Gravity Seminar: Ringdown modelling and quasi-normal mode overtones: horizon dynamics and gravitational waves, Pierre Mourier UIB

The last phase of a black hole merger is accompanied by a train of damped gravitational radiation known as the ringdown. At late enough times, it can be described linearly, as a sum of discrete modes: the quasi-normal modes (QNMs) of the final Kerr black hole. As a consequence of the black hole no-hair theorem, the QNM frequencies are fully determined by the only two parameters (mass and spin) of this black hole. Measuring these frequencies then enables tests of this prediction of GR, e.g. by comparing the frequencies of multiple QNMs (“black hole spectroscopy”).

It has been suggested that ringdown signals may be modelled by QNMs at early stages, already from the merger onwards, using a large enough number of modes (overtones). This could improve the sensitivity of such tests which would otherwise analyse much quieter late-time signals. I will present and discuss the results of further tests of this claim from numerical relativity results in two regimes: the local dynamics of the final horizon, and the closely related gravitational radiation itself when modelled with many QNM overtones. I will in particular discuss the stability of the recovery of individual QNM tones and the resulting prospects for spectroscopic tests of GR; as well as comparisons to a few phenomenological models beyond the linear QNMs.