A New Era of Black Holes Is Here
Astronomers have discovered a black-hole treasure trove that is changing our view of the cosmos.
When the first black-hole collision was detected in 2015, it was a watershed moment in the history of astronomy. Using gravitational waves, astronomers were observing the universe in an entirely new way. But this first event didn’t revolutionize our understanding of black holes—nor could it. This collision would be the first of many, astronomers knew, and only with that bounty would answers come.
“The first discovery was the thrill of our lives,” says Vicky Kalogera, an astrophysicist at Northwestern University and part of the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration that made the 2015 detection. “But you cannot do astrophysics with one source.”
Now, Kalogera and other physicists say they’re entering a new era of black-hole astronomy, driven by a rapid increase in the number of black holes they can observe.
The latest catalog of these so-called black-hole binary mergers—the result of two black holes spiraling inward toward each other and colliding—has quadrupled the black-hole merger data available to study. There are now almost 50 mergers for astrophysicists to scrutinize, with dozens more expected in the next few months and hundreds more in the coming years.
“Black-hole astrophysics is being revolutionized by gravitational waves, because the numbers are so big,” says Kalogera. “And the numbers are allowing us to ask qualitatively different questions. We’ve opened a treasure trove.”
On the strength of this data, new statistically driven studies are starting to reveal the secrets of these enigmatic objects: how black holes form, and why they merge. This growing black-hole inventory could also offer a novel way to probe cosmological evolution—from the Big Bang through the birth of the first stars and the growth of galaxies.
“I definitely didn’t expect that we’d be looking at these questions so soon after the first detection,” says Maya Fishbach, an astronomer also at Northwestern. “The field has exploded.”
Before black holes can be used to study the cosmos as a whole, astrophysicists must first figure out how they’re made. So far, two theories have dominated the debate.
Some astronomers suggest that most black holes originate inside crowded clusters of stars—regions that are sometimes a million times denser than our own galactic backyard. Each time a very massive star explodes, it leaves behind a black hole that sinks to the middle of the star cluster. The center of the cluster becomes thick with black holes, which are entwined by gravity into a fateful cosmic dance. Astronomers call this “dynamical” black-hole formation.
Others suggest that black-hole binaries start out as pairs of stars in comparatively desolate areas of galaxies. After a long and chaotic life together, they too explode, creating an “isolated” pair of black holes that keep orbiting each other. “There’s been this perception that it’s a fight between the dynamical and the isolated models,” says Daniel Holz, an astrophysicist at the University of Chicago.
The tendency of many theorists to advocate for just one binary-black-hole formation channel partly stems from the practicalities of working with very little data. “Each event was lovingly analyzed, obsessed over, and fussed over,” Holz says. “We would make a detection, and people would try to abstract very broad statements from a sample size of one or two black holes.”
Indeed, astrophysicists used that first detection to argue for opposing conclusions. LIGO found its first black-hole merger extremely quickly—before the official start of observation, in fact—which suggested that black-hole binary systems are very common in the universe. Since isolated black holes can form in a broad range of astrophysical environments, theories that favor isolated black holes predict that we’ll see a lot of mergers.
Others pointed out that the first merger featured unusually large black holes, and that the existence of these giants supported the dynamical theory. Such large black holes, they reasoned, could only have been made in the early universe, when star clusters are also thought to have formed.
Yet with a sample size of one, such assertions could only be educated guesses, says Carl Rodriguez, an astrophysicist at Carnegie Mellon University.
Now data from LIGO’s latest catalog show that black-hole binaries are far less common than expected. In fact, the rate of merging black holes now observed could be “entirely explained” by star clusters, according to a preprint paper posted late last month by Rodriguez and his collaborators.
In addition, the new mergers have enabled a fresh approach to the puzzle of where black holes come from. Despite their elusive nature, black holes are very simple. Aside from mass and charge, the only trait a black hole can have is spin—a measure of how quickly it rotates. If a pair of black holes, and the stars from which they form, live their whole lives together, the constant push and pull will align their spins. But if two black holes happen to encounter each other later in life, their spins will likely be unequal.
Theorists are now building models that include multiple black-hole-formation scenarios and unscrambling how each one might evolve across the universe’s history. Gravitational-wave physicists are hopeful that in the coming months and years they’ll be able to answer these questions with confidence.
“We’re just scratching the surface,” Kalogera says. “The sample is still too small to give us a robust answer—but when we have 100 or 200 of these [mergers], then I think we’ll have clear answers.
“We’re not that far away.”