The observed clustering of quasars on large angular scales is imprinted with information of our motion with respect to the cosmological rest-frame. Using the Catwise catalogue, based upon data from the Wide-field Infrared Survey Explorer (WISE), we infer posteriors for the kinematic dipole and the intrinsic quasar clustering dipole. We present a methodology for simulating dipole measurements including survey systematics such as; the survey mask, variable photometric errors, biased dust extinction modelling and contamination along the ecliptic plane. We then use Simulation-based Inference to infer our posteriors. We examine a range of different prior hypotheses, dipole estimators and magnitude cuts . When compared to measurements inferred from the Cosmic Microwave Background dipole and with a $\Lambda$CDM prior on the expected intrinsic quasar clustering dipole in agreement with past measurements we get a greater than 6 $\sigma$ tension with the CMB measured kinematic dipole when estimating the dipole for the entire sample and a greater than 9 $\sigma$ tension with a spectrally binned dipole estimator.

High-energy astrophysics has radically evolved over the last few years, with the advent of multi-messenger astronomy: violent cosmic phenomena have been observed in gravitational waves (GWs) and with electromagnetic (EM) counterparts, and associations have been reported between very high neutrinos energy (10^15 eV) and photons at different wavelengths. The next frontier of high-energy astrophysics is EeV (10^18 eV) or ultra-high energy (UHE) multi-messenger astronomy. The detection of EeV neutrinos in particular should solve the long-standing mystery of the origin of UHE cosmic rays and open a unique window to cast light on the workings of the most violent sources in the Universe.

The Giant Radio Array for Neutrino Detection (GRAND) is one of the most promising project to achieve the detection of these highly-seeked particles in the next decade. GRAND is a proposal for an array of 200’000 radio antennas deployed over 200’000km² in various favorable locations around the world, detecting cosmic particles (neutrinos, but also gamma rays and cosmic rays) of ultra high energy through the electromagnetic transient pulses they induce in the atmosphere when they reach the Earth. On the road to achieve this very ambitious project, a 300-unit prototype is being deployed in a remote area of the Gobi desert in China, complemented by a 10-units mini-array on the site of the Pierre Auger Observatory, deployed earlier this year.

In this presentation, we will detail the scientific motivation, detection principle and expected outcomes of the GRAND project. We will present the different phases of the GRAND project, its present status and work plan for the very next year, with a specific focus on the French contributions.

Since more than 10 years, an unexpected gamma-ray component over astrophysical backgrounds has been detected at GeV energies towards the Galactic center in the data of the Fermi-Large Area Telescope. Initially, this excess was considered to be hinting at GeV thermal relics annihilating in the Galactic dark matter halo. However, recent works have demonstrated that the excess is better explained by a population of millisecond pulsar-like sources in the Galactic bulge. While the excess photon flux is peaked at about few GeV, a significant high energy tail extending up to tens of GeV has been detected. Such high-energy photons are naturally explained by the inverse Compton emission of electrons and positrons emitted by millisecond pulsars.
I will present a novel study of the morphology and photon statistics of the Galactic center excess at energies above 10 GeV. This is based on an innovative method, which combines adaptive template fitting and pixel-count statistics in order to assess the role of sub-threshold point sources while minimizing the mis-modelling of Galactic diffuse emission backgrounds.

Using trajectories in quantum cosmology.
Cosmological perturbation theory aims at producing the primordial spectrum
by setting quantum vacuum initial conditions over a classically evolving background.
Although appearing potentially inconsistent, such an approach can be justified in
canonical quantum gravity for inflationary models by decohence. The situation may
not always be so clear-cut, but defining quantum perturbations in a computational
useful manner seems to require some way of defining a trajectory. Various proposals
are available, and I’ll describe two of them, namely the well-known de Broglie Bohm
eikonals and the coherent state approach with phase space formulation. I’ll introduce
both and discuss some consequences that can be more easily worked out.

In this talk, I will show you my investigation of the effect of significant small-scale primordial non-Gaussianity on structure formation and the galaxy formation process. Generically, I find a distinct and potentially detectable feature in the matter power spectrum around the non-linear scale. The feature might have interesting consequences for the S8 tension. I then show in particular that a positively skewed distribution with fNL of the order of 1000 implies that typical galaxy-sized halos reach half of their present-day mass at an earlier stage and have a quieter merging history at z < 3 than in the Gaussian case. Their environment between 0.5 and 4 viri al radii at z = 0 is less dense than in the Gaussian case and may impact the universality of the NFW profile. This quieter history and less dense environment has potentially interesting consequences in terms of the formation of bulges and bars. In particular, I show that all feedback prescriptions being otherwise identical, simulations with a positively skewed distribution , hence forming galaxies a bit later than in vanilla LCDM, allow to form simulated galaxies with more disky kinematics than in the vanilla case. Therefore, such small-scale primordial non-Gaussianities could potentially help alleviate, simultaneously, tensions in cosmology and galaxy formation. These simulations on small scales will need to be complemented with larger box "zoom-in" simulations with scale-dependent non-Gaussianities, to statistically confirm these trends and explore their observational consequences in further detail.

Over the last two decades, the standard cosmological model "Λ cold dark matter" (ΛCDM) has been firmly established by a variety of observations. Nevertheless, the nature of the dominant components of the Universe – namely dark matter and dark energy – and the process of creation of its initial conditions – namely inflation – are not yet known. Furthermore, as the precision of the data has increased, intriguing anomalies have appeared within the standard cosmological model. These anomalies arise from a discrepancy between the value of some cosmological parameters predicted from their calibration in the distant Universe, for example with the cosmic microwave background (CMB), and the measurement of these parameters from the local Universe. The two most significant tensions in modern cosmology concern the parameter that determines the expansion rate of the Universe, i.e., the Hubble parameter H0, and the parameter that quantifies the amplitude of local matter fluctuations, i.e., the parameter S8. A number of surveys dedicated to measuring large-scale structures (LSS) of the Universe (BOSS and eBOSS in particular, and the forthcoming DESI and EUCLID) can be used to weigh in on these cosmological tensions. The first axis of this thesis is based on a semi-analytical method, known as effective field theory of large-scale structures (EFTofLSS), which provides an accurate description of the galaxy power spectrum, and aims at improving cosmological constraints from LSS surveys, a major challenge in the context of the forthcoming data from DESI and Euclid. In particular, we apply an EFTofLSS analysis to describe the BOSS and eBOSS data, and demonstrate that these surveys can be used to obtain constraints that are competitive with those coming from CMB data. We also establish the self-consistency of this theory within the ΛCDM model, as it appears at first sight that the different EFTofLSS parametrizations proposed in the literature seem to provide different constraints on cosmological parameters. We thus identify these discrepancies as originating from subtleties of the commonly used Bayesian framework. In addition, we develop a systematic method for applying the EFTofLSS to different LSS data and obtain robust results contrasting the Bayesian framework with the frequentist approach, for an in depth cosmological inference. The second aspect of my work aims to establish the possible theoretical implications of cosmological tensions for our understanding of the dark sector of the Universe. In particular, we confront various models that can resolve these tensions with the BOSS and eBOSS data analysed under the EFTofLSS. We conclude that current clustering data are not in tension with ΛCDM, and can be used to set important constraints on model suggested to resolve cosmological tensions.

The TeV gamma-ray astronomie domain relies on two ground based detection techniques : Cherenkov telescopes (such as HESS) and particle detectors. This seminar will focus on this second category of instruments, characterised by a wide field-of-view (the whole sky above it) and a high duty cycle (24/7). I will introduce the two major instruments currently operating in the northern hemisphere, namely HAWC and LHAASO, and the future detector planned for the southern hemisphere SWGO. I will present some science cases and emphases the complementarity with Cherenkov telescope arrays.

Finding a theoretical explanation to the Hubble tension is extremely challenging, since any serious solution must also account for all other astrophysical and cosmological tests, along with experimental constraints on physics. The gravitational laws in the dark sector are much less constrained, mainly due to our ignorance of the nature of its physical origin. I will show how a non-minimally coupling of dark matter only, which satisfies all the tests in the solar system by construction, can alleviate the Hubble tension. However, given that the constraints on the expansion rate span a large range of redshifts and thus distances, this model cannot fully resolve the issue, just as the other contenders. Furthermore, one of its main features is that it predicts a lower fraction of cold dark matter today, a conclusion which is challenged by the most recent weak-lensing and supernovae results.

In the fully nonlinear regime, gravitational waves can reveal a drastic new phenomenology absent in the linearized theory. This is best illustrated by investigating exact non-perturbative radiative solutions of General Relativity. When new fields are introduced, such as in modified scalar-tensor theories, the scalar-tensor mixing can drastically modify the phenomenology of gravitational waves. So far, most of the efforts have focused on investigating the effects of these scalar-tensor mixing at the perturbative level.
In this talk, I will present a new exact radiative solution in Horndeski gravity describing the nonlinear gravitational waves emitted by a scalar pulse. I will use this opportunity to describe the non-perturbative methods allowing to analyze the gravitational wave content of a given spacetime geometry (Petrov classification, Penrose limit). Then, I will discuss how this exact new solution can be constructed using the disformal solution-generating map and comment on its new phenomenology. In particular, this analysis will show that in such higher order scalar-tensor theory, shear (tensorial modes) can be generated by a purely (time-dependent) scalar monopole, a feature which only emerges at the fully nonlinear level.
This work aims at providing a tractable toy model to explore the nonlinear radiative regime of modified gravity theories. The talk will be based on the following article: https://arxiv.org/abs/2401.05099