In a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods,...Show moreIn a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods, we search for modifications to General Relativity in the strong-field limit, for scalar as well as gravitational field perturbations. Going beyond General Relativity, Black Holes are predicted to produce echoing signals, for which we characterise observational parameters, by searching for leading order Boundary Conditions near the Black Hole horizon. For a scalar field, we discuss a parity symmetric and a shift symmetric configuration, while for a gravitational field, parity and diffeomorphism symmetry are implemented. The diffeomorphism symmetric Boundary Condition oddly seems to mix modes. Given the intimate relationship between Effective Field Theory and renormalisation techniques, we also comment on recent first principle arguments that have been brought up regarding the supposed impossibility of echo observation. We find that these arguments over-interpret a regularisation cut-off. From our perspective, the arguments still have merit, but do not form the no-go theorem that it seems to. In the end, only observation can give the decisive answer on the existence of Black Hole echoes.Show less
