Gravitational waves: the theorist’s Swiss knife

• Mairi Sakellariadou (King's College London)

Black holes with primary hair

• Christos CHARMOUSSIS

In the context of higher order scalar tensor theories we will find explicit solutions with primary scalar charge evading classical no hair theorems. The large class of theories at hand are ultra violet departures from GR and will be given by specific analytic functions and will have certain symmetries. The scalar charge will be shown to be related to a conserved Noether charge associated to the global shift symmetry of the theories at hand. At a certain limit in between the mass and charge regular black holes will be constructed and properties of the solutions will be discussed. We will then discuss axial perturbations of these theories. The effective metric thus constructed will be shown to be a very particular extension of the Einstein frame. Axial gravitons will propagate in this frame quite different to the luminous frame. We will discuss the effects of this and what bounds can be put on the hair for stability.

Spectrum and dynamics of bosonic stars and their hairy black holes

• Carlos Herdeiro

I will discuss the current understanding of the landscape of bosonic stars in simple GR models, the hairy black holes that are associated with them, and the corresponding dynamics. Some applications to both phenomenological and conceptual questions will be addressed.

Excision scheme for black hole numerical simulations

• Isabel Cordero-Carrión (University of Valencia)

I will present developments and recent applications of the excision technique in the case of the Fully Constrained Formalism. I will focus on spherically symmetric spacetimes representing the collapse of a neutron star to a black hole. I will also present a more general set up of boundary conditions to be imposed at the excised surface, an arbitrary coordinate sphere inside the apparent horizon, where a new parameter can control some physical properties. I will show exponential convergence toward the stationary solution and stable long-term evolution of the newly formed black hole. Finally, I will show the application of this technique in recent general core-collapse simulations.

Free hyperboloidal evolution and strong field initial data

• Alex Vano-Vinuales (IAC3, University of the Balearic Islands)

Gravitational wave radiation is only unambiguously defined at future null infinity -- the "location" where light rays arrive and where global properties of spacetimes can be measured. Reaching future null infinity is thus crucial for extracting correct waveforms from numerical relativity simulations of compact binaries. Hyperboloidal slices extend to null infinity while being spacelike and smooth everywhere. Among the current efforts to the hyperboloidal method, I will focus on free evolution of the conformally compactified BSSN / Z4 equations. After illustrating relevant aspects of the approach, I will focus on hyperboloidal initial data including black holes and neutron stars in spherical symmetry. I will finish with an update on the ongoing efforts towards hyperboloidal simulations in full 3D.

Stress-energy tensor in neutron-star matter

• Felipe J. Llanes-Estrada

I will provide a quick overview of the status of our understanding concerning neutron star matter, relevant to test gravity with the densest objects available in the present-day universe, and some steps which we are taking so that quantum computing may one day help.

Neutron star crust elasticity and elastic oscillations

• Hajime Sotani

The oscillation signals are important information for extracting the interior properties of the oscillating source. This technique for stars is known as asteroseismology, which is similar to seismology on Earth and helioseismology on the Sun. Since the oscillation frequencies are excited due to the corresponding physics, one could extract the properties associated with such physics once one identifies the frequencies with a specific mode. In particular, the neutron star crust has an elasticity with which the elastic modes can be excited. The neutron star crust properties relatively weakly affect the neutron star mass and radius, but their density region is around the nuclear saturation density, i.e, we can approach physics in the neutron star crust from astronomical observations and terrestrial experiments. In this presentation, we identify the magnetar QPOs with the torsional modes to extract the physical properties, and also discuss the other oscillation modes, i.e., the shear and interface modes, which may be associated with the resonance shattering in the binary neutron star evolution.

Hybrid Nuclear Matter EOS with Color Superconducting Quark Phase: Bayesian Constraints from Observations

• Alexander Ayriyan (University of Wroclaw & AANL)

We perform a Bayesian analysis of the equation of state (EOS) constraints using recent observational data, including pulsar masses, radii, and tidal deformabilities. Our focus is on a class of hybrid neutron star EOS that incorporates color superconducting quark matter, based on a recently developed nonlocal chiral quark model. The nuclear matter phase is described using a relativistic density functional approach within the DD2 class, while the phase transition between nuclear and quark matter is described using a Maxwell construction. Our analysis identifies a region within the two-dimensional parameter space, defined by the vector meson coupling and scalar diquark coupling, where the observational constraints are met with the highest probability (90% of the maximum). We present the overlap of this region with those where other properties are fulfilled: 1. A strong phase transition that produces a third family of compact stars. 2. A maximum mass of the hybrid neutron star that exceeds that of the purely nucleonic star. 3. An onset mass for quark deconfinement below one solar mass.

Black holes and spacetime energy in teleparallel gravity

• Laur Järv (University of Tartu)

Teleparallel gravity employs independent connection that is curvature free but can be characterised by torsion and nonmetricity. While it is possible to formulate families of teleparallel theories that have field equations equivalent to general relativity and only differ from it by the boundary term in the action, the extensions like scalar-tensor start to stand apart from their respective counterparts already at the level of the field equations. The talk will provide an overview of the search for nontrivial black hole solutions in extended teleparallel gravities, relevant methods and results. The last part of the presentation will briefly report the definition and calculation of quasi-local spacetime energy for Kerr spacetimes in general parallel relativity.

Fastest spinning millisecond pulsars: indicators for quark matter in neutron stars?

• Christoph Gärtlein (University of Lisbon, University of Wroclaw, University of Coimbra)

We study rotating hybrid stars, with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson and diquark couplings in the quark matter phase, we obtain different hybrid star sequences with varying extensions of the quark matter core, ensuring consistency with astrophysical constraints from mass, radius, and tidal deformability measurements. We test whether the early deconfinement phase transition is consistent with the present observational data. We show how the fastest spinning pulsars and the appearance of the quasi-radial oscillations and non-axisymmetric instabilities constrain the strongly interacting matter equation of state at zero temperature. Our findings reveal that incorporating the hybrid equation of state into the analysis of pulsars has significant implications for the constraints on the properties of strongly interacting matter and neutron stars which is of the high interest for the future SKA observations.

Stability of Neutron Stars for Causal Relativistic Viscous Fluids

• Daniel Caballero (University of Illinois Urbana-Champaign)

First order relativistic viscous theories had, until recently, been believed to not be well-behaved (i.e. causal, stable and strongly hyperbolic). Because of this, relativistic viscosity in astrophysical contexts has remained understudied. Recently, the Bemfica-Disconzi-Noronha-Kovtun (BDNK) theory has been shown to be causal, stable and strongly hyperbolic, which makes it a well-suited model for extending viscosity in the relativistic regime. In this presentation, I study the stability of neutron stars to viscous effects through perturbation theory and find necessary conditions for radial stability for the BDNK theory.

The effect of extra dimensions on astrophysical observables

• Anna Horvath

Kaluza and Klein proposed a theory with a compactified extra dimension, which may appear in high-energy phenomena, such as nuclear reactions, strong gravitational effects, or in the presence of superdense matter. In this work, I show how astrophysical observables will be modified in the presence of extra compactified dimensions. The interior of a compact star is modelled as a multidimensional interacting degenerate Fermi gas, embedded in a static, spherically symmetric spacetime with extra compactified spatial dimensions. The equation of state of this extreme medium is given and compared to the standard models of superdense matter. The modification of the mass-radius relation of compact stars is calculated and compared to realistic star models and astrophysical observation data. The interaction strength has been determined for this extraordinary matter. Constraints on the size of the extra dimension have been estimated based on pulsar measurements [1,2]. [1] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: "Application of Kaluza-Klein Theory in Modeling Compact Stars: Exploring Extra Dimensions", MNRAS, https://doi.org/10.1093/mnras/stae2637 [2] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: "The effect of multiple extra dimensions on the maximal mass of compact stars in Kaluza-Klein space-time", Accepted to International Journal of Modern Physics A, https://doi.org/10.1142/S0217751X25420047

Exploring neutron-star mergers in numerical relativity

• Masaru Shibata (Max Planck Institute for Gravitational Physics at Potsdam)

Numerical relativity plays many important roles in astrophysics and general relativity, e.g., in understanding mergers of black holes and neutron stars, formation processes of black holes for a wide variety of stellar collapse, and launching mechanisms of jets. In this talk, I will summarize our current understanding of the merger and post-processes for neutron-star binaries introducing our latest results of numerical simulations. I will pay particular attention to nucleosynthesis of heavy elements and provide predictions for the future observations and also give a speculation for the merger process of GW170817 if I have time.

Numerical relativity: Exploring new capabilities with binary black holes

• Harald Pfeiffer (Max-Planck-Institute for Gravitational Physics)

Numerical relativity has been spectacularly successful during the past decades, with numerous contributions toward gravitational wave astrophysics for binary black holes and neutron stars. Despite all accomplishments, the increasing sensitivity of gravitational wave detectors and the broader bandwidth of future detectors require significant further improvements of the numerical codes, both for black holes and neutron stars. In this talk, I will use the vacuum case of binary black holes to illustrate several dimensions along which such improvements are being pursued within the SXS collaboration. These dimensions include extension of the parameter space coverage to eccentric binary black holes and hyperbolic encounters, new computational algorithms, as well as the study of BHs in alternative theories of gravity.

The phenomenological waveforms program: challenges, recent results, and future prospects

• Sascha Husa (Institute of Space Sciences and University of the Balearic Islands)

This talks reviews challenges in the modelling of the waveforms of coalescing compact binaries as the GW community prepares for the next generation of GW observatories, and the current status and future prospects for the phenomenological waveforms program. The talk reports in particular recent progress in modelling eccentric binaries, spin precession, the memory effect, and the LISA response.

Sparse-dictionary algorithms for GW reconstruction: applications

• Alejandro Torres Forne (University of Valencia)

Sparse dictionary learning (SDL) techniques have demonstrated strong potential for extracting astrophysical signals from noise in gravitational-wave (GW) astronomy. In this talk, I will provide an overview of SDL algorithms and discuss their application to a range of GW data analysis challenges. Examples will include glitch mitigation, classification of the equation of state in binary neutron star mergers, and signal detection in the context of the LISA mission. I will highlight both recent advances and open questions, emphasizing how SDL methods can contribute to the next generation of gravitational-wave discovery.

Evolution of a black hole cluster in full general relativity

• Milton Ruiz (University of Valencia)

The dynamical interactions of compact objects in N-body clusters are of great interest for understanding the formation of black holes (BHs) in the upper mass gap, as well as intermediate and supermassive BHs. These systems are potential sources of gravitational waves (GWs) detectable by both current and future observatories. We present, for the first time, a fully general relativistic evolution of a small, collisional N-body BH cluster with arbitrary total mass M. The bound cluster is initially compact, stable, and composed of 25 equal-mass, non-spinning BHs. Unlike previous Newtonian and post-Newtonian N-body simulations, no "subgrid physics" is required to handle collisions and mergers. In full general relativity, we confirm several predictions from these earlier simulations and analytic estimates: the runaway growth of a large BH through repeated mergers; spindown of the central BH with successive captures; ejection of a BH with high asymptotic velocity following a multi-body interaction; and a regime where mergers occur primarily via direct collisions on highly eccentric orbits rather than through quasicircular inspirals. We extract the GW signal and identify several distinct features characteristic of the compact cluster regime. Our results suggest that the signal is sufficiently strong for next-generation observatories to detect similar events across much of the observable universe.

Challenges in the observation of gravitational waves from the collapse of massive stars

• Pablo Cerdá-Durán (University of Valencia)

After the first detections of gravitational waves from the collision of compact merging binaries, including both black holes and neutron stars, the next great new discovery from LIGO-Virgo-KAGRA detectors could be associated with the collapse of massive stars. With a galactic event rate of about 2-3 per century, core-collapse supernovae (CCSNe) are a primary candidate for gravitational wave detectors and their observation could unveil the mechanism for this powerful explosions as well as help us understanding the properties of matter at high densities. Unfortunately these events are extremely weak and their detection present a great challenge. I will present recent results on how to infer the properties of the nascent neutron star based in the gravitational wave signal and how these properties could be used to improve the detectability of CCSNe.

Numerical Relativity Insights into Tidal Resonance Phenomena

• Hao-Jui Kuan (Max Planck Insitute for Gravitational Physics)

Gravitational wave observations of neutron star collisions offer a unique avenue into a regime where gravity and matter entangle strongly and evolve dynamically. As these waves travel essentially undistorted through the cosmos to reach the detectors, they constitute the most promising messenger to probe into nuclear physics at extreme densities and low temperatures. In this talk, we will focus on the imprint on the waveforms left by the tidal resonance phenomenon that could happen in the last 100 milliseconds of inspiral. After introducing the analytic model for such effects, we will present recent results of numerical simulations where we confirm in part our theoretical understanding while revealing further issues that require the next level of modeling.

What if black hole spacetimes are singularity-free?

• Piero Nicolini (Trieste U. & INFN, Trieste and Frankfurt U.)

Although the Penrose-Hawking singularity theorems leave little room for the fate of collapsing matter, since the 1960s there has been great interest in the possibility of deriving black hole solutions with a regular center. In this talk, after a brief historical review, I will discuss the main features of regular black holes and their implications for fundamental physics. Finally, I will comment on the emerging new phenomenology in astrophysics and particle physics and what can be learned from such spacetimes for the future.

Doubly regular black holes

• Arthur Suvorov (University of Alicante)

Black holes in general relativity possess curvature singularities. These are not the only type of singularities that a spacetime can possess though. The Kerr solution, for example, also exhibits a *thermodynamic singularity* at a specific, non-extremal value of the spin parameter where the heat capacity diverges ("Davies' point"). Given the deep connections between black holes and thermodynamics, it seems desirable to consider objects which are not only physical-space regular but phase-space regular also. I will discuss some results regarding such "doubly-regular" holes, including context with respect to physical interpretations of the heat capacity.

Measuring the shadows of Kerr and non-Kerr compact objects

• George Pappas (AUTH)

In this talk, we will discuss the formation of black hole shadows and how some cases of non-Kerr compact objects can produce distinctive non-Kerr shadows with fractal structures. We will also discuss how these shadows can be illuminated by accretion and explore the possibilities for measuring the properties of the spacetime.

Observing horizonless compact objects by the Event Horizon Telescope

• Petya Nedkova (Sofia University "St. Kliment Ohridski")

Horizonless compact objects may produce phenomenological features which distinguish them observationally from black holes. In particular, the images of the accretion disks around them may possess a characteristic morphology including a series of central bright rings instead of a shadow. We demonstrate how the central ring structure arises relating it to the behavior of the deflection angle on the scattering geodesics and the light ring structure of the spacetime. Focusing on reflective naked singularities and wormholes we further discuss whether the central rings can be observed by the present and near-future Event Horizon Telescope (EHT) arrays. While they may be hard to be distinguished by the current capacities, the next-generation EHT will be able to detect them as qualitative deviations in the image morphology. The effect can be further confirmed by measurements of the linear polarization of the emission from the accretion disk. In these cases it forms a characteristic pattern which can serve as an independent channel for detection of horizonless compact objects.

Strong-gravity tidal interactions in hierarchical triple systems

• Marta Cocco (Department of Physics and Geology of Perugia, INFN Perugia, and Niels Bohr Institute)

Tidal interactions play a fundamental role in shaping binary systems and affect their gravitational wave (GW) signals, which is crucial for future detectors such as LISA and ET. While tidal effects on binaries are often studied in a weak-field approximation, their role in the strong-gravity regime remains largely uncharted. We present two frameworks to analyze strong-gravity tidal effects from a supermassive black hole on an inspiraling binary within a hierarchical triple system. Using the small-tide approximation, we introduce relativistic corrections at the quadrupolar level that are captured by electric and magnetic tidal moments. First, we examine the recently discovered precession resonances between the inner binary’s periastron precession and its orbital motion around a supermassive black hole. By modeling the latter as a Schwarzschild metric, we discover a significantly richer resonance spectrum than predicted by Newtonian mechanics, with distinct implications for orbital evolution. Second, we derive the first explicit metric expression for a tidally perturbed Kerr black hole by using reconstruction techniques based on the Teukolsky Master Equation. This solution, valid for general quadrupolar tidal deformations, captures spin-tidal couplings and enables us to compute shifts in the trajectories of a particle in the spacetime of the perturbed black hole. These findings open new observational pathways for detecting strong-gravity effects in hierarchical triple systems. Specifically, these effects can produce distinctive GW waveform modulations and phase shifts that could serve as smoking-gun signatures of strong-field tidal effects.

Non-convex fluid dynamics in neutron stars

• José A. Font (Valencia University)

Non-convex flows, or Bethe-Zeldovich-Thompson flows, can develop composite waves such as rarefaction shocks. This type of non-classical dynamics can be easily characterized by the so-called fundamental derivative, a quantity describing the convexity (wave structure) of a physical system and related to its equation of state. Non-convex dynamics has been analyzed for Newtonian flows and, more recently, for relativistic flows. We discuss here the effects non-convex flows may have on the dynamics of neutron stars, in the context of gravitational collapse and mergers.

Modeling the strong-field dynamics of binary neutron stars

• Sebastiano Bernuzzi (FSU Jena)

Binary neutron star mergers (BNSM) are associated with powerful gravitational and electromagnetic astronomical transients. Multimessenger observations of BNSMs promise to deliver unprecedented insights on fundamental physics questions, including constraints on dense matter models and the production of heavy elements. Detailed theoretical predictions of the merger dynamics are crucial for extracting information from such observations. This talk reviews recent progress in the modeling of BNSMs using simulations in 3+1 numerical general relativity. I will first discuss the first predictions for the complete (inspiral-merger-postmerger) gravitational-wave spectrum and their application in gravitational-wave astronomy. Afterwards, I will focus on recent results on the merger remnants and mass ejecta, the mechanisms behind kilonova light and their application to the analyses of astrophysical data.

A unified framework for magnetic field amplification in binaries involving neutron stars

• Carlos Palenzuela (Universitat de les Illes Balears)

One of the most significant open questions in the theoretical understanding of neutron stars and black hole–neutron star binaries is the amplification of magnetic fields in the remnant. This amplification initially occurs on small scales via a turbulent dynamo, primarily driven by the Kelvin–Helmholtz instability. This turbulent phase is followed by a large-scale reorganization of the field, driven by magnetic winding and possibly by other mechanisms such as the magnetorotational instability. Although current full GRMHD simulations, with typical resolutions of O(10m), are not yet capable of resolving all relevant scales, large-eddy simulation techniques—employing the gradient subgrid-scale model—allow us to accurately capture the magnetic field amplification during the firsts tens of milliseconds following remnant formation. In this work, we discuss two key outcomes of our simulations. First, we find evidence for a form of "universality" in the magnetic field strength of the newborn hypermassive neutron stars formed in BNS mergers. Second, we observe that the amplification mechanisms in the remnant disks of BH–NS mergers exhibit strong similarities to those in BNS mergers, suggesting a unified picture for magnetic field growth in both scenarios.

Tidal effects in gravitational waves from neutron star binary inspirals in scalar-tensor gravity

• Tanja Hinderer

Neutron stars are useful laboratories for many areas of physics, including subatomic physics and the coupling of nonlinear gravity with matter. To gain further insights into the fundamental information contained in observable neutron star properties and potential degeneracies between modifications to different sectors of physics, it is useful to consider theories of gravity beyond General Relativity. A feature of many such theories is the presence of extra fields, as also motivated by proposed scenarios for inflation, dark energy, or dark matter. This talk will focus on features of neutron stars in a class of scalar-tensor theories of gravity involving a scalar field. I will discuss consequences for gravitational-wave signals from binary inspirals, focusing on matter signatures from a richer set of tidal interactions that arise due to the presence of the scalar field, the associated characteristic parameters and expected imprints in gravitational waves.

Simulating and Interpreting the Multimessenger Picture of Neutron Star Mergers

• TIm Dietrich (University of Potsdam)

The correct interpretation of multimessenger data obtained from binary neutron star mergers, including gravitational waves and electromagnetic signals, requires accurate theoretical predictions that can be cross-correlated with observations. These models can be constructed by combining ab initio numerical-relativity simulations with derived analytical knowledge. In addition, an efficient Bayesian framework for multimessenger analysis is indispensable for extracting meaningful information from observational data. We will discuss how numerical-relativity simulations and multimessenger data analysis go hand in hand to provide valuable insights into neutron star mergers and fundamental physics principles.

Precessing binary black holes and mirror (a)symmetry in the Universe

• Nicolas Sanchis-Gual

Precessing binary black hole mergers can potentially excite photons from the quantum vacuum in such a way that total helicity is not preserved in the process. I will show that precessing binary black hole systems in astrophysics generate a flux of circularly polarized gravitational waves which, in turn, provides the required helical background that triggers this quantum effect. Solving the fully nonlinear Einstein’s equations with numerical relativity we explored the parameter space of binary systems and extract the detailed dependence of the quantum effect with the spins of the two black holes. We also introduce a set of diagrammatic techniques that allows us to predict when a binary black hole merger can or cannot emit circularly polarized gravitational radiation, based on mirror-symmetry considerations. According to the cosmological principle, such emission must average to zero across all binary mergers in our Universe to preserve mirror-reflection symmetry at very large scales. I will briefly discuss a new independent gravitational-wave test of this hypothesis.

Numerical Relativity Simulations of Dark Matter Admixed Binary Neutron Stars

• Violetta Sagun (University of Southampton)

Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology - a dense core or a diluted halo - may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from standard models when the tidal deformability is calculated in a two-fluid framework, neglecting the dilute and extended nature of the halo. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter's role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

Towards a more realistic asteroseismology of core-collapse supernovae

• Dimitra Tseneklidou (University of Valencia)

One of the most promising and challenging future gravitational wave (GW) sources are core-collapse supernovae. The oscillation modes of the newly born proto-neutron star (PNS) and the stalled accretion shock will be excited triggering the GW emission. Due to the stochastic nature of these signals, it is not possible to use template matching techniques. An alternative way to analyse the signal is to perform asteroseismology in order to infer properties of the PNS. The oscillations can be described by a system of partial differential equations (PDEs), which can be solved as an eigenvalue problem. In that frame, the eigenvalues are the characteristic frequencies of the oscillation modes. In this work, we relax the approximation of the hydrostatic equilibrium and allow for accretion of the PNS. By doing so, we can investigate the Standing Accretion Shock Instability and its effect on the modes.

3D collapse of rapidly rotating neutron stars into black holes in massive scalar-tensor theory

• José Carlos Olvera Meneses (Tübingen University)

We present a full 3D numerical evolution code to study rotating neutron stars in massive-scalar-tensor (MST) theories. The implementation consists in a modified version of the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formalism such that the simulations are performed in the physical Jordan frame, where the scalar field is directly coupled with the spacetime evolution. This approach allows to preserve the standard hydrodynamic evolution for matter fields, allowing eventually for the inclusion of more microphysics. As an example of a use case of this implementation, we study the problem of gravitational collapse of rapidly rotating neutron stars in MST theories by exploring the parameter space (couplings and mass of the scalar field) and its impact on the dynamical properties of the process and on the emitted scalar and gravitational radiation.

Time evolution characterization as a sieve for gravity theories

• Fethi Mubin Ramazanoglu (Koc University)

Having well posed time evolution is an important property we seek to have in our theories of nature. In this talk, I will present how some simple classical field theories can be ruled out due to a breakdown of time evolution in their dynamics. I will also speculate on using similar analyses to constrain the alternative theory landscape of gravity, helping the observational and experimental tests.

Impact of magnetic field gradients on the development of the MRI in binary neutron star mergers

• Thomas Celora (Institute of Space Sciences)

The magneto-rotational instability (MRI) is a cornerstone of accretion disk theory, and it is often invoked to explain the generation of large-scale, poloidal magnetic fields in binary neutron star mergers. However, simulations that begin with weak seed fields and follow their amplification to saturation lack convincing evidence of MRI activity, casting doubts on its role in this setting. In this talk, I will discuss how the classical MRI extends under more realistic post-merger conditions, where magnetic fields present complex topologies and field gradients are significant. In particular, I will present modified expressions for the timescale and wavelength of the fastest growing mode, along with a generalised instability criterion that captures the influence of magnetic field inhomogeneities. Finally, I will show the results of applying the extended MRI to a high-resolution simulation of a long-lived merger remnant. Our results indicate that the MRI is significantly hindered in the early post-merger phase, with favourable conditions—where the instability condition is met and the growth rate is sufficiently fast—emerging only at later stages, of the order of 100 milliseconds after merger.

Probing Magnetic Fields in Neutron Star Binaries Through Gravitational Waves

• Prasad R (International Centre for Theoretical Sciences, Bangalore)

We investigate the imprint of magnetic fields on gravitational waves from the inspiral phase of eccentric binary neutron star binaries (BNS). While neutron stars are typically observed to exhibit strong magnetic fields ranging from $10^{14}$ to $10^{15}$ G, theoretical models predict strengths up to $10^{18}$ G. BNS systems formed through dynamical capture may retain substantial eccentricity and extreme magnetic fields during their late inspiral, potentially affecting the gravitational waveform. These fields can influence the orbital dynamics through magnetic interactions and electromagnetic emission. Using a perturbative approach, we derive solutions for the equations of motion, total energy loss rate, and gravitational wave (GW) phase evolution. Our findings indicate that dephasing effects caused by magnetic fields of 10^15 G can produce sizable contributions to the gravitational wave signal detectable by third-generation gravitational wave detectors. In the deci-hertz band, our analysis suggests that even weaker fields of 10^14 G could leave a detectable imprint. Thus, future GW observations could offer a novel way to probe neutron star magnetic fields, providing insight into magnetar binaries and their formation channels.

Testing general relativity using binary black hole orbital frequency evolution on time-frequency plane

• Debtroy Das (Fudan University, Shanghai)

The orbital evolution of binary black hole (BBH) systems is determined by the component masses and spins of the black holes and the governing gravity theory. General relativity (GR) is the simplest theory of gravity that lays the foundation for successfully explaining the current gravitational wave (GW) observations. We present a method of stacking up the time-frequency pixel energies through the orbital frequency (OF) evolution with the flexibility of gradually shifting the OF curve along the frequency axis. The time-frequency spectrogram is obtained using a high-resolution Synchroextraction method. We observe a distinct energy peak corresponding to the GW signal's quadrupole mode. If an alternative theory of gravity is considered and the analysis of the BBH orbital evolution is executed following GR, the energy distribution on the time-frequency plane will be significantly different. We propose a new consistency test to check whether GR explains the BBH orbital evolution. Finally, through the numerical simulation of beyond-GR theory of gravity and utilizing the framework of second-generation interferometers, we demonstrate the efficiency of this new method in detecting any possible departure from GR.

Black Holes in Einstein-Scalar-Gauss-Bonnet Theories

• Jutta Kunz (University of Oldenburg)

Black holes represent an ideal laboratory to test Einstein’s theory of general relativity and alternative theories of gravity. Among the latter, Einstein-Scalar-Gauss-Bonnet Theories have received much attention in recent years. In this talk some properties of their black holes are recalled, which depend significantly on the coupling function of the scalar field. Linear mode stability of the black holes is addressed for some of the coupling functions. It is shown that the inclusion of an additional coupling to the curvature scalar leads to quadrupole and hexadecupole instabilities of the radially stable static black holes.

Black holes with scalar hair: from no-hair theorems to non-linear dynamics

• Thomas Sotiriou (University of Nottingham)

According to General Relativity, astrophysical black holes are remarkably simple and their properties are determined by just two quantities, their mass and their angular moment. Gravitational waves and other strong gravity observations promise to probe the nature of black holes more precisely that ever before. Any observed deviation from the simple description General Relativity provides can reveal the existence of new fundamental fields, which would signal a paradigm shift in theoretical physics, astrophysics, and cosmology. I will use the well-studied case of an additional new scalar field, coming from either an extension of the Standard Model or of General Relativity itself, as a case study to discuss 3 questions: Can new physics leave an imprint on black holes? If yes, which observations are more sensitive to this new physics? And, are all black holes the same?

Asymmetric binaries: a tale of two sides

• Andrea Maselli (Gran Sasso Science Institute)

In this talk I will discuss in which cases black holes carry a scalar charge, and the implications when the latter scales with the black hole mass. I will talk about the phenomenological consequences of these insights for the physics of compact binaries, and how asymmetric systems evolving in the LISA band are ideal sources for searches of new fundamental fields coupled to gravity. I will lay out the framework for modelling such binaries in an effective field theory approach, and present some first forecasts on LISA’s ability to constrain the properties of scalar fields from future gravitational wave observations

Numerical Relativity beyond General Relativity

• Miguel Bezares (University of Nottingham)

We will explore the long path from Einstein's equations to computational simulations. I will discuss how Numerical Relativity can serve as a tool to study nonlinear dynamics in alternative theories of gravity. In this talk, I will consider two subclasses of the broader Horndeski theory: those that have a screening mechanism and those involving scalar-Gauss-Bonnet gravity. In particular, I will discuss the rich phenomenology that can be learned, the challenges that must be overcome to extend Numerical Relativity, and the future of relativistic numerical simulations in these theories.

Non-linear dynamics beyond GR

• Llibert Areste Salo

In this talk I will discuss our recent progress in performing numerical simulations in the Einstein-scalar-Gauss-Bonnet theory of gravity, both for equal and unequal mass binary black hole systems, which result in waveforms with dephasings from GR consistent with PN calculations.

Well-posedness and gravitational collapse of self-interacting vector fields

• Marcelo Rubio

I will discuss the Cauchy problem for self-interacting massive vector fields, often facing instabilities and apparent pathologies when performing numerical simulations. After showing that these issues are due to the breakdown of the well-posedness of the initial-value problem, I will show how the pathologies can be classified, building on previous work done for 𝑘-essence, and how these issues can be avoided by "fixing the equations”, enabling stable numerical evolutions in spherical symmetry. Finally, I will display initial configurations for the massive vector field which lead to gravitational collapse and the formation of black holes in theories with cubic self-interactions.

Gravitational Waves signatures from non-trivial realisations of the Cosmological Principle

• Jose Beltrán Jiménez (University of Salamanca)

The Cosmological Principle is one of the pillars of the standard model of cosmology and it is commonly realised in a trivial way with homogeneous SO(3)-scalars. I will discuss several scenarios where the matter sector realises the Cosmological Principle in a non-trivial manner by resorting to combinations of spacetime and internal symmetries. A natural consequence of some of these scenarios is the appearance of a second helicity-2 mode in the cosmological perturbations that produces oscillations of gravitational waves with distictive signatures.

Signatures from metastable oppositely-charged black hole binaries in scalar Gauss-Bonnet gravity

• Maxence Corman (Max Planck Institute for Gravitational Physics)

Gravitational wave observations of compact objects have provided new opportunities to test our understanding of gravity in the strong-field, highly dynamical regime. To perform model-dependent tests of General Relativity with these observations, as well as to guide theory-agnostic tests, it is crucial to develop inspiral-merger-ringdown waveforms in alternative theories of gravity. In this talk, we present an example of a type of binary system in an alternative theory of gravity that undergoes a sudden state transition during the inspiral. This gives rise to telltale signatures such as a change in the multipolar character of the scalar radiation and the introduction of eccentricity in the orbit.

Binary neutron star mergers in massive scalar-tensor theory

• Alan Tsz Lok Lam (Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam)

We study binary neutron stars in the framework of Damour-Esposito-Farese-type (DEF) scalar-tensor theory of gravity with a massive scalar field. In this talk, I will start from the quasiequilibrium sequences of binary neutron stars, paying particular attention to the case where neutron stars are already spontaneously scalarized at distant orbits, i.e., in the high-coupling constant case. In particular, we are able to constrain the scalar mass from gravitational wave observations of binary neutron star mergers by inspecting the dephasing. Then, I will discuss the properties of post-merger remnants of binary neutron star mergers resulted by numerical relativity simulation and provide a distinctive signature in gravitational waves signal.

Black Hole Dynamics in Extensions of General Relativity

• Farid Thaalba

Extensions of General Relativity can give rise to black holes with nontrivial scalar structure, commonly called "hairy" black holes. This work investigates the dynamics of spherically symmetric black holes in scalar–Gauss–Bonnet gravity through fully nonlinear simulations incorporating excision techniques. These theories often suffer from an ill-posed initial value formulation, which limits their predictive power. We show that, within the framework of effective field theory, including appropriately chosen interactions, resolves this issue. Our results offer a path toward restoring predictivity in extended gravitational theories and provide new insights into their consistent formulation.

Black holes and wormholes in semiclassical gravity

• Sergei Solodukhin (University of Tours)

A black hole quantum state (Hartle-Hawking, Boulware or Unruh) is usually defined in a fixed background of a classical black hole. In my talk I will discuss the corresponding space-time geometry when the back-reaction is taken into account. The important questions include: does the back-reacted geometry always contain a horizon?; how it depends on the choice of the quantum state? and what is the right choice for the quantum state for the non-physical fields such as ghosts? I will answer these and other questions in the context of a two-dimensional dilaton gravity.

Gravitational wave tests of generic EFT-inspired theories of gravity

• Laura Bernard (LUX (CNRS / Observatoire de Paris-PSL))

Next generation of gravitational wave detectors will have the sensibility to detect potential deviations in gravitational waveforms with respect to general relativity. However, current agnostic tests of gravity with gravitational waves are plagued by a lack of realistic deviations, making it difficult to interpret such detections with respect to specific theories. In this talk, I present a dictionary that identifies the scaling of deviations with the objects’ masses and the leading order post-Newtonian corrections in generic theories constructed through the Effective Field Theory approach based on curvature. In particular, I will demonstrate that a vast set of theories only deviates from General Relativity beginning at a relatively high order. The obtained results can be readily incorporated in all gravitational wave tests already under use by current detectors.

Stationary and axisymmetric solutions in modified gravity

• Adolfo Cisterna (Institute of Theoretical Physics, Charles University)

This talk will address the study of stationary and axisymmetric exact solutions in scalar-tensor theories. We will review de Weyl problem in GR and its stationary extension, to present then a generalisation valid in the presence of scalar fields. We will show new rotating spacetimes in four and five dimensions and a systematic procedure to construct them. Extensions to generalised scalar-tensor theories are discussed as well.

Black hole scalarization at all scales

• Lodovico Capuano (Scuola Internazionale Superiore di Studi Avanzati)

The most well-studied class of theories of gravity beyond General Relativity is represented by scalar-tensor gravity, in which at least one fundamental scalar degree of freedom is included in the gravitational sector. This family is theoretically appealing due to its simplicity and due to its capability to describe cosmic dynamics at large scales. Within this class, a particularly relevant theory is the one in which the scalar field is coupled to the Gauss-Bonnet invariant. In fact, in this theory BHs can exist in different branches of solutions, including the GR solution and different "scalarized" ones. The transition of black holes from the GR branch to a scalarized one is a process known as scalarization. I will discuss this phenomenon in BH binary systems, as well as its possible implications in gravitational wave observations.

New gravity experiments

• Claus Laemmerzahl (University of Bremen)

The presentation consists of three parts: (i) new improved experiments of foundations of GR, (ii) proposals of new and improved experiments, and (iii) new thoretical frameworks to discuss these type of experiments. We discuss new tests of the equivalence of inertial, passive gravitational, and active gravitational mass, of the gravitational redshift, and also will propose an improved test of the equivalence of active and passive masses. Regarding the theoretical framework we propose a covariant formulation of the Weak Equivalence Principle which then also naturally combines with an equivalence principle for rotational motion. Furthermore, the use of a pre-metric version of the Einstein equations allows new tests of GR and also leads to new metrological triangles allowing an improved method to determine GR effects.

Black holes in modified gravity theories with large-curvature corrections

• Leonardo Gualtieri (Università di Pisa)

The direct observation of gravitational waves gives us the opportunity to test gravity in the highly dynamical, strong curvature regime probed by coalescing black hole binaries. In particular, gravitational wave detectors are sensitive to large-curvature corrections of general relativity, mostly unconstrained by other astrophysical observations. However, theoretical consistency imposes significant constraints and limitations to the form and to the scale of these corrections. I will discuss our present understanding of the large-curvature corrections to general relativity which can leave a signature observable by present and near-future gravitational wave detectors. In particular, I will discuss some recent developments in our understanding of this fascinating family of modified theories of gravity.

Black holes with electroweak hair

• Mikhail Volkov (Univerity of Tours, France)

We construct static and axially symmetric magnetically charged hairy black holes in the gravity-coupled Weinberg-Salam theory. Large black holes merge with the Reissner-Nordstr\"om (RN) family, while the small ones are extremal and support a hair in the form of a ring-shaped electroweak condensate carrying superconducting W-currents and up to $22\%$ of the total magnetic charge. The extremal solutions are asymptotically RN with a mass {\it below} the total charge, $M<|Q|$, due to the negative Zeeman energy of the condensate interacting with the black hole magnetic field. Therefore, they cannot decay into RN black holes. As their charge increases, they show a phase transition when the horizon symmetry changes from spherical to oblate. At this point they have the mass typical for planetary size black holes of which $\approx 11\%$ are stored in the hair. Being obtained within a well-tested theory, our solutions are expected to be physically relevant.

General stationary axisymmetric spacetimes: circularity and beyond

• Eugeny BABICHEV

I will discuss properties of general stationary and axisymmetric spacetimes, with a particular focus on circularity - an accidental symmetry enjoyed by the Kerr metric, and therefore widely assumed when searching for rotating black hole solutions in alternative theories of gravity as well as when constructing models of Kerr mimickers. It can be shown the local existence of a Kerr-like gauge, specified by six free functions. Within this gauge the differential circularity conditions can be solved to translate them into algebraic relations among the metric components. This result opens the way to investigating the consequences of circularity breaking in a controlled manner. In particular, I will show two simple analytical examples of non-circular deformations of the Kerr spacetime and discuss their properties.

Ringdown tests of the black-hole paradigm

• Elisa Maggio (Max Planck Institute for Gravitational Physics, Albert Einstein Institute, Potsdam)

Gravitational waves offer the promising prospect of testing one of the main predictions of general relativity, namely the presence of black holes beyond which nothing can escape. The ringdown is the final stage of a compact binary coalescence when the remnant settles down to a stationary configuration. It is modelled as a superposition of exponentially damped sinusoids whose frequencies and damping times are related to the remnant’s oscillation frequencies, the so-called quasinormal modes. In this talk, I will describe how parametrised tests of general relativity can test the black-hole paradigm by constraining deviations in the frequency and damping time of the quasinormal modes. I will also describe how false violations of general relativity can arise in the data due to missing physics in the waveform models or poorly understood noise artefacts.

The spectrum of quasinormal modes of rapidly rotating Einstein-Gauss-Bonnet-dilaton black holes

• Jose Luis Blázquez-Salcedo (Universidad Complutense de Madrid)

In this talk we will discuss the quasinormal mode spectrum of rapidly rotating black holes in Einstein-Gauss-Bonnet-dilaton theory, which is crucial for understanding the ringdown phase that follows from a black hole merger. Unlike previous studies that relied on pproximations, we compute the QNM spectrum non-perturbatively, providing robust results even for large coupling constants. Using a spectral decomposition of the metric and scalar field perturbations, we solve a system of coupled partial differential equations to determine the spectrum. We will discuss some properties of the spectrum, such as the breaking of isospectrality and dependence of the modes on the angular momentum and coupling parameter of the theory.

Advances in the Parametrized QNM Framework and New Perspectives from Bound States

• Sebastian Völkel (Max Planck Institute for Gravitational Physics (AEI))

Black hole spectroscopy, applied to the ringdown phase of compact binary mergers, is one of the most promising tools to test the nature of black holes. It allows us to quantify whether black holes and their perturbative dynamics are well described by general relativity and, thus, serve as a magnifier to explore fundamental physics. The parametrized quasi-normal mode framework has been developed as a powerful tool to capture small changes in black hole dynamics caused by small deviations from general relativity. It provides a unique role to mediate between theory-specific approaches and fully agnostic data-driven ones. In this talk, I will first review some of the seminal works and contributions of the framework. Then, I will focus on recent progress, including the framework's first systematic study in the time domain and its extension to rotating black holes. Finally, I will use the intriguing idea to relate quasi-normal modes with the bound states of an inverted potential to address ongoing discussions of spectral stability and the excitation of quasi-normal modes.

Nonlinearities in black hole ringdown

• Adrien Kuntz (CENTRA, Instituto Superior Tecnico, Lisbon)

Nonlinear effects in black hole perturbation theory may be important for describing a black hole ringdown, as suggested by recent works. I will describe a new class of "quadratic" quasi-normal modes at second order in perturbation theory. Remarkably, not only their frequency but also their amplitude is completely determined by the linear modes themselves. I will present how one can compute them using Leaver's algorithm. Quadratic modes could be used to improve ringdown models by adding nonlinear features without introducing any supplementary free parameter for data analysis purposes, or to test GR in the nonlinear regime.

Quasinormal modes of charged and static wormholes

• Pablo Navarro Moreno (Universidad Complutense de Madrid)

In this talk we discuss the quasinormal mode (QNM) spectrum of charged and static wormholes, which is interesting for understanding gravitational-wave phenomena in this kind of objects. We compute the QNMs employing a spectral method, which allows us to study in a systematic way the properties of the modes as we vary the charges of the wormholes. We discuss several properties of the QNM spectrum, such as the different families of modes we find (gravitational, scalar and electromagnetic -led modes), isospectrality, excitations, comparison with the spectrum of the Ellis-Bronnikov wormhole, and stability under linear perturbations.

A new model of spontaneous scalarization induced by curvature and matter

• Zakaria Belkhadria (Université de Genève & Università di Cagliari & GWSC & INFN)

We present a model of black hole scalarization where a scalar field couples simultaneously to the Gauss–Bonnet invariant and the electromagnetic Maxwell field. This combined interaction broadens the conditions for spontaneous scalarization. We track how the electric charge and the two coupling constants control the onset of the scalar field and uncover new solution branches with non-trivial scalar profiles. Scalarization occurs across a wide range of parameters and even with negative Gauss–Bonnet coupling at sub-extremal charge ($q$). Scalar clouds and fully scalarized black holes form above several mass thresholds; varying the Maxwell coupling or the charge makes the highest threshold mass almost three times the lowest. Multiple branches of scalarized solutions converge to the same final state, indicating non-unique growth of scalar hair. The model also produces overcharged black holes ($q>1$) while the Gauss–Bonnet coupling remains positive. Several examples show horizon areas larger than those of the Reissner–Nordström solutions with the same mass and charge. The Maxwell term shifts the scalarization onset and tends to stabilize the solutions, as seen from the evolution of the scalar charge and horizon quantities. These results provide an alternative route to scalarization, may avoid the instabilities of curvature-only or matter-only models, and open new possibilities for testing strong-gravity effects in upcoming observations.

Rotating Black Holes with Scalar Hair: Imaging in Non-Flat Target Spaces

• Galin Gyulchev (Sofia University "St. Kliment Ohridski")

We investigate the visual appearance of thin accretion disks and their embedded shadows around rotating black holes endowed with scalar hair, generated by two time-periodic scalar fields whose target space possesses non-flat geometry. These solutions extend beyond the Kerr framework and exhibit a non-trivial geodesic structure that significantly influences observable features. By varying the normalized scalar charge and the Gaussian curvature of the target space, we identify distinct transitions in both disk and shadow morphology. As the system approaches the boson star limit, the shadows fragment into highly chaotic, disconnected regions. At intermediate values of the scalar charge, a well-defined central shadow emerges, with its shape closely linked to the curvature. The accretion disk images reflect this trend: low curvature yields distorted, non-simply connected patterns, whereas higher curvature produces more coherent and symmetric configurations. For low scalar charge, the visual signatures converge toward those characteristic of a Kerr black hole.