Post-merger chirps from binary black holes as probes of the final black-hole horizon

The formation of a massive black hole (BH) by coalescence of two BHs is a fascinating cosmological event that leaves a gravitational signal that, if detected, can probe extreme gravity and the BH horizon. The authors report non-trivial features of gravitational wave signals from non-equal mass binaries that could be observed by gravitational wave detectors in the coming years, and describe their connection to the evolving shape of the new-born BH. The merger of a binary black hole gives birth to a highly distorted final black hole. The gravitational radiation emitted as this black hole relaxes presents us with the unique opportunity to probe extreme gravity and its connection with the dynamics of the black hole horizon. Using numerical relativity simulations, we demonstrate a connection between a concrete observable feature in the gravitational waves and geometrical features on the dynamical apparent horizon of the final black hole. Specifically, we show how the line-of-sight passage of a cusp-like defect on the horizon of the final black hole correlates with chirp-like frequency peaks in the post-merger gravitational-waves. These post-merger chirps should be observed and analyzed as the sensitivity of LIGO and Virgo increase and as future generation detectors, such as LISA and the Einstein Telescope, become operational.