La solution homogène et isotrope des équations d’Einstein dite LambdaCDM, assortie de ses perturbations linéaires et d’ordres supérieurs, forme le socle de la cosmologie contemporaine permettant de comprendre, dans ses grandes lignes, l’évolution et la géométrie de notre Univers. Les réflexions suivantes nous incitent à aller plus loin. Tout d’abord, l’homogénéité et l’isotropie de l’Univers ne sont que des propriétés moyennes valables à des échelles qui reculent au fur et à mesure de la découverte de structures à des décalages vers le rouge de plus en plus grands. L’avènement d’une cosmologie de précision conduit ainsi à l’émergence ou à l’amplification de tensions, voire d’anomalies, dues au décalage entre prédictions extrapolées depuis l’Univers lointain et observations à précision accrue de l’Univers proche. La Relativité Générale nous offre une manière de résoudre ces incompatibilités. Le modèle Szekeres est une solution exacte des équations d’Einstein, inhomogène et sans symétrie, capable de représenter la région de notre Univers dominée par la matière et/ou la constante cosmologique, puisque sa source gravitationnelle est un fluide de pression nulle et que la constante cosmologique peut y être incorporée. Son atout majeur est d’inclure la solution homogène et isotrope de Friedmann comme cas limite. Elle est donc susceptible de reproduire naturellemnt la transition homogène/inhomogène à l’échelle où les fonctions qui la décrivent deviennent analogues aux paramètres cosmologiques standards. Les prédictions les plus robustes de la cosmologie de l’Univers lointain restent ainsi conservées. Au cours de ce séminaire, la solution Szekeres sera présentée avec ses principales propriétés nécessaires à une utilisation dans un contexte cosmologique. Les différentes expériences qui devront être utilisées pour contraindre les fonctions-paramètres qui déterminent le modèle Szekeres d’Univers valable à précision donnée seront décrites et commentées. Les perspectives ouvertes par les réseaux de neurones pour réaliser ce travail d’analyse de grandes quantités de données seront discutées.

Tidal Love numbers provide us a handle to test the nature of compact objects, as well as theories of gravity. There have been several clarifications recently, which makes our understanding of these Love numbers better. But further investigations have led to more confusion. I plan to discuss these recent developments and the confusing nature of recent literature on these issues. I will show that the tidal Love numbers of a non-rotating black hole identically vanishes, but for a slowly rotating black hole there are other possibilities. The issues with the standard development become more apparent for arbitrary rotating black holes. Besides black holes, I will also highlight some remarkable novel features, for ultra-compact objects with non-trivial reflectivity.

In general relativity, freely-falling objects follow geodesics of the background spacetime in which they live. In a sense, this feature is a mere rephrasing of Einstein’s equivalence principle.

Lie algebras are an important class of algebras with deep applications to physics. We will give a brief overview of the mathematical definition. After this, we will discuss the current implementation in SageMath, including finite dimensional, affine, nilpotent, and other special classes. We will conclude with a discussion of the future implementations.

In high-energy and multi-messenger astrophysics, much of the phenomenology relates to the dissipation of an energy reservoir into accelerated particles that then radiate in various channels (cosmic rays, electromagnetic signals, from radio to gamma rays, and neutrinos), and most of the effort focuses on pinpointing the reservoir and the dissipation process at play Bursts of these high-energy particles often result from efficient kinetic or Poynting flux conversion in nonthermal distributions through collective plasma processes associated with astrophysical shock waves.

The LIGO-Virgo-KAGRA (LVK) collaboration has published about 90 gravitational wave observation since the first detection in 2015, majority of which have been from binary black holes. With such a statistically significant number of events it is possible to infer astrophysical distribution of black hole masses and spins. Of particular interest are spins(mis-aligned with the orbital angular momentum) of the component black holes in a binary, as they can shed light on the binary formation mechanism. Spin-misalignments of the component black holes leave their imprints on the waveform. One such modulation is due to an asymmetry between the +m and -m multipoles of the harmonic decomposition of the gravitational wave signal from the binary. This is not included in most signal models used in LVK analysis to date. I will present the first frequency-domain model of the asymmetry in the dominant co-precessing-frame signal multipole throughout inspiral, merger and ringdown. This model has been incorporated into the latest iteration of PHENOM family of precessing-binary models (PhenomXO4a) that will be used for analysing data from the fourth observing run (O4) of the LVK. I will briefly describe the features captured in PhenomXO4a and the prospect of spin measurements using this model.

Compact stars have a crucial role to play in our understanding of ultra dense and isospin asymmetric matter. Born in the extremely bright explosion of a massive star at the last stage of its life, neutron stars can present masses as high as twice the mass of the Sun, in a radius of around ten kilometers, thus gathering matter in their interior in extreme conditions of density and gravity. The nuclear physics probed by observing neutron stars is complementary in terms of density and isospin asymmetry to any terrestrial experiment. Multi-messenger astronomy is used to extract information on the interior of neutron stars, their structure and their composition. Connecting the observation of macroscopic parameters to state of the art constraints on the equation of state of dense matter can be done via hydrostatics and hydrodynamics modelling in General Relativity.

After a quick introduction to neutron star’s astrophysical features and how to model them within the theory of General Relativity, we will discuss how detection and theory of both neutron star astrophysics and nuclear physics can help us probe ultra dense matter. I will present publicly available software relevant to equation of state inference in the context of next generation of gravitational wave telescopes. Finally, we will show that almost equation of state independent relations that have been used to extract astrophysical features of neutron stars are partly irrelevant for gravitional wave signal detected with Cosmic Explorer and Einstein Telescope sensitivity.

In the era of the multi-messenger astronomy, ultra-high energy cosmic rays offer the unique opportunity to investigate the nature of astrophysical sources and of particle interactions in an energy range far beyond that covered by current particle accelerators.

In this talk, will review the recent progresses that our team has made using Gaia DR3 data. It includes the detection of a significant (Keplerian) decrease for the rotation curve of our Galaxy, and the fact that most Milky Way dwarfs are newcomers in the halo (as we already know for the Magellanic Clouds). Both discoveries lead to a considerable, downwards revision of the dynamical masses of Local Group galaxies, which will be presented. Finally, I’ll discuss possible consequences on cosmological models, as well as new observational hints that constrain the mass of dwarf irregular galaxies, such as WLM and the Large Magellanic Cloud.

In this talk, we will explore entangled relativity, a general theory of relativity that offers a more parsimonious approach compared to general relativity when integrated within a quantum field theory framework. Requiring only two universal dimensionful parameters instead of three, entangled relativity retains the key components of general relativity, including a four-dimensional spacetime manifold, a metric tensor representing the mechanic al properties of space , inertia, and gravitation, and matter fields generating curvature. The term « entangled » does not directly refer to quantum entanglement but rather emphasizes that matter fields and gravity are intrinsically intertwined and cannot be treated separately at the level of the formulation of the theory. Entangled relativity converges toward, or reduces to, general relativity in most cases but predicts a variable quantum of action (ħ) in response to gravitational phenomena, particularly in dense environments like neutron stars or the early universe. After presenting and explaining the theory, I will present a few predictions that may become observable in the foreseeable future, and which do not depend on any theoretical parameter.