### Conveners

#### Parallel session 12 (Modified Gravity)

- Session convener: José Béltran-Jimenez

Two universal predictions of general relativity (GR) are the propagation of gravitational waves (GW) along null geodesics and the isospectrality of quasinormal modes (QNM) in Schwarzschild and Kerr black holes. However, in extension of GR one generally finds that QNMs are no longer isospectral and that the GW propagation is no longer geodesic and that it exhibits birefringence ---...

In this work we characterize all the static and spherically symmetric vacuum solutions in $f(R)$ gravity when the principal null directions of the Weyl tensor are non-expanding. In contrast to General Relativity, we show that the Nariai spacetime is not the only solution of this type when general $f(R)$ theories are considered. In particular, we find four different solutions for the...

In this talk I will present a formalism to study linear perturbations of bimetric gravity on any spherically symmetric background, including dynamical spacetimes. The setup is based on the Gerlach-Sengupta formalism for general relativity. Each of the two background metrics is written as a warped product between a two-dimensional Lorentzian metric and the round metric of the two-sphere. Using...

In the realm of General Relativity (GR) and extended theories of gravity, obtaining solutions for scenarios of physical interest is a highly intricate challenge. By employing the formalism of mathematical perturbation theory within the GR framework, we have developed a systematic method to compare solutions in modified gravity theories with the field equations of GR. Within this context, we...

I will discuss black holes in the context of Einstein–aether and khronometric gravity — two closely related alternative theories of gravity that allow violations of local Lorentz invariance. Since these theories admit faster-than-light propagation, metric horizons are generically permeable and it is not clear whether proper black holes can exist; surprisingly, in some cases they do, thanks to...

This project has as its general objective the extension of the classical formulation of gravity, considering it not only as a geometric theory, but as a consequence of the laws of thermodynamics applied to accelerated observers. The general case involving curvature, torsion and non-metricity will be addressed, in the context of gauge theories of gravity, with emphasis on the complete...