Beyond Standard Model Decays During the Big Bang Nucleosynthesis Epoch

In this talk, 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, diluting decoupled particles, and will produce non-thermal high-energy neutrinos of all flavors. By considering both the production and scattering of these high-energy neutrinos, we investigate the observable consequences of this process in cosmological observables such as the number of relativistic degrees of freedom ($N_{\rm eff}$) and the sum of the neutrino mass, and discuss the possible effects of changing the time-temperature relation and an non-thermal high-energy distribution of neutrinos and anti-neutrinos on BBN yields.