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…

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