Effective Field Theory of Black Hole Quasinormal Modes in Scalar-Tensor Theories
The direct detection of gravitational waves has opened up a new window into the strong gravity regime, establishing the basis for black hole spectroscopy.
During the talk, I will focus on the ringdown phase, which is the final stage of a coalescence process, and show how it can be used as a valuable laboratory to test General Relativity and potentially constrain additional degrees of freedom in the gravitational sector. I will introduce an effective description for perturbations around spherically symmetric spacetimes in the context of scalar-tensor theories, which I will apply to study quasi-normal modes for black holes with scalar hair.
In particular, assuming the deviation of the background from Schwarzschild is small, I will use the WKB method to introduce the notion of "light ring expansion". This approximation is analogous to the slow-roll expansion used for inflation, and it allows to express the quasi-normal mode spectrum in terms of a small number of parameters. This can be thought of as a first step in describing, in a model independent way, how the scalar hair can affect the ringdown stage and leave signatures on the emitted gravitational wave signal.