Parametrising the black hole quasinormal ringdown

It is well known that black hole solutions in general relativity are surprisingly simple as shown by the no-hair theorems. Even more surprising is that for many modified gravity theories, no-hair theorems also exist. However, gravitational waves emitted during a black hole binary merger allow for discrimination between various gravity theories as the waves evolve according to different equations of motion. Of particular interest is the ringdown signal post-merger, described by a discrete spectrum of quasinormal modes. These spectra are fingerprints of the underlying gravity theory and so provide a simple yet powerful test of GR. As with all tests, the predictions made by other gravity theories need to be computed, but this is a highly laborious undertaking given the vast number of modified gravity theories that exist. We present recent work parametrising the expected deviations from the GR frequencies in a theory-independent manner.