Frans Pretorius

Testing the predictions of general relativity in the dynamical strong-field regime, in particular black holes and their dynamics, has only recently become possible with LIGO/Virgo observations of black hole mergers. I will review some of the tests that have been performed to date. However, one obstacle to extracting the most stringent constraints possible is the lack of predictions of what the coalescence phase of black hole mergers should “sound” like in modified gravity theories (or theories that claim exotic alternatives to black holes). To make things worse, there are doubts as to how well-posed modified gravity theories are that could offer merger dynamics sufficiently different from generalrelativity to be within reach of current generation detectors.One such theory is

Einstein-dilaton-Gauss-Bonnet (EdGB) gravity. I will describe early results trying to address the well-posedness of EdGB gravity in the non-linear regime. Restricting to gravitational collapse in spherical symmetry, we find that there is an uncomfortably small region of parameter space where EdGB gravity might yet serve as a foil to improve our knowledge of how well strong-field gravity is described by general relativity in nature.