Probing the horizon of black holes with gravitational waves

TL;DR

This paper explores how gravitational waves can be used to test the nature of black holes and distinguish them from horizonless compact objects predicted by some modified gravity theories.

Contribution

It provides a comprehensive overview of the phenomenology of dark compact objects and discusses how future gravitational-wave detectors can test the black hole paradigm.

Findings

Dark compact objects produce distinct gravitational-wave signatures.

Future detectors can perform model-independent tests of black hole horizons.

Gravitational-wave echoes could indicate horizonless objects.

Abstract

Gravitational waves open the possibility to investigate the nature of compact objects and probe the horizons of black holes. Some models of modified gravity predict the presence of horizonless and singularity-free compact objects. Such dark compact objects would emit a gravitational-wave signal which differs from the standard black hole scenario. In this chapter, we overview the phenomenology of dark compact objects by analysing their characteristic frequencies in the ringdown and the emission of gravitational-wave echoes in the postmerger signal. We show that future gravitational-wave detectors will allow us to perform model-independent tests of the black hole paradigm.