Probing the horizon of black holes with gravitational waves

Understand

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.