The Mystery of How Supermassive Black Holes Merge

However, modeling has shown that it is difficult to distribute enough stars to black holes to solve the final-parsec problem.

Alternatively, each black hole may have a small disk of gas surrounding it, and these disks may draw material from the wider disk surrounding the empty space carved by the holes. “The surrounding disks are fed from the wider disk,” Taylor said, and that means, their orbital energy can leak into the wider disk. “It seems to be a very efficient solution,” Natarajan said. “There is a lot of electricity available.”

In January, Blecha and his colleagues investigated the idea that a third black hole in the system could provide a solution. In some cases where two black holes are stationary, another galaxy may begin to merge with the first two, creating an additional black hole. “You can have a strong three-body interaction,” Blecha said. “It can take energy and significantly reduce the average assembly time.” In some cases, light from three holes is removed, but in others all three are combined.

Trials on the Horizon

The task now is to figure out which solution is correct, or if there are multiple processes at play.

Alonso-Álvarez hopes to test his theory by looking for a signal of self-sustaining dark matter in future pulsar data. When black holes get closer than the last parsec, they dissipate angular momentum primarily by emitting gravitational waves. But if dark matter interacts, we should see it dissipate at distances close to the parsec limit. This will make gravity waves less powerful, Alonso-Álvarez said.

Hai-Bo Yu, a particle physicist at the University of California, Riverside who is a proponent of interactive dark matter, said the idea makes sense. “It’s a way to look at the invisible properties of dark matter from gravitational wave physics,” he said. “I think that’s just interesting.”

The European Space Agency’s Laser Interferometer Space Antenna (LISA), a gravitational wave observatory scheduled for launch in 2035, may give us more answers. LISA will pick up gravitational waves emitted by merging supermassive black holes in their final days. “With LISA we will actually see supermassive black holes merging,” Pacucci said. The nature of that signal can reveal “certain features that indicate a slow process,” solving the final-parsec problem.

The first story reprinted with permission from Quanta Magazine, independent editorial publication of Simons Foundation whose mission is to improve public understanding of science by incorporating research developments and trends in mathematics and the natural and biological sciences.