Physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) have spotted a third merger of black holes, the ultraintense gravitational fields left behind when massive stars collapse. This time, the subtle tremor of spacetime that signaled the merger also revealed a key feature of the black holes: their spins, which were out of kilter. That could help reveal how the black holes paired up in the first place.
"These black holes are not like two aligned tornadoes orbiting each other, but like two tilted tornadoes," says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta and deputy spokesperson for the 1000 scientists working with LIGO. "That may have implications for formation scenarios."
In September 2015, the gigantic LIGO detectors in Livingston, Louisiana, and Hanford, Washington, sensed gravitational waves from two black holes weighing 29 and 36 times as much as the sun as they spiraled together and became one. Three months later, the detectors spotted a merger of lighter black holes. How such stellar-mass black holes form is no mystery: Each starts out as a huge star. It eventually runs low on hydrogen fuel and puffs up into a giant. A few hundred million years later, nuclear fusion in its core can no longer fight gravity, and it collapses into a black hole, typically generating a supernova explosion.
But theorists struggle to explain how such black holes could form pairs. "Whatever you cook up, it has to fulfill two things," says Selma de Mink, an astrophysicist at the University of Amsterdam. "It has to make two massive black holes, and they have to be close enough" to merge within the age of the universe. A pair of black holes could be born from massive stars that collapse while orbiting each other. Or the black holes could form first and pair later. But either scenario is trickier than it sounds.
Giant binary stars, for example, typically produce black holes lighter than the ones LIGO sees and too far apart to merge. So, according to one leading theory, the stars must start out close enough together to swap matter as they evolve. When one star collapses, the resulting black hole and the other star wind up swirling through a "common envelope" of gas—literally the outer layer of the star. Friction then saps their energy and draws them closer together. The collapse of the second star leaves two black holes in a tight orbit. However, some researchers say this common envelope scenario requires more "fine tuning" than they like.
Alternatively, black holes could form first and hook up later. When two wandering black holes cross paths in space, however, they just swing around each other and go their separate ways. To form a pair, a least one other stellar object must join in the process in a so-called dynamical formation channel, says Carl Rodriguez, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge.
For example, two binaries consisting of a star and a black hole could meet. In a complicated exchange, the black holes could pair, while throwing out the stars. Then encounters with other stars could siphon off more energy and angular momentum and pull the black holes closer. Modeling shows that such encounters could take place in dense star clusters. But some researchers question whether the clusters can produce as many black hole pairs as LIGO seems to see.
How two black holes paired should show through in their spins. If the black holes started out as paired stars, then they should spin in the same direction as their orbital axis. If the black holes formed before they paired, then they could spin in any direction. "If the black holes were not spinning in the same direction as the orbit, that would probably be a pretty good indicator of the dynamical formation channel," Rodriguez says.
On 4 January, LIGO spotted black holes of 31 and 19 solar masses spiraling together 3 billion light-years from Earth. By comparing the second-long ripple picked up by the detectors with previously calculated "waveforms," the LIGO team determined how closely the black holes' spins aligned with their orbital axis. Black holes with randomly aligned spins merge relatively quickly, Cadonati explains. But if the spins are aligned with the orbital axis, the extra angular momentum slows the merger, stretching it out a few more orbits. (Similar analyses of the previous events were ambiguous.)
LIGO researchers found that the black hole spins were not aligned, and that there's an 80% probability that at least one of them spun in generally the opposite sense of the orbital motion. In this case, at least, the dynamical pairing scenario seems more likely.
With just one event to go on, it's too early to say which scenario is more common overall, Cadonati says. "We are going to have to see more of these things in order to constrain models," she says. Seeing enough of them may take time. LIGO will end its current run in August, says David Shoemaker, a physicist at MIT and spokesperson for the LIGO scientific collaboration. Researchers will then spend 12 to 18 months trying to boost the machines' sensitivity, which has improved only slightly since the 2015–16 run.