Cosmology Seminar

The next cosmology seminar takes place on Thursday 19th of May, seminar room E349 at 11am. Our guest speaker will be Hugo Roussille from APC (Paris, France).

At 11:00, room E349, Hugo Rousille (APC), will be talking about

Linear perturbations of black holes in DHOST gravity

General relativity can be tested at many scales using various physical systems. A particularly interesting probe is the study of the ringdown phase of a binary black hole merger, during which the gravitational waves emitted by the newly-formed black hole are well described by perturbation theory over a single black hole background. The features of these waves depend heavily on the theory of gravity underlying the solution and the solution itself. Therefore, they can be used both to test GR and rule out some black hole solutions. We focus in this talk on the study of new behaviours for these waves in scalar-tensor theories in the Degenerate Higher-Order Scalar-Tensor (DHOST) class. In such theories, the usual dichotomy between axial (odd parity) and polar (even parity) perturbations no longer enables one to decouple all degrees of freedom since the even parity sector contains one additional scalar degree of freedom. Axial perturbations can still be completely decoupled: we show that they propagate in an effective metric that differs from the background metric, and is not always a black hole metric. In the case of polar perturbations, we present a systematic approach that extracts the asymptotic behaviour of perturbations at spatial infinity and near the horizon directly from the perturbed Einstein’s equations. This asymptotic behavior gives physical insight about the modes, such as their direction and speed of propagation, and allows us to rule out some solutions that explicit bad behavior. Furthermore, we can use the obtained asymptotic behavior to numerically solve the propagation equations and compute quasinormal modes in systems where this was not possible before.