Dark matter could provide supermassive black holes the brakes they need to bring them crashing together at the end of a long, spiraling journey towards their destiny.

According to new mathematical modeling, a conundrum known as the final parsec problem can be resolved by the presence of particles of self-interacting dark matter that remain clustered around the black holes, allowing them to cross the final distance between them.

It's a finding that suggests that the mysterious matter that gives the Universe its extra gravity must, therefore, be able to interact with itself, since the problem cannot be resolved with non-interacting dark matter models.

"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 physicist Gonzalo Alonso-Álvarez at the University of Toronto and McGill University. "Our calculations explain how that can occur, in contrast to what was previously thought."

Found in the hearts of galaxies, supermassive black holes present astronomers with a huge puzzle. We know that black holes of a more petite size form from the collapsed cores of massive stars that have run out of fusion fuel and kicked the cosmic bucket. These smaller masses can merge into larger ones; the most massive black hole merger detected to date produced an object with a mass equivalent to 142 Suns.

Supermassive black holes are millions to billions of times the mass of the Sun. It's reasonable to assume that they can get that large by merging with other monster-sized black holes. We've even spotted supermassive black holes circling each other after their galaxies have merged, throughout the history of the Universe, seemingly on an eventual collision course.

What isn't clear, however, is how these supermassive black holes collide. According to models, as supermassive black holes circle each other, they transfer their orbital energy to the stars and gas around them, causing their orbit to become smaller and smaller. As their separation shrinks, the amount of stuff that can steal their momentum shrinks too.

By the time they're about one parsec apart – 3.2 light-years or so – their galactic neighborhood can no longer support further orbital decay, so the orbit of the black holes stabilizes for what could be a very long period of time. How long? Well, longer than the Universe has been around for at least.

One way to determine whether supermassive black holes truly have merged in the past involves gravitational waves; vast ripples in the fabric of space-time caused by large masses when they change speed. If supermassive black holes are colliding throughout the Universe, there should be a signature background 'hum' of very low-frequency gravitational waves constantly rippling throughout the Universe.

We have, finally, detected a background gravitational wave hum. Which suggests we're missing a critical part of the supermassive black hole collision story.

This is the final parsec problem.

Dark matter might be what we're missing. Yet according to previous models of merging supermassive black holes, their gravitational interaction should also eject dark matter particles away from the system that could otherwise absorb that last bit of orbital energy.

Now, the problem with dark matter is that we don't know what it is. It doesn't interact with the normal matter of the Universe beyond its gravitational pull, making it extremely difficult to probe. We call it dark matter as a placeholder term, really, and scientists are trying to figure out its properties by studying the behavior of the Universe in other ways.

Alonso-Álvarez and his colleagues wondered if we were being too hasty throwing out dark matter as a solution, so they designed mathematical models to test it. And they found that dark matter that interacts with itself can stay in the vicinity of merging supermassive black holes – giving the black holes something to pass their last orbital energy onto so they can finally embrace, forming one extra-large supermassive black hole.

At the moment, the results are fairly theoretical, but they make predictions that can be observed. For example, the findings predict a softening of the gravitational wave background hum, of which hints have already been seen. And the results can also be used to understand the haloes of dark matter that surround galaxies throughout the Universe, since the particles must interact on a galactic scale to be able to resolve the final parsec problem.

Finally, the researchers say their findings represent a new tool for figuring out the mysteries of dark matter.

"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."

Source:

Published 9 July 2024 in Physical Review Letters; Self-Interacting Dark Matter Solves the Final Parsec Problem of Supermassive Black Hole Mergers

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.021401

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