gravitational waves from collisions of black holes. An observable quantum signal from the embedded singularities could guide us in the search for a unified theory.

This thought occurred to me during two back-to-back conferences that we hosted at Harvard University on May 7–11, 2018, one on gravitational wave astrophysics and the second the annual conference of Harvard’s Black Hole Initiative. A few days earlier, the basement at my home was flooded since the sewer pipe was clogged by tree roots, and the five hours spent with a plumber in fixing this problem led me to realize that any water going down the drain collects somewhere.

Usually the sewer pipe takes the water to a town reservoir and we do not think about where it goes because we do not see the water once it leaves our property. But because the sewer pipe at my home was clogged, the water flooded my basement and then I started thinking about the analogous problem of where the matter that makes a black hole collects. The reservoir in that case is the singularity.

True, the singularity of a stationary black hole is hidden behind an event horizon for any external observer. This “cosmic censorship” is a good reason for ignoring the observational consequences of singularities when probing the calm spacetime around isolated black holes, for example—while imaging the silhouette of Sagittarius A* at the center of the Milky Way with the Event Horizon Telescope.

But this does not imply that observers, more generally, can never study empirically the nature of the singularities. When children get a birthday present wrapped in a box, they attempt to learn about its nature without seeing it directly by shaking the box and listening to its vibrations.

Similarly, we can listen to the vibrations of a black hole horizon that is strongly shaken through its collision with another black hole, hoping to learn more about the nature of the singularities hidden inside. Future generations of LIGO detectors could serve as the “child’s ears” in extracting new information from these vibrations.

A particularly interesting question is what happens when two singularities collide. How do they merge to a single singularity, and does this process have an imprint on the gravitational wave signal that is observable by LIGO? Naïvely, one might argue that computer simulations have already calculated the gravitational wave signals from black hole collisions and there is no hint for the content of the “event horizon box” in these signals.

But existing simulations suffer from two shortfalls. First, they cut out completely the region around the singularities by postulating that this region will not have observable effects; and second—they do not incorporate quantum mechanical modifications of general relativity. If there are observable signatures of merging singularities, existing computer simulations are, by construction, blind to them.

What would a singularity look like in the quantum mechanical context? Most likely, it would appear as an extreme concentration of a huge mass (more than a few solar masses for astrophysical black holes) within a tiny volume. The size of the reservoir that drains all matter that fell into an astrophysical black hole is unknown. We could envision the remnant singularity as being a finite-size reservoir in equilibrium, similar to the halos of galaxies, where the motion of infalling particles is turned around and confined by their binding gravitational attraction.

One might assume that the outer boundary of the remnant object is some small fraction of its Schwarzschild radius, 2GM/c² (which equals three kilometers times the black hole mass, M, in solar mass units), corresponding to a universal curvature scale at which Einstein’s theory of gravity breaks down due to quantum corrections. In that case, the size of the object that replaces the singularity can be expressed in terms of its mass over the Planck mass (10⁻⁵ grams) times the Planck length (10⁻³³ centimeter, or 10⁻²⁰ times the size of the proton).

Now imagine two such singularities colliding as a result of the merger of two black holes. Although the collision of these objects might not be visible directly to an external observer (unless a “naked singularity” appears during the process), the interesting question is whether the collision will produce a transient burst of energy that is observable to the outside world through the vibrations it induces in the event horizon. Is that possible?

This is an extremely interesting question that should be discussed further. It could motivate gravitational wave observers to develop more sensitive detectors. At the very least, we might be able to outline the landscape of possibilities. Science is work in progress and most of the fun exploring it is spent in uncharted territories.

I often encourage my string-theory colleagues to contemplate testing their theory by boarding a futuristic spacecraft that will take them into the event horizon of a nearby black hole. Perhaps future extensions of LIGO can save them the expense of this lengthy one-way trip.

--Originally published: Scientific American online, May 22, 2018.

SECTION 4

Life on the Edge

Living Near a Supermassive Black Hole

by Abraham Loeb

We have known since the 1990s that planets exist around pulsars, which are extraordinarily dense objects born out of the violent explosions of stars. It is therefore reasonable to assume that planets might also exist around black holes, which, perhaps surprisingly to many people, have a much weaker impact on their environment than pulsars. It’s even possible that life may form on some of these planets, given that organisms on Earth have adapted to extreme conditions, including boiling heat, freezing cold, and acidic, highly salty and even radioactive environments.

Inhabited planets could exist near the supermassive black holes that lie at the cores of most galaxies. Our own galaxy, the Milky Way, harbors a black hole whose mass is as great as four million stars put together. Known as Sgr A* (Sgr stands for Sagittarius), its innermost stable circular orbit (ISCO) has roughly the size of the orbit of Mercury around our sun.

So, what would life be like on such a planet?

Before addressing the many health hazards for life near a black hole, we should consider the benefits. If civilizations form in, or migrate to, the vicinity of black holes, what could they do for fun and profit? The following top 10 activities come to mind:

• Using the black hole as a source of clean energy by dumping trash through the accretion disk of matter that swirls around it. Up to 42 percent of the rest mass of this trash can be converted to radiation at the ISCO of a maximally spinning black hole.
• Coupling some engineered device to the spin of the black hole, as a giant flywheel from which spin energy can be harnessed.
• Surfing with light sails on relativistic jets at speeds approaching that of light.
• Prolonging youth by visiting beauty salons to near the horizon of the black hole, where time is ticking more slowly as a result of gravitational redshift.
• Viewing the spectacle of the entire universe reflected and distorted as gravitationally lensed images around the black hole.
• Establishing an amusement park at the so-called “photon sphere,” where one could enjoy relativistic effects for fun, such as seeing oneself from behind by looking straight ahead as light circles around the black hole.
• Taking advantage of new opportunities for space travel. For example, when the Milky Way and its sister galaxy Andromeda merge billions of years from now, the two black holes at their centers will pair into a tight binary, which should act as a gravitational slingshot and eject stars or planets at up to the speed of light, as described in two papers that the author wrote with James Guillochon. Travel agencies may offer tickets to exceptional rides on ejected planets that traverse the entire universe.
• Sending criminals into the black hole as the ultimate prison with a death sentence at the singularity. The mass of the black hole will determine how much time is left for the prisoners to live. The lesser their crime, the more massive the black hole would be, extending their remaining life span after crossing the “prison walls” associated with the black hole horizon.
• Using gravitational waves from small objects orbiting the black hole for communication. Such signals cannot be blocked by any known form of matter.
• Testing fundamental aspects of quantum gravity through organized trips for string physics experimentalists.

The main danger for astronauts attempting to execute these activities stems from gravitational tides. As Albert Einstein noted in his famous thought experiment, being inside a free-falling elevator or spacecraft feels like having no gravity at all. But any difference in gravitational acceleration between your head and toes, which measures the curvature of spacetime, could potentially rip your body apart. Such tides would impose a death sentence near a stellar-mass black hole but are of no threat to the human body in the much more expansive environment around a supermassive black hole, like Sgr A*.

Correspondingly, the density of matter required to make a black hole scales linearly with its spacetime curvature. Low-mass black holes are formed through the collapse of the core of a massive star to densities far greater than that of an atomic nucleus. But to make a supermassive black hole, which is much more rarefied, it is sufficient to fill the orbit of Jupiter with liquid water. As simple as this engineering project might sound, it is by no means practical since it requires about 100 million solar masses of water. And the heat generated while pouring the water in would burn any associated facilities.

Indeed, the heat released by accreting supermassive black holes poses an existential threat to civilizations residing near the centers of galaxies. In a paper with John Forbes, we showed that a significant fraction of all planets in the universe are vulnerable to their atmospheres being stripped or their oceans being boiled off as a result of having been close to an active galactic nucleus sometime during their lives.

For the first time in human history, we now have the technology to image the silhouettes of the supermassive black holes at the centers of the Milky Way and the giant elliptical galaxy M87 on the background of the glowing gas behind them. The first such images are scheduled to be released later this year.

In the summary lecture of the 2018 conference of Harvard’s Black Hole Initiative, an interdisciplinary center that focuses on the study of black holes, I suggested that future advances in space propulsion might allow us to organize a field trip to a nearby black hole. This will provide a great opportunity to pursue some of the aforementioned activities—and perhaps even exchange notes on quantum gravity with any backpackers from other civilizations who might have already camped out there.

--Originally published: Scientific American online, March 11, 2019.