Testing gravity across scales: from black holes to cosmology
Abstract: I will present phenomenological signatures of scalar-tensor gravity in two distinct regimes: the strong-field environment near black holes and the nonlinear scales of large-scale structure. Scalar-tensor theories constitute a compelling framework for extending General Relativity and addressing cosmological tensions. However, tightly constraining these models requires robust theoretical predictions. First, I will show how an additional scalar degree of freedom alters gravitational wave emission during the black hole ringdown. By calculating the corresponding quasinormal modes, we can extract unique signatures that directly probe the underlying theory. Second, we will shift to astrophysical scales, where viable scalar-tensor theories must rely on screening mechanisms to evade stringent Solar System bounds. I will introduce a unified formalism to simultaneously model different screening mechanisms (typically studied in isolation). I will discuss a new candidate screening mechanism, as well as ongoing work on the nonlinear interplay between Chameleon and Vainshtein screening.