Dottorato in FISICA

Titolo della tesi: Quasinormal Modes of Black Holes in Einstein-dilaton Gauss-Bonnet Gravity

The detection of the first gravitational wave marked the beginning of gravitational wave astronomy, which opened up new avenues to test Einstein's General Relativity in the previously unreachable strong-field regime of gravity. Even though the gravitational waves detected so far have not shown any sign of departure from General Relativity, it is widely acknowledged that the theory is not an ultimate and complete description of gravity, highlighting the need to continue testing and developing viable extensions. One promising strategy to pursue this goal is Gravitational Spectroscopy, which involves measuring black hole quasinormal modes from the ringdown gravitational wave signal emitted in the aftermath of a compact binary coalescence. The quasinormal modes, which are the characteristic oscillation frequencies of the perturbed remnant black hole, are inherently linked to the underlying theory of gravity and as such are perfect tools to perform tests of General Relativity. As the ringdown signal is expected to be observed with more and more accuracy thanks to design-sensitivity interferometers and next-generation detectors, the possibility of performing theory-specific ringdown tests relies on theoretical predictions of quasinormal modes in alternative theories of gravity. In this Thesis, we present the first computation of the gravitational quasinormal modes of rotating black holes up to second order in spin in Einstein-dilaton Gauss-Bonnet gravity, arguably one of the simplest theories that modify the large-curvature regime of gravity and that can be tested with black hole observations. To enhance the domain of validity of the spin expansion, we perform a Padé resummation of the quasinormal modes and, from a comparison with the general relativistic case, we find that this approach should be accurate up to the astrophysically relevant spins of about 0.7. Our findings indicate that neglecting the second order in the spin could result in a serious underestimation of the effect of gravity modifications. Finally, we outline two possible strategies to perform tests of General Relativity by using a parametrized ringdown template called ParSpec. From a null test with the future detectors Cosmic Explorer and Einstein Telescope, we find that with approximately 100 observations of realistic sources, we will be able to put bounds on three beyond-GR parameters with both detectors; performing a theory-specific test of Einstein-dilaton Gauss-Bonnet gravity with the LIGO-Virgo data of GW150914 we were not able to put meaningful constraints on the theory, which are expected to improve with a higher signal-to-noise ratio and by stacking multiple events.