Supermassive black holes sometimes merge, but for decades, astrophysicists have not known how. Now, we may finally have a solution in the form of dark matter.
After a pair of galaxies smash together, their central supermassive black holes usually begin to orbit one another. Over the course of many millions of years, these colossal black holes interact gravitationally with any nearby matter, flinging it away. This process generates friction that gradually slows down the black holes, pushing them to spiral closer and closer to one another.
But by the time they get to about 1 parsec apart â that is a little more than 3 light years â very little matter remains in the area. Matter is so sparse that it could not generate enough friction to make the black holes merge in a time period less than the age of the universe. So although we know that these black holes must merge â partially because we have detected the ripples in space-time thought to result from such mergers â we donât know how. This mystery is called the final parsec problem.
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 at the University of Toronto in Canada and his colleagues have found a potential solution in the form of dark matter. The standard picture of dark matter particles states that they donât interact with one another, so they would get thrown out of the area around supermassive black holes just like the regular matter does. But the researchers calculated that, if the dark matter particles could bounce off one another, it could keep enough of them around to slow down the black holes and make them merge.
âIf the black holes are moving, then the dark matter is imparting a small friction on them through gravitational interactions, just like if you are moving through water,â says Alonso-Ălvarez. âWe were very surprised to see that these small interactions can have such a huge impact.â
For dark matter to interact with itself, there must be some new particle that mediates that interaction. These particles, called âdark photonsâ because of their similarity to particles of light, have been proposed before. In fact, self-interacting dark matter could solve some other oddities in galactic dynamics that donât quite match the standard model of cosmology.
However, it is not the only solution that has been proposed for the final parsec problem. Given the turbulence and general turmoil in most galaxies, some astrophysicists have proposed that the regular matter near supermassive black holes may simply be replenished as galaxies churn.
âI am convinced that this model would work, but Iâm not sure that itâs necessary,â says  at the University of California, Berkeley. âItâs most likely that itâs a combination of different processes that solve the final parsec problem.â
However, if this idea does hold up, it could provide a unique opportunity to study the physics of dark matter. âSelf-interacting dark matter like this might alter the accretion flows of gas into the galaxies or the dynamics of the mergers themselves, and if it does, thatâs something that we might be able to detect,â says Kelley. The researchers are working on more detailed simulations to reveal more of the effects of self-interacting dark matter on galaxies and how it would affect the supermassive black holes within.
Physical Review Letters
