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 B₀ implies that the magnetic dipole moments of any two neutrino species are bounded by μ_ν{}10^-16{{{μ}}}B$[(${10}^{{{-}}9}$ G)/${B}₀$] because of nucleosynthesis constraints on the number of neutrino flavors.