Welcoming introduction Room: Salle Jean Jaurès

Francesca Calore (LAPTh): Probing axion-like and other feebly interacting particles with high-energy astrophysics

• Francesca Calore (LAPTh, CNRS)

Room: Salle Jean Jaurès Axion-like particles (ALPs) are hypothetical pseudo Nambu Goldstone bosons predicted in numerous extensions of the standard model, which may represent viable dark matter candidates and are closely related to the axion. Axions and ALPs produced in stars, and notably in core collapse supernovae, can be detected through their coupling to photons, which induces conversion of ALPs into photon and back when travelling through external magnetic fields. In my presentation, I will illustrate how high-energy astrophysics observations of galactic and extragalactic sources can be exploited to look for specific imprints of the ALPs-photon coupling, over a broad range of wavelengths, from radio to high-energy gamma rays. I will review current constraints on the ALPs parameter space, and provide an outlook on future experiments’ sensitivity to ALPs discovery. Finally, I will present how we can constrain also other feebly interacting particles with high-energy astrophysics.

Jerome Martin (IAP): Real-space correlations of quantum cosmological perturbations

• Jerome Martin (Institut d’Astrophysique de Paris)

Room: Salle Jean Jaurès

Elias Kiritsis (APC): Holographic Cosmological Thermalization

• Elias Kiritsis (UoC and APC)

Room: Salle Jean Jaurès According to the inflationary theory of cosmology, most elementary particles in the current universe were created during a period of reheating after inflation. We couple the inflaton to a holographic strongly-coupled QFT and study thermalization after inflation. As holographic theories generically thermalize instantly, this setup provides a parametrically different paradigm for reheating after inflation.

Marc Geiller (ENS Lyon): Subleading asymptotic structure of spacetime

• Marc Geiller (CNRS, ENS de Lyon)

Room: Salle Jean Jaurès Asymptotic symmetries provide a powerful insight into the structure of gravitational radiation and memory effects in asymptotically flat spacetimes. In this talk I will review the recent developments aiming at enlarging the available asymptotic symmetries. This will be explained via a dictionary between the Bondi-Sachs formalism and the Newman-Penrose formalism, which enables to study the fine subleading structure of asymptotically-flat spacetimes.

Sami Viollet (CPT Marseille): Discreteness Unravels the Black Hole Information Puzzle: Insights from a Quantum Gravity Toy Model

• Sami Viollet (CPT Marseille)

Room: Salle Jean Jaurès The black hole information puzzle can be resolved if two conditions are met. Firstly, if the information of what falls inside a black hole remains encoded in degrees of freedom that persist after the black hole completely evaporates. Secondly, if these degrees of freedom do not significantly contribute to the system’s energy, as the macroscopic mass of the initial black hole has been radiated away as Hawking radiation to infinity. The presence of Planckian geometric degrees of freedom provides a natural mechanism for achieving these two conditions. During this talk, I will illustrate both key aspects of this mechanism using a solvable toy model of a quantum black hole inspired by loop quantum gravity. I will first argue about how some aspects of the quantum gravity dynamics of black holes emitting Hawking radiation can be modelled using Kantowski-Sachs solutions, with a massless scalar field describing the ingoing Hawking particle, when one focuses on the deep interior region r << 2M (including the singularity). Further, I will show that in the r << 2M regime, and in suitable variables, this model becomes exactly solvable at both the classical and quantum levels. The quantum dynamics inspired by loop quantum gravity is revisited. I will propose a natural polymer-quantization where the area of the orbits of the rotation group is quantized. The Dirac observable associated to the mass is quantized and shown to have an infinite degeneracy associated to the so-called ε-sectors, interpretable as Planckian geometric degrees of freedom. Suitable continuum superpositions of these are well defined distributions in the physical Hilbert space and satisfy the quantum dynamics. Finally, I'll explain how correlations between the outgoing Hawking particle and the ingoing one dissipate in favor of correlations with the Planckian geometric degrees of freedom. The latter therefore seem necessary to restore the purity of the outgoing Hawking radiation.

Eugeny Babichev (IJCLab): Black holes in scalar-tensor theories

• Eugeny BABICHEV (IJCLab)

Room: Salle Jean Jaurès I will review black holes in scalar-tensor theories. One class of theories which allows analytic construction of solutions is related to shift-symmetric and parity-preserving scalar field. The shiftsymmetry of the scalar field yields a Noether conserved current which proves extremely useful for integrating the equations of motion. Another type of symmetry is the conformal invariance of the equation of motion of the scalar field that allows to integrate equations of motion analytically in particular classes of theories. Finally, I will consider conformal and disformal transformations as a tool to obtain new non-trivial solutions in DHOST theories.

Luca Santoni (APC): Dissipative Inflation via Scalar Production

• Luca Santoni (APC, Paris)

Room: Salle Jean Jaurès I will describe a new mechanism that gives rise to dissipation during cosmic inflation. In the simplest implementation, the mechanism requires the presence of a massive scalar field with a softly-broken global U(1) symmetry, along with the inflaton field. Particle production in this scenario takes place on parametrically sub-horizon scales, as opposed to the case of dissipation into gauge fields. Consequently, the backreaction of the produced particles on the inflationary dynamics can be treated in a local manner, allowing us to compute their effects analytically. I will show the parametric dependence of the power spectrum and its deviation from the usual slow-roll expression. I will show that non-Gaussianities are always sizeable whenever perturbations are generated by the noise induced by dissipation.

Michael Volkov (IDP Tours): Black holes with electroweak hair

• Mikhail Volkov (Institut Denis Poisson, UMR-CNRS 7013, Université de Tours,)

Room: Salle Jean Jaurès We review the recent progress in constructing hairy black holes in the Einstein-Weinberg-Salam theory. These black holes are static and axially symmetric, they carry a magnetic charge and support non-linear Yang-Mills and Higgs fields outside the event horizon. Depending on the value of the magnetic charge, their mass and size vary from Planck values up to values typical for planetary mass black holes. Close to the horizon the electroweak symmetry is restored and geometry is Reissner-Nordstrom-de Sitter. The non-linear fields form an "electroweak corona" located far away from the horizon in the region where the geometry is almost flat. These solutions provide the first example of hairy black holes in a genuinely physical theory.

Dimitrios Kranas (LPENS): Entanglement generation by rotating black holes in thermal baths

• Dimitrios Kranas (LPENS)

Room: Salle Jean Jaurès It is well-known that black hole event horizons produce entangled pairs of particles via the Hawking mechanism. D. Page has quantified the entanglement produced by non-rotating black holes in isolation where the input quantum state of fields is the vacuum. In our work, we extend Page’s computations in two directions. On the one hand, we include the effect of the rotation of black holes. This results in an additional production process of entangled particles, by the ergoregion. Using tools from gaussian quantum information theory, we investigate the interplay between the horizon and the ergoregion and quantify the entanglement generated by each of the two sources. On the other hand, we study the effect of thermal baths on the generation of entanglement. We find that, depending on the temperature of the thermal bath compared to the Hawking temperature, the entanglement produced by black holes can be significantly suppressed. These results extend previous calculations to the case of rotating black holes immersed in thermal baths and are relevant extensions to discuss additional topics, such as the information loss paradox, in a more realistic framework.

Maxime Jacquet (LKB): Measuring entanglement in rotating geometries -- experiments and theory

• Maxime Jacquet (Laboratoire Kastler Brossel, CNRS France)

Room: Salle Jean Jaurès Relativistic rotating geometries like Kerr black holes are characterised by their horizon and ergosurface (the surface within which waves must co-rotate with the hole), at which vacuum fluctuations of fields yield spontaneous emission: Hawking radiation at the horizon and rotational superradiance at the ergosurface. In the relativistic context, a far away observer cannot tell the difference between the two fluxes and cannot measure entanglement, preventing the determination of the quantum statistics of the two effects in any realistic black hole geometry. However, this can be done by quantum simulating the field theory via "analogue gravity". In this talk, I will explain how we can use a quantum fluid of microcavity polaritons (half-light half-matter bosons) to create a rotating geometry whose properties we can engineer at will. I will show how we can have a horizon and an ergosurface and also present a geometry in which there is an ergosurface without a horizon inside. This rotating geometry permits the investigation of the quantum statistics of rotational superradiance independently from those of Hawking radiation. I will present experimental data including the spectrum of emission. I will also present a full quantum theory of rotational superradiance in this geometry, including new results on entanglement.

Patrick Valageas (IPhT): Some gravitational aspects of scalar field dark matter

• Patrick VALAGEAS (CEA Saclay)

Room: Salle Jean Jaurès Scalar field dark matter models (such as Fuzzy Dark Matter, axion-like particles, ultra-light DM) are interesting dark matter candidates. Their gravitational dynamics follow the usual CDM behaviours on large scales but differ on small scales because of wave effects or self-interactions. I will briefly present two aspects of their gravitational dynamics: the formation of solitons, which could play a role for the tensions of CDM on galactic scales, and the accretion onto Black Holes, which could provide a probe of such DM clouds through gravitational waves.

Jérémie Quevillon (LPSC): Controversial Trace Anomalies

• Jérémie Quevillon (LPSC Grenoble)

Room: Salle Jean Jaurès The existence of CP violating terms in the trace of the energy-momentum tensor in chiral theories coupled to gravity has no answer yet. Consequently, the neutrino could carry a source of CP or unitarity violation in its trace anomaly within the Standard Model. This talk aims to clarify the situation by presenting exclusive non-perturbative computations of the trace anomaly in curved spacetime, which should be helpful in settling the controversy.

Iason Baldes (LPENS): Primordial black holes as dark matter: Interferometric tests of phase transition origin

• Iason Baldes (LPENS)

Room: Salle Jean Jaurès We show that primordial black holes in the observationally allowed mass window with fPBH=1 formed from late nucleating patches in a first order phase transition imply upcoming gravitational wave interferometers will see a large stochastic background arising from the bubble collisions. As an example, we use a classically scale invariant B-L model, in which the right handed neutrinos explain the neutrino masses and leptogenesis, and the dark matter consists of primordial black holes. The conclusion regarding the gravitational waves is, however, expected to hold model independently for black holes coming from such late nucleating patches.

Mathias Pierre (DESY): Reheating after fragmentation

• Mathias Pierre (DESY)

Room: Salle Jean Jaurès In simple single field models of inflation, an inflaton condensate undergoes an oscillatory phase once inflationary expansion ends. In the presence of self-interactions, the inflaton field is driven into the non-linear regime by the resonant growth of its fluctuations. The once spatially homogeneous coherent inflaton is converted into a collection of inflaton particles with non-vanishing momentum. In this talk I discuss a formalism to quantify the effect of fragmentation on particle production rates and discuss consequences on the ability to successfully achieve the transition into a radiation-dominated universe.

Frédéric Vincent (Observatoire de Paris): Polarized radiative transfer and tests of gravity

• Frederic Vincent (Observatoire de Paris / LESIA)

Room: Salle Jean Jaurès The ray-tracing code GYOTO of Paris Observatory is made to compute null geodesics in arbitrary spacetimes and integrate the radiative transfer equation in various contexts. Typically, it is used to simulate images and various observables produced in the vicinity of black holes, like Sagittarius A* at the Galactic center. A recent new development of the code now allows to simulate the full polarized electromagnetic radiation. In this talk, I will present a quick reminder of electromagnetic polarization in curved spacetimes, and discuss how this might be useful to constrain gravity in the vicinity of compact objects.

Jordan Gué (Observatoire de Paris): Violation of the equivalence principle induced by oscillating rest mass and its detection in atom interferometers

• Jordan Gué (SYRTE, Observatoire de Paris)

Room: Salle Jean Jaurès We present a theoretical investigation of the expected signal produced by free falling atoms with time oscillating mass and transition frequency. These oscillations could be produced in a variety of models, in particular, models of scalar dark matter non universally coupled to the standard matter such as axion-like particles and dilatons. Performing the exact and rigorous calculations, we show that, on one hand, two different atomic species would accelerate at a different rate; and on the other hand, they would produce a non-zero differential phase shift in an atom interferometer. In this framework, we show that already existing experiments could put the best laboratory constraints on these dark matter models. Additionally, we propose an experimental variation of compact gradiometers which would be much more sensitive to these dark matter candidates and which could test the universality of free fall at an unprecedented level.

Hugo Lévy (ONERA): Numerical investigation of screened scalar-tensor theories in space-based experiments

• Hugo Lévy (ONERA / IAP)

Room: Salle Jean Jaurès Scalar-tensor theories are one of the most natural alternatives to general relativity (GR), where gravity is mediated by both a tensor field and a scalar field. Among the wide variety of scalar-tensor models proposed over the past decades, some are already ruled-out by lab experiments or astrophysical observations while others remain viable by means of screening mechanisms that dynamically suppress deviations from GR in classical fifth force searches. The hunt for such hypothetical scalar fields thus requires designing novel and intelligent experiments. Alas, this task is partly impeded by the difficulty to accurately model their effects in complex setups. This talk will showcase femtoscope — a Python numerical tool based on the finite element method for solving Klein-Gordon-like equations that arise in particular in the symmetron or chameleon models. The novelty and most important feature of femtoscope is that it includes a careful treatment of asymptotic boundary conditions — that is when the behaviour of the field is only known infinitely far away from the sources — which is essential to obtain a physically-meaningful numerical approximation. I will then discuss some recent numerical studies conducted in order to ascertain fifth force detectability in Earth orbit, with emphasis laid on the effects of departure from spherical symmetry.

Hugo Roussille (ENS Lyon): Non-linear gravitational waves in scalar-tensor theories

• Hugo Roussille (École Normale Supérieure de Lyon)

Room: Salle Jean Jaurès We construct an exact solution of Degenerate Higher-Order Scalar-Tensor (DHOST) theory which contains non linear gravitational waves. The solution is obtained by applying a disformal transformation to a similar solution in Einstein-scalar gravity. Its principal null directions are still geodesic, shear-free and twist-free. This new solution provides a first concrete arena to investigate the non linear regime of gravitational wave radiation in a wide family of higher order scalar-tensor theories. Furthermore, we present the investigation of a specific subcase of the solution which is of Petrov type D.

Camille Bonvin (Geneva University): Testing gravity with large-scale structure

• Camille Bonvin (University of Geneva)

Room: Salle Jean Jaurès To test the theory of gravity one needs to test, on one hand, how space and time are distorted by matter and, on the other hand, how matter moves in a distorted space-time. Current observations provide tight constraints on the motion of matter, through the so-called redshift-space distortions, but they only provide a measurement of the sum of the spatial and temporal distortions, via gravitational lensing. In this talk I will present a novel method to measure the time distortion on its own, and I will show that it will be detectable by future surveys like the SKA. I will then discuss new tests of gravity that can be build from this measurement.

Roya Mohayaee (IAP): Testing the cosmological principle with distant sources

• Roya Mohayaee (Institut d'Astrophysique de Paris/Sorbonne université)

Room: Salle Jean Jaurès The cosmological principle states that the Universe is statistically homogeneous and isotropic on large scales. I briefly discuss how the high redshift radio sources and quasars can be used to verify or falsify this principle, at the foundation of modern cosmology

Pierre Auclair (UCL Louvain): Spatial Curvature from Super-Hubble Cosmological Fluctuations

• Pierre Auclair (UCLouvain)

Room: Salle Jean Jaurès In this talk, based on arXiv: 2302.14530, we show how super-Hubble cosmological fluctuations on a flat FLRW background metric can be interpreted as a non-vanishing spatial curvature of the local background metric. The random nature of these fluctuations promotes the curvature density parameter to a stochastic quantity for which we derive novel non-perturbative expressions for its mean, variance, higher moments and full probability distribution. In particular, we provide novel diagrammatic methods to compute and resum the moments of the probability distribution. For scale-invariant Gaussian perturbations, such as those favored by cosmological observations, we find that the most probable value for the curvature density parameter today is -10^{-9}, that its mean is +10^{-9}, both being overwhelmed by a standard deviation of order 10^{-5}.

Guillermo Franco Abellán (GRAPPA Amsterdam): Accelerating cosmological inference from Euclid with Marginal Neural Ratio Estimation

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

Room: Salle Jean Jaurès The Euclid space telescope will measure the shapes and redshifts of billions of galaxies, probing the growth of cosmic structures with an unprecedented precision. However, the increased quality of these data also means a significant increase in the number of nuisance parameters, making the cosmological inference a very challenging task. Indeed, conventional likelihood-based methods like Markov-Chain Monte Carlo (MCMC) become extremely time-consuming when the dimensionality of the parameter space is very high. In this talk, I discuss the first application of Marginal Neural Ration Estimation (MNRE) (a recent approach in so-called simulation-based inference) to Euclid primary observables, like cosmic shear and galaxy-clustering spectra. Using expected Euclid experimental noise, I show how it's possible to recover the posterior distribution for the cosmological parameters using an order of magnitude fewer simulations than conventional likelihood-based methods.This result supports that MNRE is a powerful framework to analyse Euclid data, allowing to extend the model complexity beyond what its currently achievable with standard MCMC.

Jordan Koechler (LPTHE): X-rays constraints on sub-GeV Dark Matter

• Jordan Koechler (LPTHE - Sorbonne Université)

Room: Salle Jean Jaurès In this talk, I will present updated constraints on 'light' dark matter (DM) particles with masses between 1 MeV and 5 GeV. In this range, we can expect DM-produced e^{+-} pairs to upscatter ambient photons in the Milky Way via Inverse Compton, and produce a flux of X-rays that can be probed by a range of space observatories. Using diffuse X-ray data from XMM-Newton, INTEGRAL, NuSTAR and Suzaku, we compute the strongest constraints to date on annihilating DM for 200 MeV < m_{DM} < 5 GeV and decaying DM for 100 MeV < m_{DM} < 5 GeV. I will also discuss possible future developments of these results and this technique.

Micaela Oertel (LUTH): Neutrino-nucleon interactions in dense and hot matter

• Micaela Oertel (LUTH, Observatoire de Paris)

Room: Salle Jean Jaurès Neutrinos play an important role in compact star astrophysics: neutrino-heating is one of the main ingredients in core-collapse supernovae, neutrino-matter interactions determine the composition of matter in binary neutron star mergers and have among others a strong impact on conditions for heavy element nucleosynthesis and neutron star cooling is dominated by neutrino emission except for very old stars. Many works in the last decades have shown that in dense matter medium effects considerably change the neutrino-matter interaction rates, whereas many astrophysical simulations use analytic approximations which are often far from reproducing more complete calculations. In this talk I will discuss evaluations of charged-current processes and present a scheme which allows to incorporate improved rates into simulations and show as an example some results for core-collapse supernovae and proto-neutron star cooling.

Jean-Baptiste Fouvry (IAP): Stellar dynamics in galactic nuclei and constraining intermediate mass black holes around SgrA*

• Jean-Baptiste Fouvry (IAP)

Room: Salle Jean Jaurès Most galaxies harbor a supermassive black hole in their centre around which orbits a stellar cluster, the galactic nucleus. The unique proximity of the Milky-Way's central black hole, SgrA*, offers an extraordinary opportunity to study such a crowded environment. Although galactic nuclei are among the densest stellar systems in the universe, the steep potential well generated by the central black hole allows for efficient orbital interactions between the stars. Ultimately, this drives the relaxation of the stellar orbits through an intricate hierarchy of dynamical processes. In this presentation, I will focus on two such processes: scalar resonant relaxation through which the stellar orbital eccentricities relax, and vector resonant relaxation through which the stellar orbital orientations get reshuffled. For both processes, I will report on recent developments in kinetic theory to model these dynamics, and will present first quantitative applications of these frameworks to constrain the possible presence of intermediate mass black holes around SgrA*

Lucas Pinol (LPENS): Borel resummation of secular divergences in stochastic inflation

• Lucas Pinol (Instituto de Física Teórica (IFT), UAM-CSIC, Madrid)

Room: Salle Jean Jaurès We make use of Borel resummation to extract the exact time dependence from the divergent series found in the context of stochastic inflation. Correlation functions of self-interacting scalar fields in de Sitter spacetime are known to develop secular IR divergences via loops, and the first terms of the divergent series have been consistently computed both with standard techniques for curved spacetime quantum field theory and within the framework of stochastic inflation. Impressively, the stochastic formalism enables one to compute the time series analytically at an arbitrary order, thus outperforming other methods. However, the time series is divergent, while we also know the asymptotic result to be finite. We show that Borel resummation can be used to interpret the divergent series and to correctly infer the time evolution of the correlation functions in the transition regime. In practice, we adopt a method called Borel–Padé resummation where we approximate the Borel transformation by a Padé approximant. Beyond formal applications with test scalar fields and de Sitter spacetime, the stochastic formalism can be applied to active scalar fields during inflation, enabling one to derive non-perturbative results relevant, e.g., for primordial black hole formation.

Chiara Animali (LPENS): Stochastic inflation formalism and its application to primordial black holes

• Chiara Animali (LPENS, Paris)

Room: Salle Jean Jaurès According to the inflationary paradigm, cosmological inhomogeneities originate from vacuum quantum fluctuations, which are amplified and stretched to astrophysical distances, later seeding CMB anisotropies and large scale structures. Because of the continuous inflow of modes that cross out the Hubble radius, the inflating background gets constantly corrected by quantum fluctuations. This quantum backreaction can be described by the stochastic inflation formalism, an effective theory for the long-wavelength part of quantum fields, where amplified quantum fluctuations act as a source of stochastic noise. This backreaction effect is particularly relevant if inflation gives rise to large enough curvature perturbations, which may eventually collapse to form primordial black holes: the stochastic-delta N formalism represents a powerful tool to assess the impact of this quantum diffusion on the properties of cosmological perturbations and on the statistics of collapsed objects. In this framework, I will discuss how we can reconstruct important cosmological observables, such as the power spectrum and n-point functions, through the stochastic-delta N formalism, and their consequences for primordial black holes, in particular inflationary scenarios where quantum diffusion plays a major role in the inflationary dynamics.

Rémi Faure (IPhT): Leptogenesis during a cosmological phase-transition

• Rémi Faure (Institut de Physique Théorique (CEA Saclay, Paris))

Room: Salle Jean Jaurès Leptogenesis is an explanation for the asymmetry between matter and antimatter present in our Universe. One of its popular realisation involves new particles, right-handed neutrinos, that allow lepton-number violating processes and C and CP violation. In order to generate a final asymmetry, we also need them to be out-of-equilibrium. In our scenario, this departure from equilibrium is provided by a phase-transition that generates the mass of the right-handed neutrinos, which can extend the successful parameter space available.

Sébastien Renaux-Petel (IAP): Non-perturbative Wavefunction of the Universe in Inflation with (Resonant) Features

• Sébastien Renaux-Petel (IAP-CNRS)

Room: Salle Jean Jaurès I will describe how to obtain non-perturbative predictions for primordial correlators in large classes of inflationary theories. I will exemplify this with resonant features models, obtaining analytical and numerical results for the wavefunction of the universe, which turn out to display interesting oscillatory patterns. The methods and the results are interesting both on purely theoretical grounds, as well as phenomenologically, for instance with respect to rare events associated with primordial black holes

Pierre Vanhove (IPhT): Classical Observables from the Exponential Representation of the Gravitational S-Matrix

• Pierre Vanhove (IPhT CEA-Saclay)

Room: Salle Jean Jaurès Recent advancements in the field of scattering amplitudes have led to efficient methods for analytically evaluating two-body gravitational scattering in post-Minkowskian expansions. By utilizing an exponential representation of the S-matrix, we have successfully determined the spinless two-body scattering up to the fourth post-Minkowskian order. These results encompass both conservative and gravitational radiation effects and are applicable from small to very high relative velocities. In this presentation, we will discuss our very recent determination at the 4PM order for spinless massive bodies. Additionally, we will examine its behavior at high energies, in connexion with into the current puzzles surrounding gravitational radiation at this order.

Mariana Graña (IPhT): String theory, from the landscape to the swampland

• Mariana Grana (CEA/Saclay)

Room: Salle Jean Jaurès

Laurent Freidel (Perimeter Institute): Corner symmetry and local holography: A paradigm for quantum geometry

• Freidel Laurent (Perimeter Institute)

Room: Salle Jean Jaurès In this talk I will present an overview of the local holography program which provides a bottom-up perspective on quantum gravity design to understand the nature of quantum geometry from first principle. This new approach is deeply rooted in a renewed understanding of local symmetries in Gravity. It focuses first on the understanding of how gravitational systems decomposes into subsystems. It also provides a detailed description of the nature of entanglement of gravitational subsystems. I will emphasize the central role of the corner symmetry group attached to codimension 2 surfaces in capturing all the necessary data needed to glue back seamlessly quantum spacetime regions. I will present some elements of representation theory of these infinite dimensional corner symmetry groups for 4d gravity and show its connection with the symmetry algebra of perfect fluids. I will present recent results on the geometry and symmetry of null surfaces and the Quantization of Raychudhuri equations. If time permits I will mention some connections these results have with celestial holography and the higher spin asymptotic symmetry group.

Killian Martineau (LPSC): New findings on (ultra) high-frequency gravitational waves: on the detection by resonant cavities and generation by twisted laser pulses

• Killian Martineau

Room: Salle Jean Jaurès As underlined by recent events, numerous efforts are directed towards exploring gravitational waves (GWs) in the low frequency regime, below the LIGO-Virgo-KAGRA ranges. The high frequency regime, however, remains vastly unexplored. There is a good reason for that: no substantial signal is expected from known astrophysical or cosmological sources above few kHz. (Ultra) high frequency GWs are however expected from many beyond standard model sources: primordial black holes, topological defects such as cosmic strings, superradiance phenomena, collisions of “bubbles” arising from phase transitions in the early universe, … In this presentation I propose to present two recent works related with high-frequency gravitational waves: i) Recently, the possibility to use resonant cavities located e.g., in haloscope experiments, to search for GWs in the GHz range has gained a lot of interest, with strain sensitivities announced to be around 10^(-22). In a recent work performed in collaboration with A. Barrau, J.G. Bellido and T. Grenet, see https://arxiv.org/pdf/2303.06006.pdf, we have re-evaluated the estimated sensitivity, focusing on compact binary coalescences signals and taking the Grenoble Axion Haloscope (GrAHal) setup as a benchmark. ii) In collaboration with the laser physics department of Oxford we have investigated the gravitational waves generated by high-energy lasers carrying orbital angular momentum. Focusing on Bessel beams we provided novel analytical expressions and numerical computations for the generated metric perturbations and associated power. Findings show that properties of the generated GWs, such as frequency, polarisation states or direction of emission, can be controlled by the pulse parameters and other optical arrangements. The associated paper is to appear by mid of July.

Alberto Roper Pol (University of Geneva): LISA and γ-ray telescopes as multi-messenger probes of a first-order cosmological phase transition

• Alberto Roper Pol (APC)

Room: Salle Jean Jaurès The stochastic gravitational wave background (SGWB) produced at a first-order phase transition around the elctroweak scale is expected to be peaking within LISA's sensitivity frequency range, being a promising test of high energy physics and beyond Standard Model extensions. The contribution of magnetohydrodynamic (MHD) turbulence to the cosmological SGWB is one of the least understood sources due to the necessity, in general, to perform large-scale numerical simulations solving MHD equations. In this talk, I will review recent numerical simulations that have addressed this issue and studied the potential detectability of the resulting SGWB by space-based GW detectors like LISA. I will focus on magnetically dominated MHD turbulence and compare to astrophysical constraints that can provide a multi-messenger study of primordial magnetic fields. In particular, I will present the SGWB produced by decaying MHD turbulence, which has been validated by numerical simulations for a particular range of parameters. This model allows us to provide constraints on the primordial magnetic fields from the potential observation of a SGWB with LISA and combine the observations with those from -ray telescopes like MAGIC.