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.*