ThorGW · Testing the horizon of black holes with gravitational waves

Gravitational waves (GWs) provide a unique opportunity to test gravity in a regime inaccessible to traditional astronomical observations. One of the main predictions of general relativity (GR) is the existence of black holes (BHs) featuring a horizon beyond which nothing — not even light — can escape. Observations of GWs from the remnants of binary BH coalescences have the potential to probe the physics at the horizon of BHs.This prospect is of particular interest given some quantum-gravity theories and dark matter models that predict the presence of horizonless compact objects, known as exotic compact objects (ECOs). ECOs emit a different GW signal than BHs due to the absence of a horizon. Studying their imprints in the postmerger stage allows one to investigate the existence of horizons in compact remnants.So far, the ground-based detectors LIGO and Virgo have detected GWs from the coalescence of binary BHs and neutron stars. Several tests on the properties of the remnants have been performed, setting constraints on generic deviations from general relativity. Current constraints can be converted into bounds on the location of the horizon by modelling the variations introduced by a generic class of ECOs. However, this aim has not been achieved given the open problems in modelling spinning ECOs, which I plan to tackle in this proposal.This project will provide the first bounds on the location of the horizon with current GW observations. For this purpose, I will extend the current state-of-the-art in ECO modelling to spinning configurations and develop new data-analysis schemes relating the postmerger GW signal to the horizon properties. Finally, I will assess the prospects of detectability of BH horizons with next-generation detectors, such as the ground-based Einstein Telescope and Cosmic Explorer and the space-based Laser Interferometer Space Antenna. The outcome of this project will provide novel tests of general relativity and shed light on quantum gravity.