NEUTRINO DISPERSION IN MAGNETIZED MEDIA AND SPIN OSCILLATIONS IN THE EARLY UNIVERSE

We derive general expressions for the neutrino dispersion relation in a magnetized plasma with a wide range of temperatures, chemical potent ials, and magnetic field strengths, If the electron and proton chemica l potentials vanish, as in the early Universe, there is no magnetizati on contribution to the neutrino refractive index to leading order in t he Fermi coupling constant, contrary to claims in the recent literatur e, Therefore, as long as the magnetic field satisfies B less than or s imilar to T-2, the neutrino refractive index in the early Universe is dominated by the standard ''non-local term'', If neutrinos are Dirac p articles with magnetic moment mu, then their right-handed components a re thermally populated before the nucleosynthesis epoch by magneticall y induced spin oscillations if mu B-0 greater than or similar to 10(-6 )mu(B) G, where mu(B) = e/2m(e) is the Bohr magneton and B-0 is a larg e-scale primordial magnetic field at T-0 approximate to 1 MeV. For a t ypically expected random field distribution, even smaller values for m u B-0 would suffice to thermalize the right-handed Dirac components.