The imminent advent of 30-m class telescopes and Stage-4 cosmic microwave background observatories promises to give us precision measurements of key parameters which are set in the very early universe. For example, we may soon know to fair precision the amount of relic relativistic energy and the deuterium and helium abundances set during the time when the neutrinos fall out of thermal and chemical equilibrium. Given the excitement and ferment right now surrounding new ideas in dark matter, dark sector, and other beyond standard model (BSM) physics, we would very much like to leverage these coming measurements into deeper insights into this epoch, in effect turning the early universe into a precision BSM physics laboratory. Doing so, however, requires theorists to "raise their game” in modeling the neutrino decoupling epoch. We will discuss these issues and reveal some surprising features of the universe when it was roughly one second in age.