[Cosmology Seminar] Can we hear the event horizon? Overtones, fine structure and testing gravity with black hole ringdowns

Following the merger of two black holes, the remnant relaxes by ringing like a struck bell, emitting gravitational waves at a discrete set of complex frequencies known as quasinormal modes. Measuring these frequencies is a programme called black hole spectroscopy, by analogy with its atomic counterpart, and it offers one of the cleanest probes of general relativity in the strong-field regime. Yet there is much we still do not understand, including which ringdown features are detectable, when we should look for them, and what else ringdown can tell us about black holes and gravity. 

In this talk I will describe a fully Bayesian framework for disentangling the ringdown, applied to the most accurate numerical relativity simulations currently available. I will then present two recent results relevant to current observations. The first is a search of LIGO-Virgo-KAGRA data for isospectrality breaking, in which the mode spectrum splits in two (much as fine structure splits atomic spectral lines). The second is an analysis of whether the ringdown signal can probe physics significantly inside the light ring, close to the event horizon, and whether this might have already been done for GW250114.