EREP 2026, Spanish & Portuguese Relativity Meeting

25 May 2026, 00:00 → 29 May 2026, 23:59 Europe/Madrid

Universidad de Murcia, Campus de la Merced (Murcia, Spain)

The Spanish-Portuguese Relativity Meetings (EREPs) are a long-standing scientific tradition that began in 1977. Since then, they have served as a key forum for the Portuguese and Spanish communities working in General Relativity and Gravitation, promoting collaboration and the exchange of ideas across a broad range of topics. Organized annually by different research groups from both countries, EREPs have become the most prominent conference on gravitation and relativity in the Iberian Peninsula.

 

The EREP 2026 edition will take place in Murcia, Spain, from May 25th to 29th, 2026. We warmly invite you to join us for a week of stimulating scientific discussions in a vibrant and sunny Mediterranean setting. As in previous editions, EREP 2026 will bring together researchers from across the Iberian Peninsula and beyond, offering a dynamic program in a friendly and collaborative atmosphere.

 

The conference will be held on-site at the Paraninfo de la Universidad de Murcia, which is located at the city center and 40 km far from the Mediterranean Sea.

 

Please have a look at the Transportation section on how to travel to Murcia.

Registration is open until May 2 

Call for abstracts is open until April 19 (extended until April 24)

Registration fee:

• SEGRE members: 250€
• Non-members: 290€

 

Instructions for carrying out the payment will be provided after submitting the registration form.  

 

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Invited Speakers:

• Tomislav Andric (Gran Sasso Science Institute)

• Llibert Aresté Saló (KU Leuven)

• Marina David (KU Leuven)

• Maxime Gadioux (Cambridge U.)

• Robie Hennigar (Durham U.)

• Hector Olivares (Aveiro U.)

• Mairi Sakellariadou (King's College London)
• Chiara Toldo (ULB - Brussels)
• Miguel Zumalacárregui (Max Planck Institute- Potsdam)

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Public Talk

• Tomás Ortín (IFT Madrid)

 

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Scientific Committee:

• Barceló, Carlos (IAA-CSIC)
• Bastero, Mar (U. Granada)
• Beltrán, Jose (U. Salamanca, IUFFyM)
• Bueno, Pablo (U. Barcelona)
• Cardoso, Vitor (Niels Bohr Institute and Lisbon, IST Lisboa)
• Dias, Oscar (U. Southampton)
• Emparan, Roberto (U. Barcelona & ICREA)
• Figueras, Pau (QMUL)
• Herdeiro, Carlos (U. Aveiro)
• Martín Benito, Mercedes (IPARCOS, UCM)
• Martín Moruno, Prado (IPARCOS, UCM)
• Olmo, Gonzalo (U. Valencia & IFIC)
• Ortín, Tomás (IFT-UAM/CSIC)
• Rocha, Jorge (ISCTE, Lisboa)

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Organizing Committee:

• Belmonte Aix, Raúl
• Bonetti, Federico
• Cano Molina-Niñirola, Pablo A.
• Fernández Melgarejo, José J. 
• Giorgi, Giacomo
• Glampedakis, Kostas
• Molina Vilaplana, Javier
• Romano, Luca 
• Simón Félix, Elena
• Torrente-Lujan, Emilio

 

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SEGRE grants. SEGRE will offer a limited number of grants to support the participation of Ph.D. students and postdocs. Applications for financial support should be made by email (erep2026@um.es) and include a brief CV and a motivation letter (maximum 1 page for each document, preferably pdf). The deadline for grant applications is April 1st, 2026. Preference for financial support will be given to applicants who submit a contribution to the conference.

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Map_Buses_and_Tram.pdf

map_campus.pdf

map_hotels.pdf

Monday 25 May

Registration

Welcome

Invited talk: Miguel Zumalacárregui

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

1 Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

Just like light, gravitational waves (GWs) are gravitationally lensed by massive objects in the Universe. Moreover, their low frequency, phase coherence, and lack of absorption make GWs complementary to lensed electromagnetic sources. In addition to gravitational magnification and the formation of multiple images, lensed GWs exhibit genuine wave-propagation effects such as diffraction, that is, the bending of the signal’s wavefront. Lensing diffraction imprints a frequency-dependent modulation on the signal, encoding information about the lens mass and its distribution. I will describe the rich phenomenology of lensing diffraction as a means to discover high-redshift, magnified binary mergers and to probe small-scale structures, including dark matter. In theories beyond GR, novel propagation effects such as birefringence and dispersion—polarization- and frequency-dependent phase shifts—provide new tests of cosmological gravity and dynamical dark energy. In addition to recent theoretical developments, I will present the latest analysis of GW231123, the first candidate for a diffracted and magnified compact binary coalescence.

Speaker: Dr Miguel Zumalacarregui (Max Planck Institute for Gravitational Physics - Albert Einstein Institut)

10:30 Coffee break

40' Contribution: Diego Rubiera-Garcia

Diego Rubiera-Garcia
Black hole imaging as a probe for non-Kerr physics

2 Black hole imaging as a probe for non-Kerr physics

The field of black hole imaging - the observational appearance of a black hole when illuminated by its accretion disk - has quickly become one of our main tools in order to test the strong-field regime of the gravitational interaction. In this talk I will discuss the conceptual foundations of this field and discuss the features of images of several kinds of modified black holes and horizonless ultra-compact objects.

Speaker: Prof. Diego Rubiera-Garcia (Complutense University of Madrid)

20' Contribution: Diego Sáez-Chillón Gómez

3 Compact objects physics in the realm of multi-messenger astronomy

The reconstruction of the images of the supermassive objects at the centres of the M87 and Milky Way galaxies show two objects characterized by a central depression in luminosity (the so-called shadow) and a ring-shaped light structure around them. Both properties are closely linked to the emission intensity profile of the accretion disk and to the structure of spacetime. In addition, the detection of gravitational waves has made possible to study compact objects during the merger of binary systems of -mainly- black holes in order to test the Kerr family of solutions. In this talk, I will explain, on the one hand, the spacetime effects that influence the image obtained and how it may be used in the future to discern the existence of objects beyond the Kerr paradigm. Finally, I will also present the correspondence that exists between the shadow images and the spectrum of quasinormal modes of gravitational waves when such objects are perturbed. This is expected to play a fundamental role for the future multi-messenger astronomy of ultracompact objects.

Speaker: Prof. Diego Sáez-Chillón Gómez

20' Contribution: JOAO Santos

4 Gravitational Lensing of Fast Radio Bursts as a Probe of Primordial Black Holes Abundance: a forecast for radio telescopes

The Fast Radio Bursts (FRBs) are some of the most intriguing radio phenomena measured in radio astronomy \cite{lorimer}. These energetic bursts have extremely high radio luminosities, corresponding to $\sim 10^{36} \,–\, 10^{44}$ $\mathrm{erg \,s}^{-1}$, which is not far from gamma ray bursts. Since their discovery in 2007, several investigations have been proposed to model their sources and also to use their data for astrophysical and cosmological constraints. Among such efforts, we highlight lensing effects in the propagation of the FRBs, which could be used to constrain Primordial Black Holes (PBHs) \cite{munoz, chime}. These black holes would be produced in the earliest stages of the Universe and could have masses below $1\mathrm{M_{\odot}}$ \cite{carr}. There are several surveys constraining the possible values of mass and fraction of these black holes, and among these proposals lies the lensing effects of FRBs. In this work, we briefly review some generalities about lensing effects for point sources, and we present a forecast for LOFAR, FAST, and BINGO telescopes. These radio telescopes may characterize several FRBs in the coming years. LOFAR and FAST are operating radio telescopes and are expected to be upgraded in the next few years. BINGO \cite{bingo} is a radio telescope under construction in Brazil that may be promising to detect FRBs. The forecast is based on the design features of each of these radio telescopes, and also on the current dataset of 131 confirmed FRB signals, reported by different surveys. Our forecast suggests that LOFAR would be suitable to characterize $f_{\mathrm{PBH}} \sim 16\%$ for lenses around $1\mathrm{M_{\odot}}$, while FAST and BINGO yields to $f_{\mathrm{PBH}} \sim 39\%$ for lenses with $10^{1}\,\mathrm{M_{\odot}}$ and $10^{-1}\,\mathrm{M_{\odot}}$, respectively.

Speaker: Prof. JOAO Santos (UFCG, Leibniz University and University of Heidelberg)

20' Contribution

5 Existence and uniqueness of static and spherically symmetric anisotropic fluid stars in cosmological backgrounds

Static and spherically symmetric isotropic compact stars are known to exist and be unique for a given monotonic equation of state E(P) and central pressure Pc, provided that Λ ≤ 0, or 0 < Λ < κE(r)/2 holds inside the star. In this work, we extend these results in two directions. First, we relax the standard condition for monotonicity and smoothness of E(P), and generalize the formalism for anisotropic configurations. Second, we show how for Λ > 0 uniqueness is lost, as the exterior spacetime may either be entirely vacuum, or contain a variety of thick shells of matter.

Speaker: Eneko Aranguren (University of the Basque Country (EHU))

20' Contribution: Vojtech Pravda

6 Charged black holes in quadratic gravity

We study static, spherically symmetric, asymptotically flat and asymptotically (A)dS charged black holes in quadratic gravity. Using the conformal-to-Kundt method, we simplify the field equations and derive the solutions in the form of a power series with coefficients determined by a recurrent formula. In addition to charge and mass, these black holes possess one additional parameter – the Bach parameter, corresponding to the value of an invariant of the Bach tensor on the horizon. In the asymptotically flat case, this Bach parameter has to be fine-tuned in order to achieve asymptotic flatness. In contrast, for the asymptotically (A)dS case, there exist parameter regions (specifically for a sufficiently large cosmological constant) where this fine-tuning is not necessary, and the Bach parameter becomes a new, free parameter of the black hole.

Based on: Phys. Rev. D 110 (2024) 4, 044069, and Phys. Rev. D 113 (2026) 2, 024040

Speaker: Vojtech Pravda (Institute of Mathematics of the Czech Academy of Sciences)

13:00 Lunch

Invited talk: Tomislav Andric

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

7 The Einstein Telescope: Scientific Reach and Technological Challenges of a Third-Generation Gravitational-Wave Observatory

The Einstein Telescope (ET) is a proposed third-generation ground-based gravitational-wave observatory designed to extend the sensitivity of current detectors by an order of magnitude and to access frequencies down to a few hertz. This improvement will dramatically increase the observable volume of the Universe and enable precision studies of compact-object populations, cosmology, and fundamental physics.

In this talk, I will review the scientific potential of ET, including its capability to observe binary black holes and neutron stars across cosmic history, perform high-precision tests of general relativity, and probe the properties of dense matter and the early Universe. I will discuss how ET compares to the current generation of detectors and highlight key expected gains in detection rates, parameter estimation, and multi-messenger astronomy.

I will also present an overview of the current status of the ET project and outline the main technological challenges that must be addressed to achieve its target sensitivity. These include low-frequency noise mitigation (seismic and Newtonian noise), thermal noise reduction through cryogenic operation and improved materials, and quantum noise suppression via advanced interferometric techniques. Particular emphasis will be placed on the interplay between detector design, environmental noise, and control systems.

Speaker: Tomislav Andric (Gran Sasso Science Institute)

EREP_2026_TAndric.pdf

Coffee break

Gong show

8 Gongshow: Julen Estonba-Loinaz, Oskar Borgvall González, Matúš Papajčík, Natasha Riahi, Thomas Lovo, Izarne Martínez Donato, Álvaro Martínez, Elena Simón Feliz, Pelayo Del Valle Calzada, Pau López Oliver, Daniel Peñalver, Lucía Castells, Aitor Vicente-Cano

Welcome Cocktail

Tuesday 26 May

Invited talk: Robie Hennigar

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

9 How to Build Regular Black Holes

Regular, singularity-free black holes have been studied for more than sixty years as a possible resolution of the black hole singularity problem. Progress in this direction, however, has been limited by the lack of a robust dynamical mechanism for their formation. I will argue that, if regular black holes are relevant to the ultimate resolution of spacetime singularities, they are most likely to arise from resummed higher-derivative corrections of the kind expected in quantum gravity. I will discuss a recent explicit realization of this idea, survey the main results obtained so far in this framework, and highlight the major open problems that remain.

Speaker: Robie Hennigar

10:30 Coffee break

40' Contribution: Adolfo Cisterna

Diego Rubiera-Garcia
Black hole imaging as a probe for non-Kerr physics

10 New Dynamical Black Hole Solutions: Extending the Spectrum of Exact Primordial Objects

Constructing exact solutions describing compact objects embedded in dynamical cosmological environments remains a major challenge in General Relativity, with most known results restricted to highly symmetric settings. In this talk, we present new exact solutions and solution-generating techniques that extend the analytical description of dynamical black holes in cosmological backgrounds.
First, we introduce a novel method for the self-interacting Einstein–scalar system that generates non-stationary, axisymmetric geometries. Using this approach, we obtain the first exact solution describing a dynamical axisymmetric black (or white) hole embedded in an expanding or contracting cosmology. Its dynamical trapping horizons are characterized using the mean curvature vector, which generalizes the Kodama vector beyond spherical symmetry and allows a foliation-independent identification of trapped and untrapped regions.
We then construct a new exact solution of the Einstein–scalar–Maxwell system describing a dynamical black hole interacting with a time-dependent external electromagnetic field. The configuration is obtained by dressing a Schwarzschild black hole with a radially and temporally varying scalar field within the Fonarev framework, producing a time-dependent generalization of the Fisher–Janis–Newman–Winicour solution. An external electromagnetic field is subsequently generated through a Lie point symmetry extending the Harrison transformation to dynamical settings. The resulting spacetime features a dynamical horizon, an axisymmetric electromagnetic field, and asymptotics combining Friedmann–Lemaître–Robertson–Walker and Levi–Civita geometries.
We analyze the geometric and physical properties of these solutions, including their horizon structure and asymptotic behavior. Notably, time dependence can cloak curvature singularities that would otherwise be naked in stationary limits. These results provide new analytical tools to explore dynamical compact objects in cosmology, with potential applications to primordial black holes.

Speaker: Adolfo Cisterna (Institute of Theoretical Physics, Charles University)

20' Contribution: Ángel Murcia

11 Regular black holes in four dimensions: construction and formation

The celebrated works by Penrose and Hawking established that black holes in general relativity are inevitably doomed to contain singularities in their interiors. This is no longer the case for regular black holes, for which no singularities appear. Recently, it has been shown that the Schwarzschild singularity may be resolved after the addition of an infinite tower of higher-curvature corrections to the Einstein-Hilbert action. Nevertheless, this explicit construction only works for space-time dimensions larger or equal than five. In this talk, I will explain how regular black holes from pure gravity may also be built in four dimensions, present along the way two mechanisms for their dynamical formation. This talk will be mainly based on Phys.Rev.D 113 (2026) 2, 024019.

Speaker: Ángel Murcia (Instituto de Física Teórica)

20' Contribution: Ernesto Contreras

12 When regular black holes grow hair: scalarization beyond singular spacetimes

Regular black holes provide a compelling framework to address spacetime singularities through nonlinear electrodynamics, yielding geometries that remain well-behaved at all scales. In this work, we investigate their response to scalar field perturbations and the onset of spontaneous scalarization. Using a general inverse approach, we reconstruct the electromagnetic sector associated with a given solution and introduce a non-minimal scalar coupling. We show that this interaction can trigger a tachyonic instability, leading to scalarized black hole solutions and a non-trivial branch structure, including regions where scalarized configurations are thermodynamically preferred. We also discuss observational signatures, finding small but potentially measurable deviations in shadow size and quasi-normal mode spectra.

Speaker: Ernesto Contreras (Universidad de Alicante)

20' Contribution: Tomáš Málek

13 Quasi-topological gravity beyond spherical symmetry: Taub–NUT, NHEK, and swirling universe

We present a classification of four-dimensional gravitational theories exhibiting integrability properties similar to quasi-topological gravity, focusing on metrics that share the symmetries of Schwarzschild and Taub–NUT solutions with spherical, hyperbolic, and planar horizons, as well as their double Wick–rotated counterparts, including B-metrics, the near-horizon extremal Kerr geometry, and the swirling universe. These symmetry classes exhaust all cases with four Killing vectors and three-dimensional group orbits that admit consistent symmetry reductions at the level of the Lagrangian, in the sense of the principle of symmetric criticality. Restricting to theories constructed solely from the Riemann tensor, we demonstrate that analyticity in the Riemann tensor is compatible only with theories leading to third-order field equations, which reduce to second order after a trivial integration.

Speaker: Tomáš Málek

20' Contribution: Cristóbal Laporte

14 Macroscopic Microscopic Effects from Quantum Gravity

Quantum gravity corrections at the classical level are typically expected to be negligible. In this talk, I will present a mechanism by which even parametrically small corrections to the Schwarzschild solution, Planck-suppressed at asymptotic scales, become dominant in the near-horizon region. By analyzing linear perturbations around the Schwarzschild background, we find that higher-derivative terms, which introduce additional propagating degrees of freedom, are exponentially amplified as one approaches the horizon. Once the couplings controlling these terms are switched on, however small, their contribution grows and qualitatively modifies the near-horizon geometry: the classical event horizon is removed and replaced either by a naked singularity or by a wormhole-like structure. We illustrate this mechanism in several extensions of general relativity, including quadratic gravity, perturbative counterterms, and Einsteinian cubic gravity, and discuss the implications for constraining the Wilsonian effective action of gravity with observational data.

Speaker: Cristóbal Laporte (University of Barcelona)

Lunch

Invited talk: Hector Olivares

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

15 Simulating and Observing Black Hole Mimickers

Extensions of general relativity and of the Standard Model of particle physics predict the existence of exotic compact objects (ECOs) that can achieve compactness high enough to mimic black holes. For the first time, advances in observational techniques such as very-long-baseline interferometry (VLBI) allow us to probe supermassive black hole candidates at high spatial resolution and test some of these predictions. In this context, understanding the astrophysics of ECOs becomes essential for predicting their observational appearance and identifying signatures that distinguish them from black holes. In this talk, we review the current state of the art in simulating the observational properties of these objects in the electromagnetic channel and discuss their viability in light of the most recent observations.

Speaker: Hector Olivares (Universidade de Aveiro)

16:30 Coffee break

20' Contribution: Filipe Moura

16 Quasinormal modes, greybody factors and shadow of charged accelerating black holes

We show that the quasinormal modes, in the eikonal limit, for accelerating (non-rotating) black holes, are related to the angular velocity of the circular null geodesics and to the corresponding Lyapunov exponent, in the same way as the ones for spherically symmetric black holes are. We compute those quasinormal modes and greybody factors for neutral and charged accelerating black holes, considering massless test scalar fields, and we show that the results are universal for perturbations of any spin. We also determine the radius of the shadow cast by these black holes.

Speaker: Filipe Moura (ISCTE - Instituto Universitário de Lisboa)

20' Contribution: Francisco Silva

17 Accelerating and Superaccelerating Black Holes, Quasinormal Mode Families, and the Stability of the Cauchy Horizon

In this talk, we investigate the horizon structure of accelerating black holes described by the C-metric, including its superaccelerating limit. We then analyze the stability of the Cauchy horizon in these configurations using Christodoulou’s formulation of Strong Cosmic Censorship.

To this end, we classify the families of quasinormal modes of the spacetime and study their properties for scalar, electromagnetic, and fermionic perturbations.

Our results show that Strong Cosmic Censorship can be violated in near-extremal regimes for scalar and electromagnetic perturbations, while it appears to be preserved for fermionic perturbations. Moreover, we identify horizon configurations with non-near-extremal charge that also violate Strong Cosmic Censorship.

Speaker: Francisco Silva (Faculdade de Ciências da Universidade de Lisboa)

20' Contribution: Igor Kanatchikov

18 Non-Keplerian Galaxy Rotation Curves as a Testing Ground for Precanonical Quantum Gravity

We show that a variety of non-Keplerian galaxy rotation curves, together with the corresponding modifications of the Newtonian potential and dynamics, including MOND, qMOND, and mMOND-type behaviors, can be derived for test particles propagating on a background quantum geometry associated with a fluctuating spin connection, as described by precanonical quantum gravity (pQG).

In particular, we identify a quantum gravitational origin of Milgromian dynamics and derive a relation between the Milgromian acceleration scale $a_0$ and the cosmological constant. The smallness of these quantities is traced back to the precanonical quantization scale $\varkappa$, which enters the formulation of precanonically quantized Einstein--Yang--Mills theory and appears in estimates of the mass gap in quantum Yang--Mills theory.

These results suggest that pQG provides a first-principles framework for describing the very weak-field regime of galactic dynamics, offering a theoretically motivated alternative to phenomenological dark matter models and modified gravity scenarios.

Based on: I. Kanatchikov et al., arXiv:1212.6963, 1512.09137, 1706.01766, 2308.08738, 2311.05525; IJGMMP 14 (2017) 1750123; J. Phys.: Conf. Ser. 3017 012031 (2025); EPL 150 59002 (2025); Mod Phys Lett A 2541012 (2025); and work in progress (with V.A. Kholodnyi, J. Kozicki, and M.E. Pietrzyk).

Speaker: I Kanatchikov (Natl. Quantum Information Center in Gdansk, Poland)

Public Talk: De Pitágoras a los agujeros negros: 2500 años de música, física y matemáticas

Convener: Tomas Ortin Miguel (Consejo Superior de Investigaciones Cientificas (CSIC) (ES))

Wednesday 27 May

Invited talk: Marina David

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

19 Amplification of new physics in the quasinormal mode spectrum of near-extremal black holes

Black hole spectroscopy is a central part of testing gravity in the strong-field regime and plays a key role in gravitational wave astronomy. With the next generation of detectors, we have a unique opportunity to test modifications of General Relativity (GR). This motivates a timely question from an effective field theory perspective: where are higher-derivative corrections most strongly amplified, while the expansion itself remains reliable? In this talk, I will show that this question can be answered by investigating the quasinormal modes of highly-spinning (i.e. near extremal) black holes. I will show how perturbatively-small higher-derivative corrections to the Einstein-Hilbert action can lead to order-one modifications of the quasinormal mode spectrum of near-extremal Kerr black holes. These effects originate from shifts in the critical point controlling the bifurcation structure of Kerr quasinormal modes, which reorganizes the spectrum and changes the number between zero-damped and damped modes. We show that these large effects can take place in a regime in which the higher-derivative expansion remains under control.

Speaker: Marina David

10:30 Coffee break

40' Contribution: Sumanta Chakraborty

Diego Rubiera-Garcia
Black hole imaging as a probe for non-Kerr physics

20 Dynamical Tides in General Relativity

Tidal Love numbers provide us a handle to test the nature of compact objects, as well as theories of gravity. There have been several clarifications recently, which makes our understanding of these Love numbers better. But further investigations have led to more confusion. I plan to discuss these recent developments and the confusing nature of recent literature on these issues. I will show that the tidal Love numbers of a non-rotating black hole identically vanishes in the static case, which may not be true in the dynamical scenario. While for rotating black holes the situation is a bit more involved. Besides, I will also discuss fermionic perturbations leading to non-zero static Love numbers for black holes. I will also highlight some remarkable novel features, for ultra-compact objects as well as for quantum BHs.

Speaker: Dr Sumanta Chakraborty (IACS)

Dynamical Tides in General Relativity

20' Contribution: David Pereñíguez Rodríguez

21 Nonlinear QNMs and Ringdown Isospectrality from Penrose Limits

At high frequencies, the dynamics of black holes can be captured by Penrose-limit plane waves adapted to the photon ring. Using that, I will show that gravitational electric–magnetic duality is restored on-shell in that limit, and that this implies QNM isospectrality. I will also show how this perspective on the high-frequency regime can be used to compute exactly nonlinear QNM solutions. I will conclude by discussing consequences for EFT extensions of GR.

Speaker: David Pereñíguez Rodríguez

20' Contribution: Pratik Wagle

22 The sound of broken symmetries

Gravitational-wave observations provide a powerful probe of gravity in the strong-field regime, particularly through the ringdown phase of binary black hole mergers, where the signal is governed by damped quasinormal modes. Recent progress has extended quasinormal-mode calculations beyond general relativity using the modified Teukolsky formalism and spectral methods in several theories, including higher-derivative gravity, scalar Gauss-Bonnet gravity, and dynamical Chern-Simons gravity. In theories such as scalar Gauss-Bonnet and dynamical Chern-Simons gravity, additional scalar degrees of freedom couple to spacetime curvature and generically break isospectrality, so that odd- and even-parity modes acquire different oscillation frequencies and damping times. In this talk, I will discuss how this breaking of isospectrality imprints itself on gravitational-wave observables, and more broadly how the extra degrees of freedom modify the ringdown signal. I will show that isospectrality breaking can induce a characteristic beating pattern in the plus and cross polarization modes. These effects have important implications for waveform modelling in beyond-GR theories and for strong-field tests of general relativity.

Speaker: Pratik Wagle (Max Planck Institute for Gravitational Physics)

20' Contribution: Marta Cocco

23 Tidal perturbations of Kerr black holes and implications for extreme-mass-ratio inspirals

Tidal interactions can significantly impact the dynamics of binary systems and their gravitational-wave signals. In particular, for extreme-mass-ratio inspirals (EMRIs), a fully relativistic description is required to capture the interplay between spin, curvature, and orbital dynamics in the strong-field regime.

We study the response of a Kerr black hole to external tidal fields and its implications for EMRIs. Using metric reconstruction in the Newman-Penrose formalism, we obtain analytic solutions of the Teukolsky equation for static, quadrupolar vacuum perturbations and reconstruct the corresponding metric.

As an application, we derive a secular Hamiltonian governing the long-term dynamics of a test particle in a tidally deformed Kerr spacetime. In particular, we compute tidal-induced shifts of the innermost stable circular orbit and the light ring, and analyze their dependence on the black hole's spin. These results provide an analytic framework for tidal interactions in Kerr spacetimes, directly applicable to EMRI modelling and gravitational-wave physics.

Speaker: Marta Cocco (University of Perugia and Niels Bohr Institute)

20' Contribution

24 A new approach for semiclassical backreaction in evaporating black holes

We present a closed-form framework for the renormalized stress-energy tensor of a scalar quantum field in arbitrary $3+1$-dimensional curved spacetimes through a reformulation of the Hadamard scheme. This approach allows one to construct explicit expressions bypassing the standard mode-sum procedure and rendering the semiclassical field equations more tractable. As an application, we construct explicit families of renormalized stress-energy tensors in Schwarzschild spacetime, evaluated in Unruh-like and Boulware-like quantum states. The Unruh-like family encompasses the Hawking flux at infinity and the associated ingoing negative-energy flux at the horizon. Our results provide a new route to the study of semiclassical backreaction in four dimensions, extending to realistic spacetimes the level of analytical control previously available only in two-dimensional models.

Speaker: Francisco Javier Marañón González (Universidad de Valencia - IFIC)

Lunch

Invited talk: Llibert Aresté Saló

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

25 Fully non-linear compact binary mergers beyond General Relativity

I present the first numerical simulations of binary black holes and binary neutron star mergers in scalar–Gauss–Bonnet gravity performed in the modified puncture gauge, which is a well-motivated theory from an effective field theory point of view. Implemented within both GRChombo and MHDuet Numerical Relativity codes, our modified formulation enables stable evolution through merger and post-merger phases. These results open the door to systematic studies of scalar-Gauss-Bonnet effects on the gravitational wave signal and remnant dynamics in this class of modified gravity theories.

Speaker: Llibert Aresté Saló

Coffee break

20' Contribution: Pantelis Pnigouras

26 Asteroseismology with binary neutron stars

During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817—the first direct detection of gravitational waves from a neutron star binary—this can lead to constraints on the neutron star equation of state. As detectors become more sensitive, the hydrostatic response of the neutron star to the tidal field of its companion (equilibrium tide) needs to be supplemented by dynamical effects, such as oscillation mode resonances triggered by the orbital motion (dynamical tide). We will demonstrate how dynamical tidal effects can serve as a tool for asteroseismology studies, in order to extract information about the neutron star internal structure. These studies are highly relevant for the future of gravitational-wave observations, since the effects of tides on the gravitational-wave signal are among the scientific goals of next-generation gravitational-wave detectors, like the Einstein Telescope.

Speaker: Pantelis Pnigouras (University of Alicante)

20' Contribution: Zakaria Belkhadria

27 New Aspects of spontaneous scalarization of black holes beyond General relativity

Spontaneous scalarization is a mechanism that allows black holes to develop a non-trivial profile of a scalar field “scalar hair” because of tachyonic instabilities, enabling tests of gravity beyond General Relativity. Motivated by stability and threshold issues in Gauss-Bonnet scalarization, we propose a new model characterized by two nonminimal couplings of the scalar field to both Gauss-Bonnet curvature and a U(1) gauge field (e.g. electromagnetic field). The presence of two distinct sources of tachyonic instability broadens the conditions for spontaneous scalarization. We track how the electric charge and the coupling constants govern the onset of the scalar field and derive new solution branches with nontrivial scalar profiles. Numerical integration shows multiple coexisting scalarized black hole solutions with adjustable thresholds, influenced by the relative strengths of curvature and matter couplings. We examine their scalar charge, horizon properties, and thermodynamic characteristics, demonstrating how the model can selectively activate or suppress scalarization phenomena. The matter source term modifies the scalarization onset and promotes stable solutions, as indicated by the evolution of the scalar charge and horizon quantities. These findings suggest an alternative approach to scalarization, may avoid the instabilities of curvature-only or matter-only models, and offer new ways to test strong-gravity effects in upcoming observations.

Speaker: Zakaria Belkhadria (Université de Genève & Università di Cagliari)

20' Contribution

28 Acoustic Black Holes in BECs with an Extended Sonic Region

In the context of Hawking-like radiation in sonic black holes formed by BECs we investigate the modifications of the emission spectrum caused by a finite width of the sonic transition region connecting the subsonic to supersonic flow [1].

Acoustic black holes formed by Bose-Einstein condensates (BEC) undergoing transonic motion have shown the presence of Hawking-like radiation [2,3,4]. The detection is however indirect, since what has been measured are the correlations between the Hawking particles and their negative energy partners [4,5]. The transition from subsonic to supersonic flow usually occurs in an infinitely thin surface, the sonic horizon of the acoustic BH metric according to the gravitational analogy. In this work we study a particular case in which this transition occurs in a region of finite extension, where the flow velocity equals the speed of sound.

We study a simple toy model where the condensate is step-wise homogeneous, i.e. the sound speed profile is a step-wise function and the density of atoms in the condensate remains constant in all regions. Particularly, the model has two semi-infinite regions, one subsonic one supersonic, representing the inside and outside of a black hole, separated by a sonic region of finite length, representing a thick horizon [6].

We show that the real solutions to the dispersion relation in the sonic region are two, as in the subsonic region. This implies that it is the boundary between the supersonic and the sonic regions that produces the spontaneous emission of photons.

We find that:

• Hawking-like emission decreases as the length $a$ of the sonic region increases (as $a^{-2}$ for large $a$); the thick horizon produces a “gray body factor” (Figure 1).
• For sufficiently large values of the sonic region width ‘a’ we see a transition from a (small frequency) $\omega^{-1}$ thermal behaviour to a non-thermal $\omega^{-1/3}$ one. This behaviour is in agreement with the results in a model with two semi-infinite regions, one supersonic and the other sonic.
• For sufficiently large values of $a$ we also find oscillations in the number of Hawking phonons emitted, i.e. suggesting that the sonic region acts as some sort of cavity.

References:
[1] D. Peñalver, M. De Vito, R. Balbinot, and A. Fabbri, Acoustic black holes in BECs with an extended sonic region, Phys. Rev. D 112, L021701 (2025).
[2] W. G. Unruh, Experimental Black-Hole Evaporation?, Phys. Rev. Lett. 46, 1351 (1981).
[3] S. W. Hawking, Particle creation by black holes, Commun.Math. Phys. 43, 199 (1975).
[4] L. J. Garay, J. R. Anglin, J. I. Cirac, and P. Zoller, Sonic Analog of Gravitational Black Holes in Bose-Einstein Condensates, Phys. Rev. Lett. 85, 4643 (2000).
[5] J. R. Muñoz De Nova, K. Golubkov, V. I. Kolobov, and J. Steinhauer, Observation of thermal Hawking radiation and its temperature in an analogue black hole, Nature 569, 688 (2019).
[6] I. Carusotto, S. Fagnocchi, A. Recati, R. Balbinot, and A. Fabbri, Numerical observation of Hawking radiation from acoustic black holes in atomic Bose–Einstein condensates, New J. Phys. 10, 103001 (2008).

Speaker: Mr Daniel Peñalver Mares (Universitat de València - IFIC)

Thursday 28 May

Invited talk: Mairi Sakellariadou

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

29 Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA's O1-O4a runs

I will summarise the results of the LIGO-Virgo-KAGRA paper arXiv:2510.26848 [gr-qc]
In this paper we derive gravitational-wave background energy density upper limits from the O1-O4a data to constrain parameters associated with various possible processes in the early Universe: first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, second-order scalar perturbations, primordial black hole binaries, and parity violation.

Speaker: Prof. Maria Sakellariadou (King's College Londom)

Coffee break

40' Contribution: José Beltrán Jiménez

Diego Rubiera-Garcia
Black hole imaging as a probe for non-Kerr physics

30 Isotropic universes with a preferred direction

The Cosmological Principle establishes the symmetries of the Universe on large scales. The inflationary paradigm and observations suggest that the universe has nearly flat spatial sections and, consequently, it has an approximate Euclidian symmetry group. Most cosmological scenarios realise this symmetry in a trival manner, although some non-trivial realisations have also been explored. All these realisations occur off-shell so they are not subject to the specific dynamics of the model. I will present a class of scenarios with an on-shell realisation of the symmetries so the compliance with the Cosmological Principle only occurs for solutions of the equations of motion. This on-shell realisastion has remarkable consequences like the mixing of different helicity cosmological perturbations or preferred direction effects on an an exact FLRW geometry.

Speaker: Jose Beltrán Jiménez (University of Salamanca)

20' Contribution: Lucía Castells

31 Fluctuation-induced gravitational waves in a non-conformal strongly coupled plasma

We perform a detailed holographic analysis of linear fluctuations in a strongly coupled plasma subject to a first-order phase transition. This analysis is done through the characterization of the energy-momentum tensor correlation functions across the phase diagram. Particular attention is given to the behavior of modes near the spinodal region of the phase transition, where the instability under sound modes amplifies scalar excitations. This set of fluctuations seeds the emission of gravitation waves in strongly coupled plasmas via different mechanisms: thermal emission and emission associated to phase transitions in the spinodal regime. These results illustrate how fluctuation dynamics and correlation functions encode information about both equilibrium and non-equilibrium gravitational-wave sources in strongly coupled systems.

Speaker: Lucía Castells (University of Barcelona)

20' Contribution: Ruchika Ruchika

32 2D BAO vs 3D BAO: Hints for new physics?

As next-generation telescopes and observational surveys continue to expand the boundaries of our understanding, tensions and discrepancies between observational datasets are becoming increasingly prominent. In this work, we focus on one such discrepancy: the differences between 2D and 3D Baryon Acoustic Oscillation (BAO) measurements. Without extending beyond the standard $\Lambda$CDM framework, we systematically study and highlight this discrepancy in different parameter spaces. By analyzing BAO observational datasets from two distinct methodologies (2D and 3D) alongside the Pantheon Plus SNIa sample, we identify a significant systematic difference: 2D BAO measurements consistently yield higher values of $hr_d$ compared to both 3D BAO and DESI analyses. While 2D BAO measurements appear to bridge the Hubble tension by simultaneously accommodating both a higher $H_0$ value (aligning with SH0ES) and a larger sound horizon $r_d$ (matching Planck), this apparent reconciliation comes at the cost of introducing tension with the well-constrained Planck measurement of $\Omega_{m0}h^2$. This behavior arises because of systematically higher values of the product $H_0r_d$ observed in 2D BAO analysis compared to 3D analyses. Therefore, given these systematic differences, we advocate for careful consideration when using 2D BAO measurements to address the Hubble tension, suggesting that understanding the origin of this 2D-3D discrepancy should be a priority for future investigations.

Speaker: Ruchika Ruchika (University of Salamanca)

20' Contribution: Marcello Musso

33 The Minimum Energy Principle - Linking protohalo shapes to infall anisotropy

Inferring abundance, position and size of cosmic structures from the initial conditions is a long standing effort in theoretical cosmology.
It allows (semi-)analytical predictions of mass function, bias and correlation functions of dark matter haloes, and an intuitive understanding of the formation of the cosmic web, a complementary tool to numerical simulations. Standard methods assume that dark matter haloes originates from critically high peaks of the initial matter density field,which act as seeds for the structure formation process. Although simple and intuitive, this approach suffers from numerical and conceptual inconsistencies. I will review the standard analytical methods, highlighting their advantages and shortcomings. I will then show how many difficulties can be overcome by focusing on the initial energy of the protohalo patch, rather than its mean density. This approach improves substantially the results of protohalo abundance and mass, and provides a first-principles prediction for protohalo shapes.

Speaker: Marcello Musso

20' Contribution: Adrian del Rio

34 Electromagnetic duality anomaly in waveguides: a non-inertial analogue of gravitational chiral effects.

Maxwell's equations in vacuum exhibit a duality invariance under electric-magnetic rotations. This is a Noether symmetry of the source-free Maxwell theory in any curved spacetime, and implies that the circular polarization state (the Stokes V parameter) of classical electromagnetic waves is conserved during propagation, even in the presence of strong gravitational fields.

Remarkably, quantum vacuum fluctuations of the electromagnetic field, when enhanced by gravitational effects, can break this symmetry. In previous work, we showed that the vacuum expectation value of the Noether charge operator is no longer conserved, and its time evolution is governed by the spacetime geometry [arXiv: 1607.08879, 1810.08085]. This constitutes a quantum anomaly for spin-$1$ fields---a direct analogue of the Adler-Bell-Jackiw anomaly for spin-$1/2$ fermions in gauge fields, previously unknown for photons. It was further shown that this effect arises when the background geometry develops a chiral structure, for instance when the gravitational field carries a flux of circularly polarized gravitational waves [arXiv: 2002.01593, 2106.08350].

In this talk, I will present a tabletop realization of this gravitational phenomenon in a non-inertial setting. I will show that, when the electromagnetic field is probed by observers undergoing relativistic helical motion inside a cylindrical waveguide, the vacuum expectation value of the duality charge fails to be conserved in time. The underlying mechanism involves two distinct ingredients: (i) a mismatch between early- and late-time notions of positive frequency due to the observers’ acceleration, which leads to photon pair creation, and (ii) a chiral structure of the late-time observer frame, which induces a spectral asymmetry between right- and left-handed modes due to frame-dragging effects inside the waveguide. As a result, a net helicity imbalance is generated from the quantum vacuum [arXiv: 2505.20409, 2512.04188].

This photon helicity non-conservation reflects a relativistic quantum effect, providing a novel manifestation of a quantum anomaly in flat spacetime, and opening new avenues for exploring chiral quantum effects in controlled photonic systems.

Speaker: Adrian del Rio (Universidad Carlos III de Madrid)

20' Contribution

35 A new class of rotating charged black holes in the external Bertotti-Robinson (electro)magnetic field

We present a large family of twisting and expanding solutions to the Einstein-Maxwell equations of algebraic type D, for which the two double principal null directions (PNDs) of the Weyl tensor are not aligned with the null eigendirections of the Faraday tensor. In addition to systematically deriving this new class, we present its various metric forms and convenient parameterizations. We show that in Boyer-Lindquist-type coordinates these solutions depend on 7 parameters, namely the Kerr and NUT (Newman-Unti-Tamburino) twist parameters a and l, mass parameter m, acceleration $\alpha$, strength of the Maxwell field |c|, and angular parameters $\beta$ and $\gamma$ that represent two duality rotations of the Faraday tensor, which include the rotation between the electric and magnetic charges generating the aligned part of the Maxwell field. This coordinate parameterization, analogous to the Griffiths-Podolsky form of the Plebanski-Demiański solutions, allows us to perform various limits, explicitly identify the subcases, and determine the physical interpretation of the new class. Interestingly, by considering the limit with no acceleration ($\alpha\to 0$), one obtains either the famous Kerr-Newman-NUT black holes (if the parameter $|c|$ remains constant) or the novel Kerr-Bertotti-Robinson black holes, announced recently in our work [ Phys.Rev.Lett. 135 (2025) 18, 181401] (if $\alpha\to 0$ while $\alpha |c|=const.$). We may thus conclude that this new class of spacetimes represents twisting charged accelerating black holes, immersed in an external magnetic (or electric) field. In the non-twisting subcase, we obtain the previously known solution of Van den Bergh-Carminati.

Speaker: Hryhorii Ovcharenko (Charles University, Faculty of Mathematics and Physics, Institute of Theoretical Physics, V Holesovickach 2, 18000 Prague 8, Czechia)

EREP_pres_Ovcharenko.pdf

Lunch

36 Segre Meeting

Social Dinner

Friday 29 May

Invited talk: Maxime Gadioux

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

37 Constructing numerical counterexamples to the third law of black hole mechanics

A few years ago, Kehle and Unger proved that, by sending finely-tuned pulses of a charged scalar field into a black hole, it is possible to form an extremal Reissner-Nordström black hole in finite time, i.e. the third law is false. The proof makes use of characteristic gluing: a framework to build solutions to Einstein's equations by gluing two regions of spacetime along a null cone. In this talk, I will present numerical implementations of characteristic gluing. Firstly, I will revisit the model of Kehle and Unger and numerically construct solutions to the characteristic gluing problem. I will show how these solutions depend on the parameters of the theory (charge and mass of the scalar field, cosmological constant), and discuss the similarities and differences between solutions of different levels of regularity. In the second part of the talk, I will apply characteristic gluing to the vacuum Einstein equations in 5d, and will present a solution that forms an extremal Myers-Perry black hole in finite time. This is the first counterexample of the third law in vacuum gravity and shows that the law is false independently of any matter model. This talk is based on work done with John Crump, Harvey Reall and Jorge Santos.

Speaker: Maxime Gadioux (University of Cambridge)

Coffee break

40' Contribution: Javier Olmedo

Diego Rubiera-Garcia
Black hole imaging as a probe for non-Kerr physics

38 Hawking-Partner Entanglement in Expanding Cavities

We investigate quantum entanglement in a one-dimensional cavity with an accelerating boundary, serving as a model of Hawking-like particle creation. Employing logarithmic negativity, we study correlations between selected modes and the rest of the field, and for pure states we reconstruct partner modes using the Hotta-Schützhold-Unruh relation. By analyzing vacuum, squeezed, and thermal initial states, we show that the dynamical cavity acts as an effective squeezing device and that Hawking-partner pairs closely resemble two-mode squeezed states. Our simulations indicate that entanglement is dominated by low-energy modes, while ultraviolet modes contribute negligibly to purification. The results also show that entanglement decreases with increasing mode energy in both small- and large-acceleration regimes, as well as we highlight the limitations of a purely pairwise description in the presence of multimode correlations.

Speaker: Javier Olmedo (Universidad de Granada)

20' Contribution: Maciej Ossowski

39 Isolated horizons: Extremal and Petrov type D

Isolated horizons provide a local generalization of black hole horizons that does not require the existence of a global symmetry and admits matter content arbitrarily close to the surface. Remarkably, in General Relativity, the geometries of Petrov Type D and extremal isolated horizons are governed by equations stated on a 2D local section of the horizon, namely the Petrov type D Equation and the Near-Horizon Geometry Equation. I will present a general solution of the said equations for spherical horizons, and use them to construct a horizon with product topology and possibly a conical singularity or a novel type: a non-singular, compact horizon with topology of a non-trivial U(1) bundle. Finally, I will discuss their embeddings into the Plebański-Demiański spacetimes with the NUT parameter.

Speaker: Maciej Ossowski (Jagiellonian University)

20' Contribution: Marco de Cesare

40 Scalar field scattering from a Schwarzschild-de Sitter black hole

I will discuss the scattering problem for a massless scalar field in a Schwarzschild-de Sitter geometry. A rigorous application of the method of matched asymptotic expansions allows us to solve analytically the low-frequency s-wave dynamics, and connect the scalar’s evolution in the proximity of the black-hole horizon with that on cosmological scales. The scattering coefficients, greybody factors, and Wigner time delay are computed explicitly. We consider both small and large black holes (relative to the cosmological horizon), extending previous studies confined to the small black-hole regime. In addition, for small black holes we perform a calculation that remains agnostic about the relative size between the ratio of the geometry’s horizons and the scalar’s frequency in units of the black-hole radius. When the two are comparable, we find that the greybody factor is symmetric under the exchange of the scalar frequency and the cosmological horizon radius.
Based on arxiv:2511.09168 (to appear in PRD), in collaboration with M. Miranda and A.P. Porfyriadis.

Speaker: Marco de Cesare (Scuola Superiore Meridionale (SSM))

20' Contribution

41 Jacobson's thermodynamic approach to classical gravity applied to non-Riemmanian geometries: remarks on the simplicity of Nature

Our purpose is to give a step forward in the search of nature's options to describe classical gravity. Two reasonable hypotheses are invoked: the spacetime is described by a smooth manifold, which might or might not be Riemannian, and the classical gravitational dynamics is intimately related to the laws of thermodynamics (Jacobson's programme). We have found that the Einstein-Hilbert's action is the only one that may describe gravity in the Riemannian case whereas, in presence of torsion, an extra quadratic torsional term is needed (when the Lanczos-Lovelock requirements are also held). We have also found that the same strategy cannot be followed in the full non-Riemannian case as the two approaches are mutually inconsistent since the presence of non metricity forbids a thermodynamic equilibrium description.

Speaker: Jhan Nicolás Martínez Lobo (Universidad Industrial de Santander)

workshop-erep (1).pdf

20' Contribution

42 Aristotelian geometry and the conformal approach to coupling matter

In this talk, I will give a brief introduction to p-brane Aristotelian geometry, the underlying geometry of physical models without boost symmetry, and take the first steps in constructing matter-coupled Aristotelian gravity theories through a conformal approach. I will distinguish between three cases of Aristotelian gravity: electric, magnetic, and electric-magnetic, and then discuss how to couple these backgrounds to matter fields, focusing on quadratic derivative models, such as the coupling to massive Aristotelian electrodynamics, as well as some higher-derivative models related to fracton theories.

Speaker: Giacomo Giorgi (Universidad de Murcia)

Lluís Bel award

Lunch

Invited talk: Chiara Toldo (participating online)

Miguel Zumalacárregui
Gravitational lensing of waves: a new window into astrophysics, dark matter & gravity

43 Thermodynamics of near-extremal black holes

From the perspective of classical gravity, a black hole is the simplest object we know of. At the same time, it possesses huge entropy, hinting at an incredibly complex microstructure: understanding this fact falls in the realm of quantum gravity. In this talk I will review recent results concerning the microscopics and the thermodynamics of black holes close to extremality. In the first part, I will describe how recently developed techniques allow to compute the quantum corrections to the entropy of near-extremal black holes, both by making use of an effective near-horizon theory (Jackiw-Teitelboim gravity) and by regularizing certain zero-modes appearing in the gravitational path integral in the near horizon geometry. I then will show that the quantum-corrected near-extremal entropy exhibits 3/2logT behavior characteristic of the Schwarzian model, and predicts the lifting of the ground state degeneracy for the extremal black hole. I will highlight some subtleties that arise when dealing with the Kerr solution, and I will briefly comment on results for AdS4 black holes which admit a supersymmetric limit, and near extremal de Sitter ones.

Speaker: Chiara Toldo

Coffee break: Farewell coffee

44 Farewell coffee