Welcome address

• The TUG Collective

Room: Amphi Claude Bloch

The S-matrix bootstrap and other bootstraps: a lightning review

• Piotr TOURKINE (LAPTh, CNRS, Annecy)

Room: Amphi Claude Bloch

Wheeler-DeWitt equation for flat minisuperspace models and emergence of time

• Herve Partouche (CPHT-Ecole Polytechnique)

Room: Amphi Claude Bloch The wavefunction of the universe is a solution of the Wheeler–DeWitt equation obtained through the canonical quantization of the Hamiltonian. Different ways of ordering the operators in this Hamiltonian then lead to an ambiguity in the equation. From the path-integral perspective, the wavefunction is also ambiguous, due to the multitude of possible choices for the path-integral measure. We have shown how to completely resolve these ambiguities in simplified cases, where the universe is assumed to be closed and homogeneous. Once the correct expression of the wavefunction is established, we demonstrate how the notion of time in the classical trajectory emerges from the wavefunction, even though the latter does not involve the notion of time in any way.

Effective Microstructure

• Raphaël Dulac (IPHT- CEA Saclay)

Room: Amphi Claude Bloch Large families of smooth, horizonless microstate geometries have been constructed over the last 20 years. These solutions are often quite complicated, and working with them can be a technical challenge. However we will show that there are intermediate effective descriptions that can capture specific stringy and geometrical features of heavy pure states.We will exhibit some effective descriptions based on families of multi-centered solutions. These solutions are generically not smooth, but they capture important physical features, like the migration of the momentum charge, inherent in their smooth counterparts. Our examples include new fully-back-reacted shockwaves on circular supertubes, two-charge (D1-P or F1-P) probes of circular supertubes and spectral flowed supertubes. Our results suggest that effective descriptions can be extended to more general microstates, including non-supersymmetric solutions. We expect our results, and generalizations, will be invaluable in more phenomenological studies of microstructure.

A smooth bounce in radiation domination

• Siméon Vareilles (Centre de Physique Théorique)

Room: Amphi Claude Bloch We introduce time as a “test field” in minisuperspace quantum cosmology. We argue that the time dependent wavefunction of the universe satisfies a Schrödinger equation and select solutions where the time field has negligible backreaction on the metric. By applying this idea to a radiation-dominated epoch, we model show that the problem reduces to that of a quantum particle in an $s$-wave state scattering off an attractive power-law potential $V(r)\sim -r^{-2/3}$. Quantum mechanically, this potential admits bound states and yields a smooth, unitary bounce—absent in the classical theory—where the uncertainty and expectation value of the scale factor are comparable. By choosing a large initial variance, the bounce can be made arbitrarily smooth, with a correspondingly soft evolution of the mean Hubble rate.

Examining the Anomalous Nature of Chiral Effects in Thermodynamics

• Diego Saviot

Room: Amphi Claude Bloch Quantum anomalies give rise to novel transport phenomena, including the generation of a current in a relativistic fluid due to the presence of magnetic field or vorticity. I will present an exclusive and direct computation of the chiral anomaly within the path integral for a massless fermion on a generic electromagnetic and curved background, including local temperature and chemical potential. I will identify new thermodynamical contributions to the anomaly which induce the Chiral Separation and Vortical Effects. Additionally, I will show that the anomaly fully vanishes at global equilibrium.

Symmetries of asymptotically flat spacetimes

• Céline Zwikel (Collège de France)

Room: Amphi Claude Bloch

The Born-Oppenheimer approximation in quantum cosmology

• Patrick PETER (CNRS)

Room: Amphi Claude Bloch Cosmology usually assumes quantum perturbations on top of a mostly classical background. In quantum cosmology, one quantizes the background as well and then write the full quantum state as $|\Psi_\text{tot}^{(\text{BO})}\rangle = |\Psi_\text{bckg} (a,t)\rangle \otimes |\Psi_\text{pert}[v(x^i,t);a(t)\rangle$, in which the unique background state $|\Psi_\text{bckg} (a,t)\rangle$ solves the zeroth-order Schrödinger equation and provides, often by means of taking an expectation value $\langle a \rangle$,a time dependent scale factor $a(t)$ to be plugged back into the second order perturbation equations. This so-called Born-Oppenheimer approximation is only an approximation, and going beyond it naturally induces the appearance of non-gaussianities, in practice a nonvanishing trispectrum with a universal shape.

Perturbative signatures of a superposition quantum universe

• Lisa Mickel (Institut d'Astrophysique de Paris)

Room: Amphi Claude Bloch In the quest of finding a quantum description of the early universe, we consider a quantised flat FLRW background together with quantum perturbations. We compute quantum trajectories for a universe that can be in a superposition of semiclassical background (and perturbation) states and investigate how the evolution of cosmological perturbations is influenced by the quantum nature of the background. In addition to probing the quantum nature of our universe, our results thereby pave the way to obtain insights into the physical consequences of ambiguities in the quantum theory.

An Open System Approach to Gravity

• Thomas Colas (DAMTP - University of Cambridge)

Room: Amphi Claude Bloch Several open problems in cosmology — including the nature of inflation, dark matter, and dark energy — involve spacetime-filling media with unknown microphysics, and can be probed so far only through their gravitational effects. This observation motivates a systematic open-system approach, in which gravity evolves in the presence of a generic, unobservable environment. In this talk, I will present a general framework for open gravitational dynamics based on General Relativity and the Schwinger-Keldysh formalism. As an application, I will describe the most general conservative and dissipative dynamics of scalar and tensor perturbations during single-clock inflation. I will conclude by discussing future prospects for modelling dissipative effects in the late universe.

Primordial black holes beyond spherical symmetry: Improved compaction function and exact solutions

• Jibril BEN ACHOUR (ENS de Lyon / Arnold Sommerfeld Center (Munich))

Room: Amphi Claude Bloch Understanding how the presence of primordial black holes (PBH) affects the cosmic history is a crucial endeavor for the refine our cosmological models. When studying PBHs, a key difficulty is that the notion drawn from the study of asymptotically flat and stationary black holes, such as the Schwarzschild and Kerr black hole solutions, are no longer available. PBH are by definition asymptotically FLRW and fully dynamical. Therefore, a characterization of their properties requires new tools to compute their quasi-local mass, characterize their dynamical horizons and define their temperature. Additionally, a central question is how to provide a gauge-invariant characterization for an inhomogeneity embedded in a cosmological spacetime to form a black hole. The current criteria based on the compaction function and the Misner-Sharp mass has revealed very useful but is limited to spherically symmetric collapse. In this talk, I will present two new results allowing to characterize asymptotically FRW black holes and thus PBH. First I will introduce a new notion of quasi-local mass adapted to asymptotically FLRW geometries which hold beyond spherical symmetry. Then I will discuss how this can be used to provide an improved notion of compaction function for PBHs beyond spherical symmetry. Finally, I will present new exact analytical solutions of General Relativity describing asymptotically flat axisymmetric black holes solutions embedded in cosmology and discuss how they can be understood as exact model of PBHs. They will serve to confront and test the new definitions introduced in this talk.

Cosmic strings: a decade in the gravitational wave era

• Pierre Auclair (UCLouvain)

Room: Amphi Claude Bloch Ten years after gravitational waves were first detected, we are now able to strongly constrain the existence of cosmic strings, and even see some of their signatures emerging in our observations. In this review presentation, I would like to provide an update on research into cosmic strings: how these objects form, how we understand their dynamics and their gravitational wave signatures. In particular, I would like to focus on the various points of contention within the community and the efforts being made to resolve them.

Momentum Spaces For Quantum Field Theory in de Sitter

• Arthur Poisson (IAP)

Room: Amphi Claude Bloch Cosmic inflation is the leading paradigm for describing the initial conditions of cosmological perturbations. The search for the most natural theoretical description is therefore a very important matter. Due to the relevance of quantum effects, this amounts to determining how to formulate quantum field theory in a quasi-de Sitter space-time, where the lack of time-translation invariance renders flat-space methods based on energy inapplicable. In this talk, I will explore two methods for constructing the Hilbert space of a quantum field theory in exact de-Sitter spacetime, based on its isometry group.

Mapping inflationary fields with plane waves

• Nathan Belrhali (Institut d'Astrophysique de Paris)

Room: Amphi Claude Bloch Inflation is an accelerated expansion era introduced at the beginning of universe timeline to address causality problems of the FLRW model. The simplest realization of inflation is a scalar field theory minimally coupled to gravity. This scalar field has a background classical part that drives the accelerated expansion, and quantum fluctuations that, stretched by this expansion, give rise to large scale structures. These fluctuations can be described as quantum fields that propagate in a curved time-dependent space-time. I will show how this complicated time-dependent propagation can be decomposed over plane waves by an integral transformation, introducing a convenient dual space for expressing physical quantities in inflation. In particular, we can map systematically n-point functions of inflationary fluctuations in terms of the corresponding flat space observable with linear shifts using this transformation.

Time-reversed Stochastic Inflation

• Baptiste Blachier (Université Catholique de Louvain)

Room: Amphi Claude Bloch Based on ArXiv 2504.17680, B. Blachier, C. Ringeval (2025) Cosmic inflation may exhibit stochastic periods during which quantum fluctuations dominate over the semi-classical evolution. Extracting observables in these regimes is a notoriously difficult program as quantum randomness makes them fully probabilistic. However, among all the possible quantum histories, the ones which are relevant for Cosmology are conditioned by the requirement that stochastic inflation ended. From an observational point of view, it would be more convenient to model stochastic periods as starting from the time at which they ended and evolving backwards in times. In some cases, it is even compulsory to compute observables with respect to local observers in order to avoid gauge artefacts. We present a time-reversed approach to stochastic inflation, based on a reverse Fokker-Planck equation, which allows us to derive non-perturbatively the probability distribution of the field values at a given time before the end of the quantum regime. As a motivated example, we solve the flat semi-infinite potential and express the probability distribution of the quantum-generated curvature fluctuations. We show that even when allowing eternal inflation to occur, the reverse-time scheme cure divergences: the probability distribution is finite, exhibits heavy tails and some features of Gaussian statistics are recovered for small curvature perturbations. This reverse-time stochastic-formalism enables a path-by-path conditioning by the lifetimes of the stochastic trajectories, and express probabilities in number of -folds in reference to the end of inflation hypersurface, hence it could be applied to any inflationary potentials and quantum diffusion eras, including the ones leading to the formation of primordial black holes.

WIMPs and New Physics Interpretations of the PTA Signal are Incompatible

• Yann Gouttenoire

Room: Amphi Claude Bloch In order to explain the large amplitude of the nano-Hertz stochastic gravitational wave background observed in pulsar timing arrays (PTA), primordial sources must be particularly energetic. This is correlated to the generation of large density fluctuations, later collapsing into ultra-compact mini-halo (UCMHs). I will show that if dark matter is made of WIMPs, then photon and neutrino fluxes from UCMHs produced by curvature peaks, first-order phase transition and domain wall interpretations of the PTA signal, exceed current bounds.

Burdening (or not) gravitational waves in the presence of primordial black holes

• Mathieu GROSS (IJCLab)

Room: Amphi Claude Bloch In this talk, I will present the gravitational wave signal expected from an early universe filled with an inflaton and some primordial black holes (PBH). I will present the 4 sources that are expected and explain how they are affected by the modification of the PBH evaporation through the "memory burden" effect.

Quantum gravity and asymptotic symmetries

• Simone Speziale (CPT Marseille, CNRS)

Room: Amphi Claude Bloch

Tensions in Cosmology: the central role of Type Ia Supernovae (remote)

• Mickael Rigault (IP2I)

Room: Amphi Claude Bloch

Beyond ΛCDM: Lessons and Anomalies from Recent Observations

• Rodrigo Calderon (Institute of Physics, Czech Academy of Science)

Room: Amphi Claude Bloch The standard cosmological paradigm, ΛCDM, has been remarkably successful in describing the universe. However, recent observations present intriguing challenges that may hint at a more intricate dark sector than conventionally assumed. In this talk, I will discuss recent constraints on dark energy derived from various cosmological probes, focusing on the latest Baryonic Acoustic Oscillation measurements from the Dark Energy Spectroscopic Instrument (DESI DR2). I will explore what these findings reveal about the nature of dark energy and their implications for our understanding of the cosmos. Finally, I will outline possible directions for future research and exploration—examining potential connections with observational systematics and unresolved anomalies.

Interacting dark sector with dark matter entropy couplings

• Elsa Teixeira (LUPM Montpellier)

Room: Amphi Claude Bloch The persistent discrepancies between predictions of the standard cosmological model and high-precision measurements across multiple probes remain a significant challenge in modern cosmology. Over the past decade, mounting evidence for tensions in key cosmological parameters - derived through both model-dependent and independent methods - has motivated the exploration of extensions to the standard paradigm. Among the most compelling directions is the dark sector, whose fundamental nature remains largely unknown. In particular, the microphysical properties of dark matter are still poorly understood beyond its gravitational role. In this talk, I will introduce a new class of interactions between dark matter and dark energy, formulated within a relativistic fluid framework. In this scenario, dark matter carries non-trivial intrinsic entropy with scale-dependent fluctuations, encoding additional internal degrees of freedom. Coupling this entropy to a scalar-field dark energy component gives rise to distinctive signatures in the growth of cosmic structure, opening a novel window into dark sector physics and on new physics in the standard model as a way of reconciling the observed cosmological tensions.

Invisible neutrino decays revisited

• Guillermo Franco Abellan (GRAPPA Institute, University of Amsterdam)

Room: Amphi Claude Bloch A new tension is starting to emerge between the tight cosmological upper bounds on the total neutrino mass and the lower limits from oscillation data, with potentially far-reaching implications for cosmology and particle physics. Invisible neutrino decays provide a compelling particle physics scenario to explain this tension. In this talk, I will present updated constraints on a framework where neutrinos decay non-relativistically into dark radiation, showing that the mass bound from *Planck 2018+DESI BAO DR2* is relaxed to $\sum m_\nu < 0.24 \ \rm{eV}$, in full agreement with oscillation data. I will also report the first late-time cosmological analysis of neutrino decays consistent with the measured mass splittings, showing that this scenario can only slightly alleviate the mass bounds. These results were possible thanks to new neural network emulators, which are ~200 times faster than the full Boltzmann solutions. I will end by discussing how future experiments aiming at directly detecting the cosmic neutrino background (e.g., PTOLEMY) have great potential to probe invisible neutrino decays.

Static quadratic Love numbers of neutron stars

• Filippo Vernizzi (IPhT - CEA Saclay)

Room: Amphi Claude Bloch

Cosmology with strong lensing images

• Julien Larena (LUPM, Université de Montpellier)

Room: Amphi Claude Bloch

Simulating structure formation in modified gravity

• Michel-Andrès Breton (CEA)

Room: Amphi Claude Bloch

Smooth sailing or ragged climb? — Increasing the robustness of power spectrum de-wiggling and ShapeFit parameter compression

• Katayoon GHAEMI

Room: Amphi Claude Bloch ShapeFit is a novel approach alternative to Full Modeling, and has been gaining popularity for analyzing the large scale structures of the universe. This approach provides information on the slope of the matter power spectrum at the pivot scale, m. There are two crucial steps to obtain this additional information: de-wiggling the power spectrum and calculating the derivative at the pivot scale. In this work different de-wiggling and derivative methods were compared and examined to study their impact on the obtained value of the slope. A systematic uncertainty of σ = 0.023|m| + 0.001 is derived by studying the behavior of the slope values in different cosmologies within and beyond LCDM.

Constraining the origin of supermassive black holes and modelling their growth

• Maxime Trebitsch (LUX - Observatoire de Paris - Sorbonne Université)

Room: Amphi Claude Bloch

Black holes with primary scalar hair

• Christos CHARMOUSSIS (IJCLAB)

Room: Amphi Claude Bloch

The separate universe approach : Jordan frame vs Einstein frame

• Hugo Holland (Institut d'Astrophysique Spatiale)

Room: Amphi Claude Bloch In this talk I will present ongoing work on the study of the validity conditions in the Jordan and the Einstein frame. Notably I will present the general conditions we can derive and look at a specific two field model in both frames.

Dynamical studies of dark matter - selected pieces

• Jonathan Freundlich

Room: Amphi Claude Bloch

Does the outer Milky Way lack Dark Matter?

• Gary Mamon (IAP)

Room: Amphi Claude Bloch Using Gaia proper motions, several studies in the last few years have pointed to a declining rotation curve of the Milky Way beyond 20 kpc. One study found that this decline is Keplerian like in our Solar System, suggesting a sharp truncation of the dark matter density profile. This would require to either drop the standard model of the mass density of the Universe dominated by cold dark matter, or that our Galaxy has suffered from tidal truncation from a more massive and concentrated object. I will describe the results of a new re-analysis of the Gaia data, which would be the first bayesian analysis on a star by star basis, properly including (for the 1st time) all terms in the stationary axisymmetric Jeans equation of local dynamical equilibrium, using a novel, yet general set of priors.

On-shell approach to gravitational radiation beyond General Relativity

• Adam FALKOWSKI (IJCLab)

Room: Amphi Claude Bloch

Evolution and spectrum in non-normal dynamics: a (black hole) gravitational case

• José-Luis Jaramillo (U. Bourgogne)

Room: Amphi Claude Bloch

Non-minimal light-curvature couplings and black-hole imaging

• Héloïse Delaporte (University of the Faroe Islands)

Room: Amphi Claude Bloch Non-minimal couplings between the electromagnetic field strength and the spacetime curvature are part of the effective field theory of gravity and matter. They alter the local propagation of light in a significant way if the ratio of spacetime curvature to the non-minimal coupling is of order one. Spacetime curvature can become appreciable around black holes, and yet the effect of non-minimal couplings on electromagnetic observations of black holes remains underexplored. A particular feature of the non-minimal coupling between the electromagnetic field-strength and the Riemann tensor is that it generates two distinct photon rings for different polarizations. Working within the paradigm of lensing bands, and focusing on the $n=1$ photon ring, we investigate which values of the non-minimal coupling can be robustly excluded.

Exact solutions in Entangled Relativity

• Maxime Adrien Raphael Wavasseur (Universitat de Barcelona - ICCUB)

Room: Amphi Claude Bloch We will present several exact solutions of Entangled Relativity, a recent general theory of relativity that is more parsimonious in terms of constants and natural units than General Relativity, while still recovering its well-tested phenomenology. These solutions range from spherical black holes to traversable wormholes that satisfy the Null Energy Conditions, including slowly rotating charged black holes and compact objects with scalar charge embedded in external electromagnetic fields à la Melvin. We will also explain why the ratio between R and the on-shell matter Lagrangian, which defines a new gravitational scalar degree of freedom in the theory, is always well-defined and finite. The talk will be based on results from publications 2407.17846, 2411.09327, 2502.13829, as well as on a spin-off of the findings presented in 2502.07206.

A new integrable parametrization for deformations to the Kerr photon ring

• Hugo Roussille (École Normale Supérieure de Lyon)

Room: Amphi Claude Bloch The first direct electromagnetic observation of a black hole (BH) was done by the EHT collaboration in 2019, triggering the possibility to test General Relativity (GR) in its strong field regime by probing the curvature of spacetime close to BHs. The observed image contains in particular the "critical curve", which is the imprint of the photons emitted by the accretion disk and trapped around the BH for several orbits. Measuring this critical curve and comparing it to the GR prediction constitutes a new kind of test of this theory: it is therefore crucial to compute this observable for beyond-GR BHs. In this work, we compute analytically such a critical curve for the "Kerr off-shell" family of spacetimes which describes a broad class of rotating BHs beyond the Kerr solution. This family of solutions is the most general extension of Kerr that preserves its "Killing tower" of symmetries, making the geodesic motion integrable. We study several concrete examples, studying in particular for the first time solutions with polar deformations with respect to Kerr. We conclude on the feasibility of a test of GR using future critical curve observations.

Ergoregion Instabilities in a Horizon-less Black Hole Analogue

• Paula Calizaya Cabrera (Laboratoire Kastler Brossel)

Room: Amphi Claude Bloch Rotational superradiance has long been associated with black holes—but it can actually emerge in any system featuring an ergoregion. The aim of this work is to show that rotating polariton quantum fluids provide a powerful platform to observe this phenomenon, as here one may study ergoregions in isolation (i.e without a horizon). Because ergoregions are intrinsically unstable, they generate dynamical instabilities that are tightly linked to superradiance. We demonstrate via numerical simulations that such instabilities are indeed present in this rotating system of polaritons, showing that experimental measurement of superradiance in such a system may soon be achieved.

Testing theories of gravity with planetary ephemerides

• Olivier Minazzoli (ARTEMIS - OCA)

Room: Amphi Claude Bloch We will explore how planetary ephemerides can be used to test alternative theories of gravity within the solar system, highlighting common mistakes found in the literature when attempting to constrain the parameters of these theories. This talk is based on our eponymous review with Agnes Fienga, published in Living Reviews in Relativity.

Experimental observation of superradiance in stationary and in dynamically unstable spacetimes

• Maxime Jacquet (Laboratoire Kastler Brossel, CNRS France)

Room: Amphi Claude Bloch Superradiance is a universal amplification phenomenon. In the instance of rotational superradiance, amplification occurs when a spatial region of a rotating system supports negative energy waves. Rotational superradiance is thus predicted to occur in compact objects in astrophysics, and may be observed experimentally in vortices in quantum fluids. In this talk, I will report on our recent observation of rotational superradiance in an experiment with a quantum fluid of light [1,2,3]. I will first present data in configurations where the vortex is dynamically stable (stationary) and then data in which rotational superradiance destabilises the vortex, ie when an ergoregion instability is triggered. In recent calculations, we predicted that entanglement generated by superradiance could be observed in this system [4], so our ability to trace superradiance from the linear regime of interactions to the nonlinear regime opens the way to investigations of entanglement beyond the non-perturbative regime. [1] Falque et al, PRL 2025 [2] Guerrero et al, arxiv 2025 [3] Guerrero et al, arxiv 2025 [4] Delhom et al, PRD 2024

Search for high frequency gravitational waves in electromagnetic cavities

• Jordan Gué (IFAE, UAB)

Room: Amphi Claude Bloch Recently, there has been a great interest in searching for gravitational waves with frequencies much higher than the realm of current ground based detectors, typically in the MHz-GHz range. For the most part, the signals expected in this frequency range would be signs of physics beyond the Standard Model. Among many experimental possibilities, one promising way of detection relies on the use of electromagnetic cavities. In this talk, I will review the various effects of GW on cavities and I will discuss the current progress on the modeling of the full response of the cavity to GW.

Slowly rotating black holes in theories of gravity

• Hugo Candan (LUX, Observatoire de Paris PSL & IJCLab, Université Paris-Saclay)

Room: Amphi Claude Bloch We are entering an era of high-precision measurements of black holes, either through gravitational waves (LVK, and soon LISA) or direct imaging (EHT). It is therefore interesting to study black hole spacetimes within both General Relativity (GR) and alternative theories, in order to confront them with observations. In GR, the Kerr spacetime, which describes rotating black holes, is the most general astrophysically relevant solution. However, in modified gravity theories, very few rotating solutions are known, even though real black holes always possess non-zero spin. Finding exact rotating solutions is known to be a very complicated task, but approximate analytical solutions can be obtained in the slow-rotation regime. Here, we present a general method to compute first-order slowly rotating black holes in the class DHOST theories.

Quantum (and classical) detection of gravitational waves: scope and limitations

• Paolo Bilisco (IPhT - Université Paris-Saclay)

Room: Amphi Claude Bloch LIGO, VIRGO and Kagra (just to name a few) represent outstanding feats of engineering that have launched us in a new era of gravitational-wave (GW) detection. Even so, we may wonder whether their sensitivity is enough to detect very high-frequency signals of beyond-Standard-Model origin, such as those sourced by primordial stochastic GW backgrounds, primordial black holes, or black hole superradiance. In this talk, I will argue that the sensitivity of present-day and near-future GW detectors will most likely prove insufficient in this regard. I will present the theoretical framework in which this question can be rigorously addressed and where to formally prove some heuristic expectations from the literature. I will also review these expectations and suggest that highly-controlled quantum resources will probably be needed to outrun the current sensitivity window of GW searches in the ultra-violet region.