Magnetically induced neutrino oscillations and neutrino refractive effects in the early Universe

We investigate neutrino spin precession in primordial magnetic fields, taking previously ignored neutrino refractive effects into account. A present-day intergalactic field strength ${\mathrm{B}}_0$ implies that the magnetic dipole moments of any two neutrino species are bounded by ${\mathrm{\mu }}_{\nu }$≲${10}^{\mathrm{-}16}$ ${\mathrm{\mu }}_{\mathrm{B}}$[(${10}^{\mathrm{-}9}$ G)/${\mathit{B}}_0$] because of nucleosynthesis constraints on the number of neutrino flavors.