Black-hole spectroscopy: excitation, detection and parameter estimation

Gravitational-wave detectors have just started listening to a full symphony produced by some of the most violent events in the universe: mergers of black holes. In this talk, I will focus on the final stages of the black hole coalescences – the so-called ringdown phase – when this symphony is most melodious. Linear perturbation theory predicts that, at late times, the ringdown consists of a superposition of exponentially damped sinusoids called quasinormal modes (QNMs).
Recently, it has been shown that the post-peak gravitational waveform can be modeled entirely using pure linear theory with the inclusion of enough overtones. This is surprising given that binary black hole mergers are expected to be highly nonlinear. In this talk, I will revisit this key question: can we reliably conclude that the entire post-peak waveform is a superposition of overtones predicted by the linear theory? Are higher overtones physical, or do they just overfit the nonlinearities in the merger signal? 
As current gravitational-wave detectors undergo technological improvements, we will soon reach an era when it would be routine to identify black hole merger remnants by measuring their quasinormal mode frequencies.  I will address the question of event rates and detectability of QNMs with current and planned detectors. We will also discuss how detecting multiple ringdown modes can help parameter estimation by breaking various degeneracies. This degeneracy breaking will be very important when detectors see only the ringdown (i.e., for intermediate-mass black hole mergers observed by ground-based detectors and for the heavier supermassive black hole mergers observed by LISA). I will discuss methods to harness the vast potential of ringdown harmonics in the estimation of the properties of the black holes that produced the signal.