Answer:
When the black holes get close to each other, they fuse into one, bigger black hole. The new black hole’s radius will be roughly the sum of the two original black hole’s radii, making the new black hole encompass a much bigger volume. The fusion is somewhat like what two water droplets would do in space when they get close.
Also importantly, the black holes emit copious amounts of gravitational waves as they close in on each other. This can turn a few percent of their mass into pure energy radiated away as gravitational waves.
We detected the collision of two black holes for the first time not too long ago, in 2015, following the construction of the Advanced LIGO gravitational-wave observatories. With continued technological improvements, we will go from this first detection to a discovery every week in the next few years. While we observe these collisions, we don’t yet know what cosmic process brings the black holes close to each other so they can collide. Observing these collisions can also help us answer a range of outstanding questions, such as how black holes work as cosmic particle accelerators, or whether Einstein’s General Theory of Relativity is the correct description of nature. Black hole collisions can even help us better map how the Universe is expanding.
