Cosmic microwave background Stage-IV experiments and thirty-meter-class
telescopes will come on line in the next decade. The convolution of these data
sets will provide on order 1% precision for observables related to neutrino
cosmology. Beyond Standard Model (BSM) physics could manifest itself in slight
deviations from the standard predictions of quantities such as the neutrino
energy density and the primordial abundances from Big Bang Nucleosynthesis
(BBN). In this talk, I will argue for the need for precise and accurate
numerical calculations of BBN. I will first show the detailed evolution of the
neutrino spectra as they go out of equilibrium with the plasma. The spectra
are important in changing the ratio of neutrons to protons. I will show how
sensitive the primordial mass fraction of helium is to the weak interaction
rates which evolve the neutron-to-proton ratio. Finally, I will present an
example of how BSM physics can affect BBN by instituting an asymmetry between
neutrinos and antineutrinos, commonly characterized by a lepton number.