Gravity Beyond Einstein: Gravitational-Wave Signatures of New Physics

forthcoming

The detection of gravitational waves (GWs) from binary black holes (BBHs) allows us to test the predictions of General Relativity (GR) in a highly-warped regime for the first time in history. Input from fundamental physics indicates that GR is not a complete theory, and therefore it must undergo modifications that would become relevant in extremely powerful gravitational fields. The next generation of GW observatories will probe extreme gravitational phenomena with a new level of precision, and hence opens a new window to observe the effects of these modifications. This project will describe the most relevant smoking-gun signatures of modifications of GR on GW observations of BBHs, therefore enabling the experimental search of new physics. By working within the framework of effective field theory (EFT) —which captures the most general modifications of GR in an agnostic way — we will analyze the impact of beyond-GR physics on several key observables. We will provide a thorough analysis of the quasinormal modes (QNMs) of black holes — which control the ringdown part of the GW signal — introducing a novel formulation of perturbation theory as well as a correspondence between QNMs and modified geodesics. We will obtain the QNM frequencies of black holes of arbitrary rotation (including the challenging case of highly rotating black holes) and we will also analyze the amplitudes of QNMs. This will allow us to accurately model the ringdown signal and to identify resonances that would amplify the effects of new physics. We will also describe the inspiral of a BBH in the EFT extension of GR, accounting for corrections to radiation reaction and tidal effects — which we will analyze in detail by determining the tidal deformability of rotating black holes. In this way, we will obtain inspiral waveforms for spinning BBHs beyond GR. Our results will make it possible to perform precision tests on modifications of GR with the next generation of GW observatories.