Binary Black Hole Mergers: Physical Mechanisms

During inspiral, two horizons orbit a common center of mass and gradually lose orbital energy. Near merger, spacetime becomes strongly nonlinear and numerical relativity is required. The final remnant settles toward a Kerr black hole through damped ringdown oscillations.

Gravitational radiation removes energy and angular momentum, raising orbital frequency as separation decreases. The final collision forms a distorted common horizon, and the remnant radiates remaining asymmetry as gravitational waves until only mass and spin dominate its exterior state.

Detected chirp waveforms match general-relativistic templates across inspiral, merger, and ringdown. Independent detectors observe consistent signals, and inferred final masses are smaller than the initial mass sum by an amount corresponding to energy radiated in gravitational waves.

Radiation carries away energy and angular momentum, increasing orbital frequency until strong nonlinear spacetime dynamics dominate. Numerical relativity is needed through merger, after which perturbation theory describes ringdown.