Cosmology seminar [2pm]: Beyond linear ringdown

Gravitational waves (GWs) emitted by a perturbed black hole (BH) ringing down is typically modeled analytically using first-order BH perturbation theory, which has been shown to work unexpectedly well even near the merger. In this talk, I will first show analytical arguments that suggest nonlinearities indeed have a seemingly small effect on the GW radiated to asymptotically far observers. Nevertheless, nonlinearities may still be resolvable and we search for them in high accuracy Numerical Relativity (NR) simulations. Focusing on a specific second-order nonlinearity, we show it to be present in binary simulations with a wide range of mass ratios, and confirm its physical origin with a consistency test. By including this nonlinear effect, we find that GW analytical models can improve their fittings to simulations by up to two orders of magnitude.