Probing the horizon of black holes with gravitational waves

Winner of the Competition “Prize for PhD Thesis 2023” arranged by Sapienza University Press. Black holes are the most compact objects in the Universe. According to general relativity, black holes have a horizon that hides a singularity where Einstein’s theory breaks down. Recently, gravitational waves have opened the possibility of probing the existence of horizons and investigating the nature of compact objects. This is particularly interesting given some quantum-gravity models which predict the presence of horizonless and singularity-free compact objects. This thesis derives the gravitational-wave signal emitted by horizonless compact objects for model-independent tests of the black hole paradigm.