Strong gravity physics of dynamical horizons in black hole mergers and its imprint in their gravitational radiation

Abstract

In a binary black hole merger, the inspiral portion of the gravitational 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. In this thesis, 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. The shear behaves very much like a compact binary coalescence waveform with increasing frequency and amplitude, and the parameters of the system estimated from the horizon agree with those estimated from the waveform. We then track the source multipole moments of the dynamical horizons in simulations of several binary black hole mergers and calculate the corresponding Love numbers for axisymmetric modes. Unlike the (field) tidal Love numbers of non-spinning black holes, these do not vanish. We present evidence that, at least for modest mass ratios, their behaviour is universal. We extend this to the non-axisymmetric moments and show that the dynamical horizons encode detailed information about the dynamics of the binary, and that their source multipole moments are strongly correlated with the gravitational field at null infinity. Finally, we study the outer common dynamical horizon formed during the merger and its relationship with the outgoing gravitational waves, and show how the deformed common horizon approaches the final Kerr state by absorbing gravitational radiation with signatures of quasi-normal ringing. Its gravitational field shares a similar multipolar structure as that at future null infinity, and we point out some limitations, inconsistencies, and anomalies that need to be addressed.