What phenomenon is produced when two black holes merge and ripple spacetime?

When two black holes merge, they produce gravitational waves that ripple through spacetime.

According to Einstein’s general theory of relativity, accelerating masses can disturb the geometry of spacetime and send those disturbances outward at the speed of light. A pair of orbiting black holes becomes especially powerful near the end of its life: the objects spiral inward, orbit faster, merge, and settle into a single rotating black hole. This sequence is called the inspiral, merger and ringdown.

Gravitational waves are not ordinary sound waves, gamma-ray bursts or streams of particles. They are alternating stretches and squeezes of space itself. On Earth, instruments such as LIGO detect them with laser interferometers whose arms change by an extraordinarily tiny amount when a wave passes.

The first direct detection, GW150914, arrived in 2015 after traveling about 1.4 billion light-years. The event confirmed a major prediction of general relativity and opened gravitational-wave astronomy. Black-hole mergers may emit little or no visible light, so these ripples can reveal otherwise dark cosmic events that telescopes cannot see.