Kinetic theory of neutrinos and nuclei in the 'early' universe*

We wish to use astronomical observations of the early universe at high precision as an extraterrestrial laboratory to study an array of fundamental and applied questions in nuclear and particle physics. Suppose, for example, we want to constrain a particular beyond-standard model scenario that might affect the amounts of light elements created in the Big Bang. Here, as in other scenarios we might consider, we are faced with solving two challenging, tightly coupled problems. That of the non-equilibrium evolution of the neutrino distributions in energy and flavor and, second, determining the nuclear abundances, which depend sensitively on weak interactions with neutrinos, by solving a stiff nuclear reaction network. We overview work on this topic, highlighting recent advances in our understanding of neutrino flavor evolution in the presence of their collisions with each other and matter in the early universe. We demonstrate, by concurrent solution of the neutrino and matter plasma evolution, percent-level effects on predicted deuterium abundances due to non-equilibrium distortions of the neutrino spectra, an order of magnitude larger than previous estimates. Preliminary results for coherent neutrino flavor evolution in the presence of collisions are also discussed.