Vitor Cardoso
Title: The sound of black holes
Abstract: One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes. Is the strong field dynamical regime of gravity well described by General Relativity? I will summarize the status of black hole spectroscopy and attempts at probing near horizon physics.
Laura Sberna
Title: The black hole ringdown and quasinormal modes
Abstract: When two black holes merge, the resulting remnant emits gravitational waves as it settles into a stationary configuration — a phase known as the black hole ringdown. This stage is governed by the black hole’s quasinormal modes (QNMs), which encode key information about its mass, spin, and potentially deviations from general relativity. In this talk, I will explore the properties of QNMs, including their orthogonality, their utility in constructing mode-based perturbative expansions, and the physical mechanisms by which they are excited. In particular, I will present recent work on the excitation of QNMs by a plunging point particle, highlighting deviations from the standard description of the ringdown.
Andrea Sanna
Title: The Fellowship of the Curvatures: f(R) meets Gauss-Bonnet
Abstract: General Relativity is expected to break down in the high-curvature regime. Beyond effective field theories with higher-order operators, it is crucial to identify consistent nonperturbative theories including higher-curvature terms. Two well-studied cases are f(R) gravity and Einstein–dilaton–Gauss–Bonnet (EdGB) gravity. The former shares GR's vacuum solutions, while the latter faces well-posedness issues in the strong-coupling regime. We show that combining them yields genuinely new phenomena beyond simple superposition. This framework naturally extends EdGB gravity to include arbitrary higher-curvature terms. Focusing on quadratic and quartic corrections, we find: (i) black holes are modified by f(R) terms, unlike in pure EdGB; (ii) the solutions preserve key nonperturbative EdGB features, such as minimum mass and multiple branches; (iii) a mechanism suppresses Ricci-scalar divergence in the interior; yet (iv) the singularity and elliptic regions remain similar to EdGB. Thus, adding higher-order terms does not resolve the theory's ill-posedness at the nonperturbative level (based on: arXiv:2510.17965).
Maxime Gadioux
Title: Non-smooth horizons in Kerr black hole mergers
Abstract: Dynamical black holes are known to develop non-smooth structures on their horizon. We begin by reviewing a classification of all generic non-smooth structures that may appear on black hole horizons in four-dimensional spacetimes. Introducing a time function, we describe how two of these features – namely creases and caustics – evolve, and in particular discuss processes known as ‘perestroikas’, where the non-smooth structure on a horizon cross-section changes qualitatively. We then study the merger of two Kerr black holes in the extreme mass ratio limit, and focus on the creases and caustics that are present on the horizon. We explain how our results differ from an older analysis of the same system by Emparan et al., and show that these novel results are consistent with the properties of creases expected generically. This talk is based on work done with Harvey Reall and Robie Hennigar.
Miguel Zilhão
Title: Exploring the Dynamics of Hairy Black Holes with Numerical Relativity
Abstract: We report on the numerical evolutions of black holes with resonant hair and synchronized hair. In the "very hairy" regime, where a small horizon lies inside a bosonic star containing most of the energy, they deviate sharply from Kerr, but their dynamics remain unexplored. We present 3D evolutions of such systems, demonstrating that these solutions are dynamically unstable and decay to bald black holes via two distinct mechanisms. This fate is likely to be generic for sufficiently hairy BHs in the broader class of models with synchronized or resonant hair, but possible exceptions may exist.
