Beyond Standard Model Decays During the Big Bang Nucleosynthesis Epoch

We explore Beyond Standard Model (BSM) physics models of out-of-equilibrium particle decay in the early universe around the time of Big Bang Nucleosynthesis (BBN). In particular, we look into the decay of massive neutral fermions (e.g., ``sterile neutrinos'') into Standard Model particles that will heat the photon-electron-positron-baryon plasma during the BBN epoch and produce non-thermal high-energy neutrinos and antineutrinos of all flavors. By considering the production and scattering of these high-energy neutrinos, we investigate the implications of this process on cosmological observables such as the number of relativistic degrees of freedom, $N_{\rm eff}$, and the sum of the neutrino masses, and further discuss possible effects of changing the time-temperature relation and a non-thermal high-energy distribution of neutrinos and antineutrinos on BBN yields.