However, modeling has shown that it is difficult to scatter enough stars towards black holes to solve the final parsec problem.
Alternatively, each black hole could have a small disk of gas around it, and these disks could absorb material from a larger disk surrounding the hollow region carved out by the holes. “The disks around them are powered by the wider disk,” Taylor said, and that means, in turn, that their orbital energy can escape into the wider disk. “It seems like a very efficient solution,” Natarajan said. “There is a lot of gas available.”
In January, Blecha and his colleagues explored the idea that a third black hole in the system could provide a solution. In some cases where two black holes are stalled, another galaxy may begin to merge with the first two, bringing an additional black hole with it. “You can have a strong three-body interaction,” Blecha said. “It can steal energy and significantly shorten fusion times.” In some scenarios, the lightest of the three holes is ejected, but in others all three merge.
Trials on the horizon
The task now is to figure out which solution is correct or whether multiple processes are at play.
Alonso-Álvarez hopes to test his idea by looking for a signal of self-interacting dark matter in upcoming pulsar time array data. Once black holes get closer than the final parsec, they lose angular momentum mainly by emitting gravitational waves. But if self-interacting dark matter is at play, then we should see it absorbing some of the energy at distances around the parsec limit. This in turn would create less energetic gravitational waves, Alonso-Álvarez said.
Hai-Bo Yu, a particle physicist at the University of California, Riverside and a proponent of self-interacting dark matter, said the idea is plausible. “It’s an avenue to look for microscopic features of dark matter through gravitational wave physics,” he said. “I think it’s just fascinating.”
The European Space Agency’s Laser Interferometer Space Antenna (LISA) spacecraft, a gravitational wave observatory due to launch in 2035, could give us even more answers. LISA will pick up strong gravitational waves emitted by merging supermassive black holes in their final days. “With LISA we will actually see the merger of supermassive black holes,” Pacucci said. The nature of that signal could reveal “particular features that show the slowing down process,” solving the final parsec problem.
Original story reprinted with permission from Quanta magazine, an editorially independent publication of Simon Foundation whose mission is to improve public understanding of science by covering research developments and trends in mathematics, physical and life sciences.