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How Do Merging Supermassive Black Holes Pass the Final Parsec?

Galaxies have been merging into ever-bigger structures over the course of cosmic history. When galaxies merge, the supermassive black holes that sit in their centers must eventually merge, too, forming an even more gargantuan black hole.

For decades, though, a question has vexed astrophysicists: How can supermassive black holes get close enough to spiral together and coalesce? In calculations, when the converging holes reach the so-called final parsec — a distance of about one parsec, or 3.26 light-years — their progress stalls. They should essentially orbit each other indefinitely.

“It was thought in-spiral times could be as high as … the age of the universe,” said Stephen Taylor(opens a new tab), an astrophysicist at Vanderbilt University. “People were concerned that you might not get any merging black holes.”

Evidence has accrued that they do merge. Last year, observations of the subtle movements of pulsating stars known as a pulsar timing array revealed a background hum of gravitational waves in the universe — ripples in the fabric of space-time. These gravitational waves most likely come from tightly orbiting supermassive black holes within a parsec of each other that are close to merging. “This was our first evidence that black hole binaries do overcome the final-parsec problem,” said Laura Blecha(opens a new tab), an astrophysicist at the University of Florida.

So how do they do it?

Astrophysicists have a new suggestion: Dark matter could sap angular momentum from the two black holes and nudge them closer.

Dark matter is the term for the undiscovered 85% of matter in the universe. We can see its gravitational effects on galaxies and cosmic structure, but at the moment we can’t work out what it is. The simplest hypothetical particles that could comprise this invisible form of matter would be no help in facilitating black hole mergers. But this summer, a group of physicists in Canada argued(opens a new tab) that something more complex called self-interacting dark matter could. These particles could drag on the supermassive black holes enough to drop them within a parsec of each other. If this explanation is right, it will “tell you that dark matter is not as simple as we thought,” said Gonzalo Alonso-Álvarez(opens a new tab), a theoretical physicist at the University of Toronto and one of the authors.

Then in September, a separate group of physicists pointed out(opens a new tab) that another dark matter candidate sometimes called fuzzy dark matter could do the trick as well.