TBA

In stellar evolution, one of the fundamental ingredients is the chemical composition. At the stellar surface, the chemical abundances change during the life of the stars due to the presence of the chemical transport mechanisms. One of these processes is atomic diffusion. This process presents great success in simulating solar-type star evolution.
However, for F-type stars, the inclusion of atomic diffusion shows unrealistic chemical variations at the stellar surface, indicating the need for competing transport processes. In this talk, we present the possibility of parameterizing the effects of the competing processes using the turbulent mixing. We successfully found that it is possible to avoid the chemical over-variations at the stellar surface. We also found that the effects of radiative acceleration in F-type stars can be reproduced using turbulent mixing.
Then, we explored the performance of using the parameterised turbulent mixing in the inference of the global properties from a sample of observed Kepler stars. We also compared its results with the current models that neglect atomic diffusion in F-type stars. We found that considering turbulent mixing and atomic diffusion induce a difference in the inferred age up to 20%.

The modeling and interpretation of molecular spectra from interstellar environments require the knowledge
of both radiative and collisional processes of energy transfer between chemical species. Accurate
state-to-state rate coefficients of interstellar molecules in collision with the main collisional partners H2,
H and He are then necessary.

Such collisional data can be obtained through quantum time-independant scattering calculations by
resolving the close-coupled equations. This method is nowadays one of the most accurate for describing
collisions at very low temperature. However, species such as radicals possess very complex energetic
structures because of the presence of non zero electronic and even nuclear spins. Also, accurate description
of fine and hyperfine transitions implying two interacting molecules is a true theoretical challenge.
Then, this method is not fully applicable in terms of memory and CPU time.

This thesis work consists in quantifying the collisional excitation of these open-shell molecules in astrophysical
media. The development of methodological and numerical tools are then required to overcome
these problems and to characterize physical and chemical processes as precisely as possible. The provided
collisional data are expected to interpret observations and corroborate experiments performed in
laboratory.

The redshifted 21 cm signal from neutral hydrogen is a very useful probe of the cosmic evolution. A particularly interesting phase of this evolution is the Epoch of Reionization (EoR), when the first ionizing sources were born in the early universe. The current and the upcoming low frequency radio telescopes, like the upgraded GMRT (Giant Metrewave Radio Telescope) or the Square Kilometre Array (SKA), are expected to play a crucial role in studying cosmic evolution during the EoR and the post-EoR era, and thus addressing many important but unresolved issues. One of the challenging aspects in this regard is the fast and efficient data processing necessary for detecting the weak cosmological signal in presence of the strong, confusing foreground. However, the foreground is interesting on its own right. In this talk, I will share some of our recent efforts to study the low radio frequency foreground using data from the uGMRT, an SKA pathfinder, as stepping stones towards constraining the 21 cm signal. This work may help us to understand the near-future pathways of studying the atomic hydrogen power spectrum from the EoR/post-EoR era; it also allows us to study, in details, the properties of a large population of extragalactic sources as well as of the interstellar medium of the Milky Way.

Reconciling the standard model's theoretical predictions with precision measurements from multiple probes stands as one of the most pressing challenges of modern cosmology. During the last decade, increasing evidence for persistent discrepancies in the value of cosmological parameters when inferred from model-dependent and -independent probes opened the door for a new paradigm. In this talk, I will address whether (part of) the cosmological tensions can be attributed to missing physics in the standard model. In particular I will focus on tests of different models with interactions between dark energy and dark matter.

What does an accurate model of the universe consist of? First, it must rely on the correct fundamental laws of physics, as we know them today. Second, because we only observe the universe from a specific vantage point at a specific time in its history, we need to make some assumptions about the symmetries and large-scale
properties of the universe to be able to extrapolate from our observations to what distant regions of the universe might look like. Finally, once the theatre has been set by the fundamental laws and the cosmological hypotheses, we need to describe all the observations we make in models that are detailed enough to not introduce theoretical biases in our interpretation and analysis of observations. This accurate modelling of small but subtle effects is also essential to help uncover new phenomena by pointing to observables challenging minute aspects of the model.
Over the years, I have been involved in investigating all three of these pillars of cosmology with a constant focus on trying to test their limits and robustness. In this defence, I will specifically focus on:
1. tests of the cosmological principle;
2. light propagation in a lumpy universe;
3. line-of-sight effects in strong gravitational lensing.

Ultra-light bosons such as axions and dark photons are copiously produced around black holes due to the superradiant instability, at the expense of the black hole’s mass and spin. Here I present two new signatures of black-hole superradiance that can be used to look for these particles. First, I show that the superradiant spin-down of super massive black holes is uniquely imprinted in measurements of stellar tidal disruption event (TDE) rates. Upcoming TDE rate measurements by the LSST could reveal or set constraints on the existence of axions or dark photons with masses around 10^-19 eV. And second, I show that kinetically-mixed dark-photon superradiant clouds around stellar mass black holes lead to a new type of electromagnetically bright and possibly pulsating astrophysical source. These “new pulsars”, which are realized for dark photons with masses around 10^-13 eV, can be looked for using X-ray and radio telescopes, and by performing GW searches of anomalous pulsars.

We are living exciting times: we are now able to probe the most violent events of the Universe with diverse messengers (cosmic rays, neutrinos, photons and gravitational waves). One challenge to complete the multi-messenger picture resides in the highest energies, as no ultra-high energy neutrinos have been observed yet. This challenge could be undertaken by the GRAND (Giant Radio Array for Neutrino Detection) project, which aims at detecting ultra-high energy particles, with a colossal array of 200'000 antennas over 200'000 km2, split into ~20 sub-arrays of ~10'000 km2 deployed worldwide. In this talk, we will present preliminary designs and simulation results, plans for the ongoing, staged approach to construction, and the rich research program made possible by the proposed sensitivity and angular resolution.

After giving a short introduction on gamma-ray bursts (grb) prompt phase, I will discuss two aspects of the presence of protons in the jet. Firstly, I will present a model of emission based on proton synchrotron and the production of pairs via the Bethe-Heitler process. Secondly, after reviewing why grbs are good candidates to be the site of ultra-high energy cosmic ray (UHECR, around 10^20 eV) acceleration, I will show how the characteristics of grb emission constrain the maximum energy that UHECR can reach in their jet to be less than 10^19 eV for fiducial grb parameters. Finally, I will explain why a specific class of grbs, namely low-luminosity grbs, could still satisfy the constraints. However, I will show that their contribution to the total flux of UHECR is necessarily limited