Sterile neutrino decay during the weak decoupling epoch and its effects on the early Universe and Big Bang Nucleosynthesis

The hot and dense early Universe combined with the promise of high-precision cosmological observations provide an intriguing laboratory for Beyond Standard Model (BSM) physics. We simulate the early Universe to examine the effects of the decay of thermally populated sterile neutrino states into Standard Model products around the time of weak decoupling. These decays deposit a significant amount of entropy into the plasma as well as produce a population of high-energy out-of-equilibrium active neutrinos. As a result, we can constrain these models by their inferred value of N_eff, the effective number of relativistic degrees of freedom. In our work, we explore a variety of models with N_eff values consistent with CMB observations, but with vastly different active neutrino spectra which will alter the time-temperature relation and lepton capture rates on free nucleons.  We discuss how these effects of sterile neutrino decay challenge the standard cosmological model and affect Big Bang Nucleosynthesis (BBN).