Big Bang Nucleosynthesis, the Cosmic Neutrino Background, and Beyond Standard Model Physics During the Weak Decoupling Epoch*

Big Bang Nucleosynthesis (BBN) couples nuclear astrophysics with cosmological models, weak interaction physics, and possibly Beyond Standard Model (BSM) physics. While BBN is often viewed as a concordance between independent cosmological observations and theory, the long-standing lithium problem and a newly considered helium anomaly raise intriguing questions. Improved sensitivities in cosmic microwave background observations and from 30 meter-class telescopes coupled with improvements in measuring nuclear reaction rates bring forth the interesting possibility that BBN may probe physics beyond the Standard Model and/or beyond the standard cosmological model. This talk will explore the effects of the decay of sterile neutrinos during the weak decoupling epoch into Standard Model particles, including active neutrinos. BBN is sensitive to the interplay between nuclei, the neutrino sector, and time-temperature relation around this epoch in the early universe. The result is that BBN is a sensitive probe to BSM physics relevant in this epoch.