R. Aoude

A. Ochirov

How gravitational waves are absorbed by a black hole is understood, for the first time, through effective on-shell scattering amplitudes.

Gravitational-wave data has sparked theorists to develop new methods in general relativity. Many use scattering amplitudes to study conservative black-hole interactions. We present the first study of non-conservative gravitational-wave absorption using on-shell scattering amplitudes. In the process, we develop a novel covariant approach to spin-weighted spherical harmonics, which has applications both in and beyond GR. 

We study gravitational absorption effects using effective on-shell scattering amplitudes. We develop an in-in probability-based framework involving plane- and partial-wave coherent states for the incoming wave to describe the interaction of the wave with a black hole or another compact object. We connect this framework to a simplified single-quantum analysis. The basic ingredients are mass-changing three-point amplitudes that model the leading absorption effects. As an application, we consider a non-spinning black hole that may start spinning as a consequence of the dynamics. The corresponding amplitudes are found to correspond to covariant spin-weighted spherical harmonics, the properties of which we formulate and make use of. We perform a matching calculation to general-relativity results at the cross-section level and derive the effective absorptive three-point couplings. 

Gravitational partial-wave absorption from scattering amplitudes