Sebastian Völkel, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam, Germany — July 8, 2024

Gravitational wave measurements from merging compact objects provide unprecedented opportunities to advance our understanding of fundamental physics, astrophysics, nuclear physics, and cosmology. This talk reviews the emerging relevance of gravitational waves across disciplines. Then, it focuses on black hole spectroscopy, a key element of modern black hole physics, and the post-merger period known as ringdown. Theoretical studies predict that linear perturbation theory becomes an accurate description at late enough times, allowing for spectroscopic studies of the remnant object. One would expect that the characteristic frequencies and damping times, the so-called quasi-normal modes, are uniquely related to the final mass and spin of the black hole. However, quasi-normal modes can differ if the final object is not a Kerr black hole. Describing deviations from general relativity is challenging and calls for state-of-the-art modeling. Recent studies have highlighted that even spectroscopy within general relativity can be more complex than anticipated. Almost a decade after the first direct gravitational wave measurement, gravitational physics enters a crucial stage, with systematic effects potentially rivaling theoretical violations of Einstein's theory. Thus, stringent tests of general relativity require comprehension of all these aspects, as my research contributions aim to address.