How have gravitational waves provided evidence for black hole mergers?

• A. Indirect inference from cosmic microwave background fluctuations.
• B. Observations of characteristic waveforms from LIGO/Virgo that match the inspiral, merger, and ringdown of merging black holes.
• C. Observations of gamma-ray bursts associated with star formation.
• D. Detection of a continuous gravitational wave signal from a single rotating black hole.

Answer: (C)

Gravitational waves reveal black hole mergers by carrying a waveform that exactly matches the prediction for two black holes spiraling together, merging, and then settling into a single hole. As the pair orbit, they radiate gravitational waves, losing energy and pulling the orbit tighter. This creates a distinctive chirp: the waves rise in frequency and grow in amplitude as the black holes get closer. When they finally coalesce, a sharp, complex merger signal appears, followed by a smooth ringdown as the new black hole settles into a stable state. Observatories like LIGO and Virgo have detected such waveforms with high confidence, and the data line up remarkably well with numerical simulations of binary black hole mergers in general relativity. From the shape and timing of the signal, we also infer the masses and spins of the holes and the amount of energy emitted as gravitational waves—evidence that a true black hole merger occurred. Electromagnetic gamma-ray bursts, while interesting phenomena, are not the signature of a black hole merger in the way these gravitational-wave fingerprints are. A continuous signal from a single rotating black hole would imply a different, steady source rather than a one-time merger event. So the clearest evidence is the gravitational-wave waveform that tracks the inspiral, merger, and ringdown predicted for merging black holes.