Big-Bang Nucleosynthesis marks an important era in the history of the Universe. During a relatively brief period of time, the light elements were created in a hot and expanding environment, the aftermath of which has left imprints on the abundances of those elements today. Recent advancements in measurements and theoretical predictions of the primordial abundances of light elements have allowed Big-Bang Nucleosynthesis to become a powerful cosmological probe in testing our understanding of the early Universe. An important implication of this is that any physics scenario beyond the Standard Models of particle physics and cosmology, that alters the course of events at that time, could be subject to strong constraints by this probe. In this thesis, three such scenarios are studied in detail. In the ﬁrst case, we consider Heavy Neutral Leptons that are motivated by their ability to account for neutrino masses and the baryon asymmetry of the Universe. We derive bounds on their masses and lifetimes by developing a high-precision Boltzmann code that simulates the early Universe in their presence. In the second case, we discuss the cosmological consequences of thermal dark sectors, which include thermal dark matter candidates and their mediators with the Standard Model sector, and put constraints on their masses. Finally, in the last case, an alternative expansion history through a time-varying gravitational constant is considered. The limits obtained on the time evolution of the gravitational constant can then be applied to different classes of models that predict such an effect. Throughout the thesis, we also comment on the complementary nature of the derived bounds to those from other cosmological, astrophysical and laboratory probes, and discuss what future improvements are required in order to push to envelope.
|Date of Award
|1 Aug 2022
|Diego Blas (Supervisor) & Malcolm Fairbairn (Supervisor)