Evrim Yazgin
Cosmos science journalist
New research may have found a link between supermassive black holes and dark matter particles which might solve an issue which has irked astrophysicists for decades: the “final parsec problem.”
Making gravitational waves
Last year, an international team of researchers discovered a background “hum” of gravitational waves. They hypothesised that this background signal is emanating from millions of merging pairs of supermassive black hole.
Supermassive black holes are hundreds of thousands to billions of times larger than our Sun.
But theory says that such mergers between pairs of supermassive black holes shouldn’t be able to occur.
What’s the problem?
Physicists have to rely on theoretical simulations to understand the dynamics of black hole pairs as it is extremely difficult to spot them.
Models show that, as pairs of supermassive black holes spiral closer together, they shed energy. Initially, that energy is transferred to surrounding material like gas and dust. But when they get close enough, there should no longer be any material left to take on the energy from the black holes – the celestial monsters would have absorbed or dispersed all of the matter around them.
This is what is known as the “final parsec problem.” That is because, according to the simulations, the black holes stall their approach at a distance of a parsec – equivalent to about 3 light-years, or the estimated width of our solar system including the Oort Cloud.
Not only does the final parsec problem raise doubt over merging supermassive black holes being the source of the gravitational wave background. It also conflicts with the theory that supermassive black holes are formed by the merging of smaller black holes.
A dark solution
The new study, published in the Physical Review Letters journal puts forward a solution to the final parsec problem.
“We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and coalesce,” says co-author Gonzalo Alonso-Álvarez from the University of Toronto and McGill University, Canada. “Our calculations explain how that can occur, in contrast to what was previously thought.”
Previous models have showed when supermassive black holes got to within a parsec of each other, they began to interact with the dark matter cloud or halo around them. The gravitational forces of the spinning black holes throws the dark matter away, meaning the energy from the black holes cannot be transferred and the distance between them stops shrinking.
The new model, however, incorporates an interaction between dark matter particles and the black hole pair.
“The possibility that dark matter particles interact with each other is an assumption that we made, an extra ingredient that not all dark matter models contain,” says Alonso-Álvarez. “Our argument is that only models with that ingredient can solve the final parsec problem.”
Not only does the new model provide a potential answer to the final parsec problem, it might also give some insight into the nature of dark matter which should make up 80% of all matter in the universe, but has avoided detection.
“Our work is a new way to help us understand the particle nature of dark matter,” says Alonso-Álvarez. “We found that the evolution of black hole orbits is very sensitive to the microphysics of dark matter and that means we can use observations of supermassive black hole mergers to better understand these particles.”
