The orientation of merging black holes’ spins helps reveal how the pair of black holes formed, and physicists have now probed the spin distribution statistically.
Less than 5 years ago, physicists rocked the scientific world when they first spotted gravitational waves—fleeting ripples in space and time—set off when two gargantuan black holes billions of light-years away swirled into each other. Since then, scientists have detected a scad of similar events, mostly reported event by event. Today, however, researchers with a global network of gravitational wave detectors announced the first major statistical analyses of their data so far, 50 events in all. Posted online in four papers, the analyses show that black holes—ghostly ultraintense gravitational fields left behind when massive stars collapse—are both more common and stranger than expected. They also shed light on mysteries such as how such black holes pair up before merging.
The new studies, posted on the physics preprint server arXiv, “are super-important,” says Carl Rodriguez, an astrophysicist at Carnegie Mellon University who was not involved in the work. “With an individual event, there’s only so much you can do in comparing to astrophysics models. But with a catalog you can not only begin to constrain the theory, you can start to understand the landscape.” Selma de Mink, an astrophysicist at Harvard University, says she and her colleagues have been waiting to do their own analyses of the data trove. “There will definitely be a flurry of papers that are rushing to take the first stabs at the data.”
The observations come from three huge L-shaped optical instruments called interferometers that can measure the infinitesimal stretching of space itself by a passing gravitational wave. Two of those detectors belong to the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors with arms 4 kilometers long in Louisiana and Washington state that spotted the first gravitational waves in 2015. The third detector is Virgo, an interferometer near Pisa, Italy, that has 3-kilometer-long arms and joined the hunt for gravitational waves in 2017.
LIGO and Virgo had already spotted 11 events including one merger of neutron stars, an event that may shed light on how the universe forges heavy elements. Now the team has catalogued 37 additional black hole mergers, one likely neutron star merger, and one possible merger of a black hole and neutron star from the first half of its third observing run, from April through September 2019.
Analyses of all 50 events show that when it comes to black holes, “the diversity is surprisingly large,” says Frank Ohme, a gravitational wave astronomer at the Max Planck Institute for Gravitational Physics. From details of the mergers’ chirp-like signals, scientists can calculate the masses of the colliding black holes. They expected to find a “mass gap” between about 45 and 135 solar masses—the result of particle physics processes that should blow apart stars within a certain mass range before they can collapse into black holes.
However, LIGO and Virgo have now spotted mergers involving black holes squarely within the gap, including one with a mass of roughly 85 solar masses. De Mink, who models the evolution of black-hole pairs from binary star systems, says that accounting for the interlopers will be challenging. The mass gap is “such a clear prediction from the models that it’s hard to believe that there’s not a feature there” in the mass spectrum, she says.
Similarly, scientists expected another forbidden range below 5 solar masses, based on previous observations of individual black holes peacefully orbiting normal stars. But at least one hole in the catalogue appears to fall below that limit. “How do you describe the boundaries of this population?” Ohme asks. “It’s not such a clear picture anymore,” he says.
Their new ability to take a census of black holes has also enabled researchers to probe whether black holes in a merging pair point in the same direction as they orbit each other—a potential clue to how the pair came together in the first place. If the spins align with the orbital axis, the black holes might have formed from a pair of stars that were born together, naturally acquired matching spins, and remained companions after they collapsed. If the spins point in different directions, the black holes might have formed first and then somehow paired later. Which formation channel dominates is a subject of intense debate.
In particular, if one of the black holes spins in the opposite sense of the orbit, the pair would more likely come from the mingling of black holes that had already formed. But it’s very hard to tell for sure if that’s happening from the warble of a single signal, says Maya Fishbach, an astrophysicist and LIGO member from Northwestern University. However, by analyzing the events en masse, scientists have teased out evidence that at least some of the mergers involve reversed spins. That result in turn suggests that black-hole pairs form in more than one way, Fishbach says. “It seems like there might be multiple things going on.”
Rodriguez notes that the overall rate of black hole mergers that LIGO and Virgo see seems to roughly match the rate he predicted in his model in which already-formed black holes find each other and pair in knots of old stars called globular clusters. “I shouldn’t toot my own horn—but I totally am going to,” he says. But he notes that the data are also consistent with such a mechanism producing just a quarter of the mergers, Rodriguez notes.
Researcher have even been able to probe how the number of black hole mergers may have changed over cosmic time, Fishbach says. The rate is expected to be higher in the early universe, when the pace of star formation was also higher. But previous data allowed that rate to be up to 100,000 times higher than it is currently. Now, scientists have seen enough far-flung events to say that the rate of mergers 8 billion years ago was no more than 10 times what it is now, Fishbach says.
LIGO and Virgo scientists owe their scientific bounty to the increasing sensitivity of their detectors, which has enabled them to spot ever fainter and more distant events. Now they are eager to build up their catalogue even further. With more events, they find a correlation between spin alignment and the masses of the black holes that could help reveal whether the heaviest might themselves have formed through mergers. (If the two black holes spins aren’t aligned, then they may not have form from an isolated pair of stars, and theorist wouldn’t necessarily have to explain how a collapsing star could produce such a heavy black hole.) “We’ve answered a lot of questions we didn’t even know we had,” Fishbach says, “but we raised even more. This is just the beginning of the science.”
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