Black Holes

Dark Matter Could Help Solve the Final Parsec Problem of Black Holes

When galaxies collide, their supermassive black holes enter into a gravitational dance, gradually orbiting each other ever closer until eventually…merging. We know they merge because we see the gravitational beasts that result, and we have detected the gravitational waves they emit as they inspiral. But the details of their final consummation remain a mystery. Now a new paper suggests part of that mystery can be solved with a bit of dark matter.

Just as the famous three-body problem has no general analytical solution for Newtonian gravity, the two-body problem has no general solution in general relativity. So, we have to resort to computer simulations to model how black holes orbit each other and eventually merge. For binary black holes that are relatively widely separated, our simulations work really well, but when black holes are close to each other things get complicated. Einstein’s equations are very nonlinear, and modeling the dynamics of strongly interacting black holes is difficult.

As a result, the simulations we do have don’t show the black holes merging. Instead, they inspiral until they are about a parsec apart, then stabilize. This is known as the final parsec problem. One idea to solve the problem is to introduce dark matter into the mix. After all, cold dark matter is nearly everywhere according to the standard cosmological model, so it likely plays a role in the mergers of supermassive black holes. But so far it seems cold dark matter isn’t the answer. Simulations using it have the same final parsec problem as general relativity alone.

How fuzzy dark matter can increase inspiralling. Credit: Koo, et al

In this new study, the team considers a variation on dark matter known as fuzzy dark matter. It’s similar to standard cold dark matter except it is made of low-mass scalar particles. Since these particles wouldn’t interact with each other by anything other than gravity, they wouldn’t clump in quite the same way as regular dark matter, and thus have a more “fuzzy” distribution.

Fuzzy dark matter was first proposed to address what is known as the cusp problem of dwarf galaxies, which is a weak point for dark matter. Here the authors show that fuzzy dark matter can increase the rate of orbital decay for black holes, particularly the largest of supermassive black holes. It could explain some of the monsters we’ve observed at the heart of some elliptical galaxies.

But the work doesn’t prove fuzzy dark matter is the solution. The details of the final stage of inspiralling black holes are still, shall we say, a bit fuzzy. It will take direct observations of supermassive black holes to either prove or rule out the idea. Fortunately, future NANOGrav observations, or those of the planned LISA gravitational wave observatory should be able to see such mergers. Then our understanding of how the largest black holes in the cosmos merge won’t be quite so fuzzy.

Reference: Koo, Hyeonmo, et al. “Final parsec problem of black hole mergers and ultralight dark matter.” arXiv preprint arXiv:2311.03412 (2023).

Brian Koberlein

Brian Koberlein is an astrophysicist and science writer with the National Radio Astronomy Observatory. He writes about astronomy and astrophysics on his blog. You can follow him on YouTube, and on Twitter @BrianKoberlein.

Recent Posts

China Names its Capsule and Lander for its Upcoming Human Lunar Missions

In a recent announcement, the Chinese Space Agency (CSA) unveiled the names for its forthcoming…

14 hours ago

If Exoplanets Have Lightning, it’ll Complicate the Search for Life

Discovering exoplanets is almost routine now. We've found over 5,500 exoplanets, and the next step…

17 hours ago

Electrodes in Spacesuits Could Protect Astronauts from Harmful Dust on Mars

To quote NASA associate administrator Jim Reuter, sending crewed missions to Mars by 2040 is…

17 hours ago

Odysseus Moon Lander Sends More Pictures — and We Know Where It Is

Four days after Intuitive Machines' Odysseus lander made an off-kilter touchdown on the moon, the…

20 hours ago

Astronomers Discover a New Meteor Shower. The Source is Comet 46P/Wirtanen

Like many of you, I love a good meteor shower. I have fond memories of…

1 day ago

Surprise! Japan’s SLIM Moon Lander Wakes Up After a Freezing Night

Japan's space agency didn't expect its wrong-side-up SLIM moon lander to revive itself after powering…

1 day ago