Self-Consistent Evolution of Magnetic Fields and Chiral Asymmetry in the Early Universe

We show that the evolution of magnetic fields in a primordial plasma, filled with standard model particles at temperatures T10 MeV, is strongly affected by the chiral anomaly---an effect previously neglected. Although reactions, equilibrating left and right electrons, are in thermal equilibrium for T80 TeV, a left-right asymmetry develops in the presence of strong magnetic fields. This results in magnetic helicity transfer from shorter to longer scales and lepton asymmetry present in the plasma until T~10 MeV, which may strongly affect many processes in the early Universe.