Strong gravity physics of dynamical horizons in black hole mergers and its imprint in their gravitational radiation
In a binary black hole merger, it is known that the inspiral portion of the waveform corresponds to two distinct horizons orbiting each other and that the merger and ringdown signals correspond to the final horizon being formed and settling down to equilibrium. However, we still lack a detailed understanding of the relation between the horizon geometry in these three regimes and the observed waveform. In this talk, we show that the well-known inspiral chirp waveform has a clear counterpart on black hole horizons, namely, the shear of the outgoing null rays at the horizon. We demonstrate that the shear behaves very much like a compact binary coalescence waveform with increasing frequency and amplitude. In particular, we will discuss: (a) The part of the gravitational radiation absorbed at the dynamical horizons is strongly correlated with the News of the outgoing gravitational radiation in all three phases of binary black hole mergers; (b) The common dynamical horizon that forms late into the inspiral loses hair by absorbing gravitational radiation, and approaches the isolated Kerr horizon late into the ringdown.