'Final parsec problem' that makes supermassive black holes impossible to explain could finally have a solution

Scientists have been modeling how supermassive black holes form when two smaller black holes merge. But in their simulations, most pairs of massive black holes get stuck orbiting each other indefinitely. Now, scientists may have finally found a solution to this "final parsec problem" — and it may also help uncover the identity of one of the universe's most mysterious components.

Lurking at the heart of most ordinary galaxies is a supermassive black hole (SMBH), like the one imaged by the Event Horizon Telescope collaboration in the galaxy M87. That one is about 6.5 billion times the mass of the sun, but it wasn't always so big. Astronomers think SMBHs start out much smaller and grow into behemoths through repeated mergers with other black holes.

Evidence for those colliding giants came in 2023, when scientists with the International Pulsar Timing Array collaboration announced they had found a background "hum" of gravitational waves — ripples in the fabric of space-time released during mergers of extremely massive objects. Astronomers think this background is produced by distant pairs of massive black holes as they send space "ringing" with the gravitational echo of their faraway collisions.

Eternal cosmic dance

Researchers use sophisticated computer simulations to investigate the complex dance of these circling black holes. But until now, the models have run into a problem: When the black holes get down to a separation of about a parsec — about 3.26 light-years — they get stuck, circling each other eternally.

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That's because, to collide and merge, the spiraling black holes must first lose energy and slow down. While approaching each other from many light-years apart, the black holes orbit through gas clouds and star clusters that slow their motion, causing them to spiral even closer.

But by the time they reach the last parsec, there isn't enough material left to drain their energy. Instead, the models predict that the duration of their final merger stretches to more than the current age of the universe. This has become known as the "final parsec problem."