Environmental effects in GW physics: tidal deformability of black holes immersed in matter

TL;DR

This paper investigates how matter surrounding black holes affects their tidal deformability, which is crucial for gravitational wave signals and tests of gravity theories, especially in the context of upcoming LISA observations.

Contribution

It introduces analytical models for black holes immersed in matter and analyzes their impact on tidal Love numbers, highlighting challenges for strong-field gravity tests.

Findings

Matter surroundings can significantly alter tidal Love numbers.

Black holes in realistic environments may mimic nonzero Love numbers.

Challenges exist in using tidal deformability to test gravity theories.

Abstract

The tidal deformability of compact objects by a companion has a detectable imprint in the gravitational waves emitted by a binary system. This effect is governed by the so-called tidal Love numbers. For a particular theory of gravity, these depend solely on the object internal structure and they vanish for black holes in general relativity. A measurement compatible with nonzero tidal Love numbers could thus provide evidence of new physics in the strong-field regime. However, in realistic astrophysical scenarios, compact objects are surrounded by a nonvacuum environment. In this work, we study the tidal deformability of configurations of black holes immersed in matter, focusing on two analytical models representing an anisotropic fluid and a thin-shell of matter around a black hole. We then apply our results to the astrophysically relevant case of a black hole surrounded by an accretion disk, in the parameter region of interest of the upcoming LISA mission. Our results indicate that there are challenges to overcome concerning tests of strong-field gravity using tidal Love numbers.