How Do Merging Supermassive Black Holes Pass the Final Parsec?

An editorially independent publication supported by the Simons Foundation.

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, 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, an astrophysicist at the University of Florida.

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