Despite their potential importance for understanding astrophysical jets, physically realistic exact solutions for magnetospheres around Kerr black holes have not been found, even in the force-free approximation. Instead approximate analytical solutions such as the Blandford-Znajek (split-)monopole, as well as numerical solutions, have been constructed. In this talk we consider a new approach...

Gravitational-wave observations of coalescing binary black holes allow for novel tests of the strong-field regime of gravity. Using the detections of the LIGO and Virgo collaborations, we place the first constraints on higher-order curvature corrections that arise in the effective-field-theory extension of general relativity where higher-order powers in the Riemann tensor are included in the...

We will discuss an analytic hairy black hole in a subclass of scalar tensor theories

In generic higher-order scalar-tensor theories which avoid the Ostrogradsky instability, the presence of a scalar field significantly modifies the propagation of matter perturbations, even in weakly curved backgrounds. This affects notably the speed of sound in the atmosphere of the Earth. It can also generate instabilities in homogeneous media. I will use this to constrain the viable...

Generic extensions of General Relativity aiming to explain dark energy typically introduce fifth forces of gravitational origin. In this talk, I will explain how helioseismic observations can provide a powerful and novel tool towards precision constraints of fifth forces, as predicted by general theories for dark energy, and I will discuss the implications for cosmology.

Horava Gravity is a renormalizable theory of Quantum Gravity which is expected to flow to GR in the low energy limit. This naive expectation is obstructed by a strongly coupled interaction when the parameters of the Lagrangian flow to the general relativistic values. However, when closely studied, only self-interactions of the extra scalar mode of the theory are strongly coupled. When matter...

We develop a formalism to treat higher order (nonlinear) metric perturbations of the Kerr spacetime in a Teukolsky framework. We first show that solutions to the linearized Einstein equation with nonvanishing stress tensor can be decomposed into a pure gauge part plus a zero mode (infinitesimal perturbation of the mass and spin) plus a perturbation arising from a certain scalar ("Debye-Hertz")...

In this talk we study the quasi-normal modes of spherically symmetric black holes in modified theories of gravity, allowing for couplings between the tensorial and scalar field degrees of freedom. Using the eikonal approximation and a largely theory-agnostic approach, we obtain analytical results for the fundamental mode of such black holes.

Validating the no-hair theorem with a gravitational wave observation from a compact binary coalescence presents a compelling argument that the remnant object is indeed a black hole described by the classical general theory of relativity. Validating this theorem relies on performing a spectroscopic analysis of the post-merger signal and recovering the frequencies of either different angular...

Spontaneous scalarization is a mechanism that endows relativistic stars and black holes with a nontrivial configuration only when their spacetime curvature exceeds some threshold. The standard way to trigger spontaneous scalarization is via a tachyonic instability at the linear level, which is eventually quenched due to the effect of non-linear terms. At this work (Phys. Rev. D 99, 124022...

We present a numerical investigation of the superradiant instability in spinning black holes surrounded by a plasma with density increasing when moving closer to the black hole. We try to understand whether superradiant instabilities are relevant or not for astrophysical black-holes surrounded by matter.

A modified causal structure of black holes in theories beyond general relativity might have implications for the stability of such solutions. In this talk, we explore the horizon structure of black holes as perceived by scalar fields for generalized scalar-tensor theories, which exhibit derivative self-interactions. This means that the propagation of perturbations on nontrivial field...