Neutrino Quantum Kinetics in the Early Universe*

Neutrinos decouple from the plasma at a temperature scale of 1 MeV in the early universe. The decoupling is an out-of-equilibrium process and requires the solution of the Quantum Kinetic Equations (QKEs) in a homogeneous and isotropic geometry. We present results from our code BURST on solving the complete set of QKEs relevant for neutrino energy transport. By using generalized density matrices, we follow the non-degenerate spectra of electron, muon, and tau flavor neutrinos. In addition, we are able to show how the coherence between the flavor states manifests itself in the off-diagonal elements of the density matrices. Our results have implications for both the primordial abundances predicted from big bang nucleosynthesis and neutrino observables in the cosmic microwave background.