The original version of this story appeared in Quanta Magazine.
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