Lepton (flavour) asymmetries in the early universe

At temperatures below the QCD phase transition, any substantial lepton (flavour) asymmetry in the Universe can only be present within the neutrino sector. In this talk, I discuss the limits on primordial lepton flavour asymmetries from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). This requires solving the quantum kinetic equations describing neutrino oscillations as well as the BBN reaction network. These bounds are complementary to the limits obtained from avoiding baryon overproduction through sphaleron processes at the electroweak phase transition and I will discuss implications for CP-violating BSM scenarios.