## Details

Abstract:

Guided by the principles of effective field theory, I will discuss three consecutive avenues to constrain physics beyond General Relativity with black-hole observations.

1) Shadows: Without specifying any particular gravitational dynamics, I will discuss image features of black-hole shadows in general parameterizations and their relation to fundamental-physics principles like (i) regularity (no remaining curvature singularity), (ii) simplicity (a single new-physics scale), and (iii) locality (a new-physics scale set by local curvature).

2) Stability: Specifying the linearized dynamics around black-hole spacetimes determines the onset of potential instabilities and connects to the ringdown phase of gravitational waves. I will present scenarios in which said instabilities constrain theories of low-scale dark energy and ultralight dark matter.

3) Nonlinear evolution: The larger the probed curvature scale, the better the constraints on new physics. Making full use of experimental data, thus requires predictions in the nonlinear regime of binary mergers. I will present recent progress towards achieving stable numerical evolution for the effective field theory of gravity up to quadratic order in curvature.