Black holes beyond General Relativity: stability and nonlinear evolution*

The dawn of gravitational-wave astronomy provides a unique opportunity to test General Relativity (GR) in the strong-field regime. Going beyond null tests, I will present recent progress to obtain strong-field predictions in Quadratic Gravity, i.e., for the leading-order EFT-corrections to GR.

Building on a proof by David R Noakes, I will discuss well-posedness of the initial-value-problem for Quadratic Gravity, both in spherical symmetry and (3+1) dimensions. In spherical symmetry, analytic linear stability complements stable nonlinear numerical evolution to determine the impact of EFT-corrections during spherical gravitational collapse. In (3+1) dimensions, a well-posed formulation crucially relies on strong hyperbolicity.