The first galaxies in the universe were formed over 13 billion years ago. These early galaxies are very small and irregularly shaped, and younger galaxies are much more massive and more structured. We think this evolution has happened through a process called hierarchical galaxy formation, in which smaller galaxies merge to form bigger and bigger galaxies. At the center of all of these massive galaxies are supermassive black holes which have masses of millions to billions of times that of the Sun. Following a galaxy merger, the two supermassive black holes from each galaxy together to eventually form a single even more massive black hole.
There is a lot we don’t understand about the final stages of this process, though. In particular, we know that gravitational wave emission will cause the black holes to merge at the very final stages, but what other processes bring the black holes together before that time? This is known as the “final parsec problem”. By measuring the gravitational waves produced in the final millions of years of the binary’s life, we can understand how these behemoth mergers occur, and the nature of their interactions with the host galaxy. Indirectly, gravitational waves also provide unique insights into the subtle connections between black holes and galaxies, thus providing clues to how galaxies and supermassive black holes form and co-evolve over cosmic time.