The Universe at the MeV era: neutrino evolution and cosmological observables

TL;DR

This paper investigates neutrino evolution in the early Universe at MeV temperatures, providing precise calculations of neutrino decoupling, flavor oscillations, and their impact on primordial element abundances, crucial for cosmological models.

Contribution

It introduces a new derivation of kinetic equations for neutrino evolution and applies it to improve predictions of cosmological parameters like N_eff and primordial abundances.

Findings

Achieved high-precision calculation of N_eff.

Validated an effective description of flavor oscillations.

Analyzed effects of incomplete neutrino decoupling on BBN.

Abstract

Neutrino physics in the early Universe is key to our understanding of later cosmological stages, such as primordial nucleosynthesis (BBN) or the formation of large-scale structures. The coming decade promises new experimental results to explore and constrain cosmological models even more precisely - which requires robust theoretical predictions. This PhD thesis presents a study of the evolution of neutrinos in the first seconds after the Big Bang, more precisely when the temperature of the Universe is of the order of one mega-electronvolt. This evolution is obtained numerically by solving kinetic equations for which we propose a new derivation. A first application is the calculation of the so-called "standard" decoupling in order to calculate the cosmological parameter quantifying the energy density of the primordial relativistic species, $N_\mathrm{eff}$, to a precision of a few ten-thousandths. This study has highlighted the possibility of effectively describing the phenomenon of flavour oscillations, taking advantage of the large separation of time scales involved. Such an approximation is then adapted and validated in the case of non-zero asymmetries between neutrinos and antineutrinos. Finally, we study semi-analytically the consequences of incomplete neutrino decoupling on BBN, in order to understand how the primordial abundances of helium and deuterium are affected by this physics.