# Astro Lunch

## Black holes and naked singularities quasi-normal modes: theory and observation

After the discovery of gravitational waves in 2015 from the LIGO-Virgo collaboration, the existence of black holes were finally verified through the merger of a binary system. The merging consists of three different phases: it starts with the inspiral phase, in which the two black holes are orbiting far enough from each other to be distinguished as two objects; then we have the merger phase, where the event horizons of the black holes collide. This is the phase in which most of the gravitational waves are emitted and the analysis of Einstein’s full non-linear equations is necessary; lastly, only one black hole remains and it oscillates for a finite time, emitting weak gravitational waves. This phase is known as the ringdown phase and can be analyzed throughout the study of its quasi-normal modes.

This seminar will focus on some theoretical and observational aspects of the last stage of the merging process. It will be divided in three main parts: first I will explain what is a black hole and discuss its main properties; here I will also discuss naked singularities and define what is a quasi-normal mode. Secondly, I shall present some theoretical

results obtained for both black holes and naked singularities, with respect to their quasi-normal modes. In the last part, I will talk about my on-going work, within the LIGO-Virgo collaboration, related to the detection of the ringdown phase.