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 mechanical 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 and standard quantum field theory 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.

The origin of the prompt emission in gamma-ray bursts (GRBs) remains highly debated despite decades of research. The GRB jet is initially optically thick and the trapped radiation is released when the jet transitions to the optically thin regime at the photospheric radius. Due to the high radiation pressure close to the central engine, shocks that occur before this transition are radiation mediated. In this seminar, I give an introduction to the theory of radiation-mediated shocks (RMSs) in the context of GRBs. I show that such shocks are expected in GRB jets and that the energized radiation downstream of the shock exhibits several promising features with regard to the observations. However, these shocks are computationally challenging to model from first principles. With this motivation, we have developed an approximate simulation code that is both quick and accurate. The simulation code is unique in its ability to fit an RMS model to the observational data.

Electrospheres are environments with the same origin as pulsars: a highly magnetized rotating neutron star. In pulsars, a cascade of electron-positron pair creation enriches the plasma. The plasma surrounding an electrosphere consists solely of particles that have escaped from the surface of the neutron star. Most of the 300 million presumed neutron stars in the Galaxy are not pulsars, but are surrounded by an electrosphere. Electrospheres are therefore fairly common objects, despite their low luminosity, which has so far prevented direct observation.

Electrospheres whose magnetic axis is aligned with the rotation axis have been well described for decades. The case of a magnetic axis oblique to the axis of rotation has resisted much theoretical research. We aimed at studying the physics of aligned and oblique electrospheres using a dedicated numerical simulation code which would enable ab initio simulations with realistic neutron star parameters. This code has recently been completed. Several numerical simulations have been carried out.

In this seminar, we briefly introduce the numerical simulation code and show the results of a parametric study. Many oblique electrospheres have the a structure analogous to those of the aligned electrospheres already known, but others have more specific properties. Some of them can enrich the interstellar medium with a plasma of ions and electrons.

Dark sirens are a powerful way to infer cosmological and astrophysical parameters from the combination of gravitational wave sirens and galaxy catalogues. Importantly, the method relies on the completeness of the galaxy catalogues being well modelled. A magnitude-limited catalogue will always be incomplete to some extent, requiring a completion scheme to avoid biasing the parameter inference. Standard methods include homogeneous and multiplicative completion, which have the advantage of simplicity but underestimate or overestimate the amplitude of structure at low completeness, respectively. In this talk, I will introduce a new method to complete galaxy catalogues which uses clustering information to incorporate knowledge of the large scale structure into the dark sirens method. If the structure of the true number of galaxies is sufficiently well preserved in the catalogue, our estimator can perform drastically better than both homogeneous and multiplicative completion. We lay the foundations for a maximally informative dark sirens analysis.

Cosmological surveys are probing extremely large volumes at ever increasing precision. The dramatic improvement in statistical errors allows us to detect even very subtle effects in the large-scale distribution of matter, and to thoroughly test our theory of gravity on cosmological distance scales. I will present a numerical framework for cosmological N-body simulations that offers a consistent treatment of the dynamics of spacetime as described by the full set of Einstein's equations. Such a framework can be used to compute higher-order correlations in a relativistic setting, or to predict the gravitational dynamics of exotic models that do not offer a simple Newtonian description.

It is now established that about 95% of the energy density of the Universe is made up of Dark Matter (DM) and Dark Energy (DE). Given our limited understanding of their physical nature, an intriguing possibility is to test interactions involving the dark components of the model. In this talk, I will focus on two different categories of interactions: scatter-like interactions between DM and neutrinos, and interacting DE. I will show that both offer interesting perspectives from a theoretical and observational standpoint. I will argue that small-scale Cosmic Microwave Background (CMB) observations can open novel observational windows to accurately test interactions, possibly revealing unique signatures that would be challenging to detect on larger angular scales. Interestingly, several independent CMB observations seem to hint at a mild preference for an interacting dark sector that might help with cosmological tensions.

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+/e- pairs to up-scatter low-energy ambient photons in the Milky Way via the inverse-Compton process, 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 set the strongest constraints to date on annihilating and decaying DM with masses between 150 MeV and 5 GeV.