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.
However, recent evidence has shown that these supermassive black holes do eventually merge. Last year, observations of pulsating stars known as pulsar timing arrays revealed a background hum of gravitational waves in the universe, indicating the presence of tightly orbiting supermassive black holes within a parsec of each other that are close to merging. This discovery has challenged the long-standing belief that black hole mergers were impossible due to the final-parsec problem.
Astrophysicists have proposed various explanations for how these black holes are able to overcome the final-parsec problem. One theory suggests that dark matter, the mysterious substance that makes up 85% of the universe, could play a role. Specifically, self-interacting dark matter particles could drag on the black holes and bring them closer together. This would not only solve the final-parsec problem, but also provide new insights into the nature of dark matter.
Other potential solutions to the puzzle have also been proposed, including the idea of fuzzy dark matter. This summer, a group of physicists in Canada argued that this type of dark matter could also facilitate black hole mergers. With multiple theories on the table, scientists are now working on ways to test and compare these explanations in order to gain a better understanding of the final-parsec problem and the role of dark matter in the universe.
As we continue to unravel the mysteries of the cosmos, it is clear that the study of black holes and dark matter will play a crucial role in our understanding of the universe and its evolution. The final-parsec problem is just one of many puzzles waiting to be solved, and with each new discovery, we come closer to unlocking the secrets of the universe.
