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Abstract

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 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. Here 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.
Furthermore, the parameters of the system estimated from the horizon agree with
those estimated from the waveform. This implies that even though black hole
horizons are causally disconnected from us, assuming general relativity to be
true, we can potentially infer some of their detailed properties from
gravitational wave observations.