Monday 12
Tanja Hinderer
Title: Tidal effects in black hole binaries in General Relativity and beyond
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.
Pablo Cano
Title: Amplification of new physics in the quasinormal mode spectrum of highly-rotating black holes
Abstract: The computation of quasinormal modes of rotating black holes in modified theories of gravity has been recently made possible thanks to the development of new techniques, like a modified Teukolsky equation and spectral methods. However, no method so far has been able to peek into the highly rotating regime — close to extremality. In this talk, I will consider a newly identified higher-curvature modification of GR that preserves the isospectrality of quasinormal modes in the eikonal limit. In this theory, eikonal perturbations can be described in terms of an effective scalar equation, and solving it we will obtain the corrections to the eikonal Kerr quasinormal modes for arbitrary rotation. For moderate rotation, we check that the eikonal computation gives a good approximation to the exact QNMs obtained from the modified Teukolsky equation, even for low harmonics. For high rotation, we discover that the corrections to GR become much larger and can lead to dramatic effects. Our results suggest that the observation of the ringdown of a highly rotating black hole would be a “golden event” to search for new physics.
Julio Arrechea
Title: On how a physical vacuum can change the black hole paradigm (and how does it sound?)
Abstract: The detection of gravitational-wave echoes is considered one of the smoking guns behind the presence of new physics in dark and compact objects. In this talk I will discuss how the energetic contribution from the quantum vacuum might provide the ingredients necessary to i) surpass the stellar compactness limits from general relativity, and ii) make trapped regions evaporate in timescales shorter than the Hawking time. Motivated by these findings, I will examine the echoes produced by test fields on spacetimes describing black hole mimickers sustained by semiclassical effects and on spacetimes modelling the formation and sudden disappearance of trapped regions.
Tuesday 13
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.
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.
Roberto Emparan
Title: Observing Quantum Gravity in Extremely Cold Horizons
Abstract: Recent developments have revealed that black holes near extremality exhibit large quantum fluctuations in their geometry, marking a controllable breakdown of semiclassical quantum field theory in curved spacetime. In this talk, I will discuss how these fluctuations can be revealed through scattering waves off the black hole. In particular, we find that extremely cold black holes become transparent to low-frequency light or gravitational radiation. Although such signatures are extraordinarily hard to detect, they provide concrete signatures of quantum gravity at play in near-extremal regimes.
Wednesday 14
Adam Pound
Title: Black hole binary evolution and perturbed horizons in self-force theory
Abstract: Models of asymmetric-mass black hole binaries are now well developed in gravitational self-force theory. Recent work has extended the inspiral stage of these models beyond leading order in the binary's mass ratio, and ongoing efforts have begun to extend the merger-ringdown stage in the same way. In this talk I summarize that work as well as how it can be used to study the evolution of the primary black hole's horizon. Drawing on arXiv:2109.09514, I explain how teleological effects, singularities on the horizon, and differences between the event horizon and apparent horizon are all heavily suppressed during the inspiral but should become more prominent in the merger-ringdown stage.
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).
Gabriel Andres Piovano
Title: Gravitational waves from asymmetric-mass binaries with spin and orbital precession
Abstract: Extreme and intermediate-mass-ratio inspirals (I/EMRIs) are low-frequency sources of gravitational waves that will be detectable by future space-based observatories such as LISA. These systems, formed by the capture of a small, compact object by a supermassive black hole, emit rich, complicated gravitational waveforms due to their intricated orbital dynamics.
In this talk, I will review recent work in modelling the effects of the smaller companion's spin on I/EMRI waveforms. First, I will present new results on the calculation of the small spin corrections to periodic, homoclinic, and plunging orbits. Next, I will discuss recent progress in modeling the inspiral of a test particle with precessing spin and orbital angular momentum. Finally, I will introduce a hybrid self-force and post-Newtonian waveform model designed to describe the inspiral of both asymmetric and comparable-mass binaries detectable by future terrestrial and space-based detectors.
Thursday 15
Badri Krishnan
Title: Understanding black hole boundaries quasi-locally
Abstract: It is desirable to study black hole horizons in a quasi-local framework, avoiding entirely the teleological aspects of event horizons. In this talk I will summarize the motivations and basic concepts, thereby setting the stage for the following talks in this session. This will lead us naturally to various notions such as isolated horizons, dynamical horizons and all that. These notions have been widely employed in numerical relativity, mathematical relativity and gravitational wave astronomy. I will summarize some recent results in these areas, focusing mostly on the binary black hole problem.
Béatrice Bonga
Title: Black Hole Tomography: Unveiling Horizon Dynamics Through Quasinormal Modes
Abstract: During the ringdown phase of binary black hole mergers, both the outgoing gravitational waves at infinity and the infalling radiation at the horizon exhibit quasinormal mode (QNM) structure. I will first discuss a reformulation of the QNM problem using a characteristic initial value formulation, prescribing data directly on a perturbed isolated horizon and a transverse null hypersurface. This fully four-dimensional approach reveals that QNM frequencies emerge naturally by demanding analyticity and stability toward the future, without explicitly imposing purely outgoing boundary conditions at the horizon and infinity. It also establishes "black hole tomography" in the perturbative regime: the ability to reconstruct horizon geometry from gravitational wave observations. Complementing this analytical work, I wll present numerical evidence from head-on black hole collision simulations showing that the horizon is indeed well described by QNMs.
Ivan Booth
Title: Black hole geometry and evolution: the stability operator and bifurcation theory
Abstract: Apparent horizons are the best-known examples of marginally outer trapped surfaces (MOTS). However, it is now clear that most MOTS are not apparent horizons. Large, likely infinite, families of MOTS are found in the interior of black hole mergers and these engage in a complex set of interactions and evolutions, including the ultimate dissolution of the original apparent horizons inside the final black hole. The key theoretical tool that brings order to these evolutions is the MOTS stability operator, the modern version of which was introduced by Andersson, Mars and Simon in 2005. The stability operator is best understood as the linearization of the outward null expansion for surfaces “near” an existing MOTS. As such it can tell us whether or not a given MOTS can be understood as a (local) apparent horizon, forming a boundary between trapped and untrapped regions. However it also governs how a MOTS will evolve in a changing spacetime: if the stability operator is invertible then that evolution is unique. In this talk, I will review that background and then focus on MOTS with non-invertible stability operators and so non-unique evolutions. The MOTS pair-creations and annihilations observed during black hole mergers are the best-known examples of non-unique evolutions but these are not the only possibilities. Understanding the MOTS as fixed points of the outward null expansion equations, a generalization of standard, dynamical system, bifurcation theory can be used to classify all possible non-unique evolutions. MOTS pair-creation/annihilations are then understood as examples of saddle-node bifurcations. There are other possibilities, including pitchfork and transcritical bifurcations. I apply analytical and numerical tools to identify examples of the various bifurcations in a variety of spacetimes. This theory depends only on the geometry of a MOTS and its surrounding spacetime. Hence the classification results apply not only to possible bifurcations observed in numerical time evolutions but also those that occur for any other deformation of the spacetime. In particular, the constraints on possible bifurcations are not restricted to general relativity but apply to any geometric theory of gravity.
Friday 16
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.
Pau Figueras
Title: Regularised Higher Derivative Effective Field Theories
Abstract: In this talk I will introduce the “regularisation” scheme to formulate the initial value problem for general classical effective field theories (EFTs) with higher than second order equations of motion. Examples of such theories are relativistic viscous hydrodynamics or higher derivative theories of gravity. The regularised theories are related by a field redefinition to the original truncated EFTs and they propagate massive ghost fields (whose masses can be chosen to be of the order of the UV mass scale), in addition to the light fields. I will argue that, for suitable chosen initial conditions, the ghost fields remain bounded, thus allowing one to construct low energy solutions that are consistent with the EFT expansion. I will illustrate this procedure in detail for a simple scalar field model, and I will explain how to implement it for various higher derivative theories of gravity that have attracted the attention of the community in recent years.
