What are the key phases in a gravitational wave signal from a binary black hole merger?

• A. Collision, Fusion, Afterglow.
• B. Collapse, Merging, Afterglow.
• C. Inspiral, Merger, and Ringdown.
• D. Spin-up, Spin-down, and Damping.

Answer: (C)

The gravitational-wave signal from a binary black hole merger unfolds in three well-defined stages: inspiral, merger, and ringdown. In the inspiral, the two black holes orbit each other and steadily lose energy to gravitational waves, so they spiral closer together and the waveform sweeps upward in frequency and grows in amplitude, producing a characteristic chirp. The merger is the brief, highly nonlinear moment when the two horizons coalesce into one distorted black hole; the gravitational-wave signal reaches its peak as the horizons merge and the spacetime dynamics are extremely strong. The ringdown follows, as the newly formed black hole settles into a stable Kerr state, radiating away its distortions in damped gravitational waves at characteristic quasi-normal-mode frequencies. This sequence—inspiral, merger, ringdown—captures how the signal evolves from gradual orbital decay to a violent coalescence to a calm, final ringing. Other terms don’t map onto this well-established progression of gravitational radiation, and describe aspects like spin changes rather than the distinct phases of the emission.