Why do black holes merge and produce gravitational waves?

Einstein predicted gravitational waves in 1916, but they were first directly detected by LIGO in 2015. When two black holes orbit one another, their changing mass distribution sends ripples through spacetime that carry away energy and angular momentum. That loss makes the orbit shrink and speeds up the inspiral. In the final fractions of a second, the collision produces a powerful burst of gravitational waves that can be stronger than the light from all the stars in the observable universe combined. The first detected event, GW150914, came from the merger of two stellar-mass black holes about 1.3 billion light-years away.