Laura Donnay: Null infinity and the black hole horizon: new conserved quantities from a geometric duality Room: Amphi Bio, Batiment F I will present a unified treatment for the boundary geometry of asymptotically flat spacetimes and black hole horizons. This geometric duality implies an exact inversion isometry for extremal, non-rotating horizons, which will be used to relate and derive new infinite towers of conserved quantities.

Sebastian Gurriaran: Non-linear instability of the Kerr Cauchy horizon near timelike infinity Room: Amphi Bio, Batiment F The Kerr metrics model rotating vacuum black holes, and are expected to play a central role in the long-time description of generic solutions to the Einstein vacuum equation. They present the disturbing feature that determinism breaks down inside the black hole, beyond the Cauchy horizon. Penrose's Strong Cosmic Censorship conjecture states that this behavior is however unstable and vanishes upon small perturbations. I will present a recent work which proves that, assuming a non-linear Price's law-type estimate near the event horizon, in generic perturbations of Kerr black holes ruled by the full, non-linear, Einstein vacuum equation, a singularity forms at the Cauchy horizon near timelike infinity. This singularity - at the Lipschitz level for the metric - prevents the unphysical extensions and recovers determinism.

Filippo Vernizzi: Static Quadratic Love Numbers Room: Amphi Bio, Batiment F Tidal Love numbers quantify the deformability of compact objects under external tidal fields. They are key quantities in gravitational‑wave astronomy for accurately modeling waveforms during the final stage of an inspiral and carry information about the microphysics of the object. I will present a framework for computing tidal Love numbers beyond linear order by matching relativistic perturbation theory of compact objects with the worldline effective field theory approach used to define their tidal deformability.

Alexandros Kehagias: The AdS Perspective on the Nonlinear Tails in Black Hole Ringdown Room: Amphi Bio, Batiment F Black holes gradually settle into their static configuration by emitting gravitational waves, whose amplitude diminish over time according to a power-law decay at fixed spatial locations. We show that the nonlinear tails in the presence of a quadratic source, which have been recently found to potentially dominate over the linear ones, can be simply derived from the AdS_2 × S^2 spacetime perspective with their amplitudes being related to the Aretakis constants.

Ludovic Souetre: The homogeneous Robin boundary conditions for asymptotically Anti-de Sitter spaces Room: Amphi Bio, Batiment F Modelled on the Anti-de Sitter space, asymptotically Anti-de Sitter spaces can be defined as Lorentzian manifolds that possess a timelike conformal boundary. Due to their lack of global hyperbolicity, finding asymptotically Anti-de Sitter solutions to the Einstein equations (necessarily with a negative cosmological constant) through the Cauchy problem requires tackling the latter as an initial boundary value problem. In this talk, I will present the two known types of geometric boundary conditions leading to the local existence and uniqueness of solutions in dimension 4: the Dirichlet boundary conditions, which were introduced by Friedrich in 1995, and the homogeneous Robin boundary conditions, which I introduced.

Aadharsh Raj Soundararadjan Indumathi: Flat from AdS: In Any Dimension, for Any Spin Room: Amphi Bio, Batiment F The space of solutions to the free equations of motion for massless fields of arbitrary integer spin in Minkowski spacetime is recovered as a smooth limit of the anti-de Sitter solution space for any even spacetime dimension. The infinite set of boundary data near null infinity that characterise solutions in Minkowski spacetime is obtained from an expansion of the anti-de Sitter source and vev in powers of the cosmological constant. In particular, the source gives rise to the analogue of the gravitational shear tensor, while the vev yields the analogues of the mass and angular-momentum aspects, as well as the subleading infinite tower of boundary data. These identifications are further supported by the branching of the source and vev into representations of the Lorentz algebra identified with the conformal algebra of the celestial sphere.

Vagif Tagiev: Wormholes as the black hole mimickers and their scalar Love numbers Room: Amphi Bio, Batiment F In this talk, I plan to discuss dynamical scalar Love numbers for two types of static wormhole metrics. I will also discuss how these numbers may make it possible to distinguish not only black holes from wormholes, but also different types of wormholes that appear topologically similar, at least at first glance.

Weidong Zhang: Nonlinear stability of Minkowski spacetime for massive Dirac spinor fields Room: Amphi Bio, Batiment F I will present a work in collaboration with P.G. LeFloch and Yue Ma on the nonlinear stability of Minkowski spacetime. We consider the massive Einstein-Dirac system and investigate the global evolution problem when the initial data are sufficiently close to those describing a spacelike, asymptotically Euclidean slice in Minkowski spacetime. We establish the existence of a globally hyperbolic development that remains asymptotic to Minkowski spacetime in the future timelike, null, and spacelike directions. Previous results on this problem have been limited to the massless Einstein-Dirac system. Our analysis follows closely the asymptotically hyperboloidal-Euclidean framework introduced by LeFloch and Yue Ma for the massive Klein-Gordon-Einstein system. The structure specific to spinor fields and the Dirac equation necessitates significantly new elements in the proof. In contrast with prior approaches, our treatment of spinor fields and the Dirac equation is fully gauge-invariant, relying on the formalism of Lorentz Clifford algebras, principal fiber bundles, and Dirac forms.

Eugeny Babichev: Beyond circularity Room: Amphi Bio, Batiment F The Kerr metric enjoys a special symmetry known as circularity, which is normally assumed when constructing rotating black hole solutions in alternative theories of gravity and when building Kerr-mimicker models. In this talk I will examine how justified this assumption is and what happens when it breaks. Working within a geometrically natural gauge, the circularity conditions are solved analytically and translated into algebraic relations among metric components, enabling controlled study of circularity-breaking deformations. Explicit analytical examples of non-circular Kerr deformations are presented, highlighting how the horizon can lose its Killing nature even when its location and the ergosphere remain unchanged. These results lay the groundwork for more general parametrizations of rotating black holes and their observational signatures beyond general relativity.

Jibril Ben Achour: Acceleration of charged particles in the Kerr magnetosphere: An electrogeodesic approach Room: Amphi Bio, Batiment F The launch of relativistic jets of plasma on astrophysical to cosmological scales is observed in a variety of astrophysical sources, from active galactic nuclei to X-ray binaries. While these jets can be reproduced by general relativistic magneto-hydrodynamics (GRMHD) and particle-in- cells (GRPIC) simulations of the dynamical Kerr magnetosphere, the development of analytic models to describe the physics of the jets has remained limited. A key challenge is to analytically describe the individual trajectories of accelerated charged particles, which ultimately build up the jet and emit radiation. In this talk, I will review a first simple but fully analytical model of jet launching from the Kerr magnetosphere based on the motion of charged particles. To that end, I will review the condition for the integrability of electrogeodesic motion in the Kerr monopole magnetosphere allowing one to study the ejection of charged particles near the poles. Based on this, I will discuss (i) a criterion for the rotation axis to constitute a stable latitudinal equilibrium position, thereby representing an idealized jet, (ii) the expression for the magnetic frame-dragging effect, and (iii) the condition for an asymptotic observer to measure blueshifted particles emanating from the black hole surroundings. This will show that particles can be accelerated only in a specific region whose maximal radius depends on the spin and magnetization of the black hole. Alongside these results, I will review the role of the explicit and hidden symmetries when building models for the Kerr magnetosphere. Based on: [https://arxiv.org/abs/2601.05048][1] [1]: https://arxiv.org/abs/2601.05048

Juan Valiente Kroon: Conformal geodesics and timelike infinity —revisiting the semiglobal stability of the Minkowski spacetime Room: Amphi Bio, Batiment F The seminal result by H. Friedrich on the semiglobal stability of the Minkowski spacetime shows that hyperboloidal initial data for the Einstein field equations which is suitably close to data for the Minkowski spacetime and conformally smooth gives rise to a future development which is future geodesically complete and has the same global structure as the comparable region in the Minkowski spacetime. Moreover, the resulting spacetime is conformally smooth and the generators of null infinity intersect at a point describing timelike infinity. In this talk I will show how a gauge based on the properties of a particular type of conformal invariants, the so-called conformal geodesics, gives rise to an alternative conformal representation of this class of spacetimes in which timelike infinity is described by a hyperboloid describing the endpoints of timelike geodesics. This representation provides an implementation of “scri-fixing” —that is, a gauge in which the location of null infinity is described as the locus of points with a fixed spatial coordinate. Finally, it will be shown how this conformal representation is naturally adapted to the use of ideas and methods of Melrose’s school of microlocal analysis. This observation opens the possibility of obtaining generalisations of Friedrich’s semiglobal results for spacetimes with polyhomogeneous asymptotics.

Marc Herzlich: From the ADM mass to a topological invariant of low-dimensional manifolds Room: Amphi Bio, Batiment F Initially defined in the context of General Relativity, the ADM mass is an important invariant of Riemannian asymptotically flat manifolds. Some 25 years ago, L. Habermann used it to answer a question in pure Riemannian geometry : finding a canonical metric in each conformal class of Riemannian metrics with positive scalar curvature on a compact manifold of dimension 3, 4, or 5. Despite the importance of the question in geometry, the Habermann metric seems to have escaped further notice. In a joint work with Emmanuel Humbert (Tours), we study its volume, which may lead to an interesting new topological invariant of low-dimensional manifolds.

Simon Raulot: Positive energy theorems for charged spin initial data Room: Amphi Bio, Batiment F I will present recent results on positive energy theorems for the Einstein–Maxwell equations in the setting of asymptotically flat spin initial data. A spinorial approach leads to positivity results for the total energy–momentum. We then focus on questions in the Riemannian setting. This naturally leads to the notion of charged parallel spinors, whose existence is closely tied to the equality case and model geometries such as the Reissner–Nordström and Majumdar Papapetrou solutions.

Alessia Platania: Quantum Gravity at the Crossroads: Asymptotic Safety, Effective Field Theory, and the Swampland Room: Amphi Bio, Batiment F Quantum gravity remains fragmented across scales and frameworks: in the UV, a number of alternative theories aim to describe the fundamental aspects of gravity; in the IR, EFT considerations and observations can constrain beyond GR physics in a theory-agnostic fashion. Rather than treating these domains as separate, I will argue for a unifying approach: by extending the logic of the swampland program, one can attempt to map UV theories into the corresponding “landscapes” of EFTs, thus establishing a common language to compare different quantum-gravity frameworks among each other and with IR constraints. In this talk I will use this approach to illustrate how ideas from Asymptotic Safety, the Swampland Program, and EFT can inform and challenge each other.

Maël Chantreau: Spi and symmetries at spatial infinity Room: Amphi Bio, Batiment F According to Strominger, Weinberg's soft graviton theorem is related to the fact that the BMS group is a symmetry of the S matrix. For this to even make sense, the BMS group must be promoted from two BMS groups acting on asymptotic states to a global BMS group. In this, spatial infinity plays a pivotal role as the link between the two. In this talk, we will introduce an extended boundary, called Spi, at spatial infinity. This boundary is canonically defined and possesses a strongly Carrollian structure. Preservation of this strongly Carrollian structure in a suitable way leads to the appearance of the BMS group at spatial infinity, up to some parity condition. We will show that this parity condition is related to regularity issues at Scri.

Mikhail Markov: Compactifying Gauge Theories Room: Amphi Bio, Batiment F I plan to discuss applications of the Gauge PDE formalism to the study of gauge field theories with asymptotic boundaries. The main ingredient is a notion of compactification for gauge field theories inspired by graded geometry, leading to a natural generalization of Penrose's idea of conformal compactification of gravity. As an application, the resulting framework gives rise to natural action functionals for asymptotic boundary data. In particular, it provides a derivation of actions for Carrollian field theories on null infinity from theories on Minkowski spacetime. In a certain sense, this procedure is analogous to the extraction of the logarithmic term in the renormalized on-shell action familiar from the AdS/CFT context, but it applies to arbitrary gauge field theories with asymptotic boundaries.

Peter Horvathy: Memory Effect in Gravitational Memory: an approximate description Room: Amphi Bio, Batiment F Abstract: The large-distance behaviour of a sandwich gravitational wave by a continuous but not necessarily smooth profile provides us with an approximate analytic description of particle motion in a gravitational wave, as spelled out for the Pöschl-Teller profile. Our approximate models are consistent with the Carroll symmetry. Authors: Q-L Zhao (Hangzhou), P.-M. Zhang (Zhuhai), M. Elbistan (Istanbul); P. Horvathy (Tours), presented by P. Horvathy

Pierre Vanhove: Exact Metric solutions from a cubic action Room: Amphi Bio, Batiment F In this talk we describe a method for deriving exact metric solutions from a cubic worldline action in every dimensions from an amplitude computation. With this formalism one can derive the geodesic motions of a test particle in such background. The same formalism is suitable for the self-force expansion in a post Minkowskian formalism. This is based on work with Stavros Mougiakakos and work in progress.

Armand Coudray: Existence and uniqueness of solutions to the conformal constraint equations Room: Amphi Bio, Batiment F During the talk, I will present the results obtained in collaboration with Romain Gicquaud concerning the classical construction of initial data using the conformal method, which was originally proposed by Holst, Nagy and Tsogtgerel and later refined by Maxwell. This method transforms the usual constraint equation for initial data in general relativity into a set of two coupled nonlinear elliptic PDEs. Our work revisits the standard proof by removing certain assumptions. In particular, I will explain how our proof guarantees the uniqueness of solutions to the equations of the conformal method as soon as a bound is imposed on the physical volume and how it provides an explicit construction of solutions.

Tamanna Jain: Post-Minkowskian, Effective-one-body formalism, and Numerical Relativity : beyond GR+Standard Model Theories Room: Amphi Bio, Batiment F In this talk, I will present our recent work on post-Minkowskian EFT for scalar-tensor theories and boson stars. We first derive the analytical expressions of the scattering angle using PM-EFT techniques for both the cases, and in particular provide the first analytical treatment of boson stars as a two-body problem. We then derive the effective-one-body description of these systems. We then compare analytic results to the scattering angle extracted from sequences of numerical-relativity simulations at fixed energy, varying impact parameter, and coupling strength. We find excellent agreement for both the cases. In particular for boson stars, the very good agreement exhibits the attractive (repulsive) effect of in-phase (out-of-phase) binaries. For small impact parameters, where the stars approach more closely before separating to infinity, the scalar-field interaction is found to dominate.

Celine Zwikel: New symmetries at spatial infinity Room: Amphi Bio, Batiment F I will investigate the asymptotic symmetries of four-dimensional asymptotically flat spacetimes at spatial infinity using covariant phase space methods. I will show that novel symmetries can be realized—beyond those identified at null infinity—namely logarithmic translations and log supertranslations. The associated charges are finite and conserved, and we show that their algebra admits non-trivial central extensions.

Julio Parra Martinez: Black holes and the renormalization group Room: Amphi Bio, Batiment F Tidal Love numbers quantify the finite-size properties of Black Holes and Neutron Stars, such as absorption, dissipation and their response to external fields. Perhaps surprisingly, even in classical general relativity, they undergo renormalization group running due to nonlinearities. In this talk I will explain some exact results about their running, which can be extracted using scattering amplitudes in black-hole perturbation theory and point-particle effective theories (EFT). Due to the universality of the EFT, these results also have applications to the physics of neutron stars, binaries and their signals in gravitational wave observatories. The framework I will present also provides a way to calculate the precise values of both static and dynamical Love numbers of black holes in various dimensions, avoiding ambiguities in other methods.