Abstract

Experimental evidence1 for a non-zero neutrino rest mass, mv≠0, while still controversial2, poses intriguing cosmological questions3–10. One reassurance is that the possibility of non-zero masses does not necessarily alter previous predictions of the standard big-bang model for the primordial 4He abundance, Y. We show here that a non-zero neutrino rest mass, mv ∼ 10eV, may provide the most stringent upper limit proposed to date on the strength of any ordered, primordial magnetic field: B0≲4 × 10−9/mv (eV) G. This limit is set by requiring that previous predictions of the standard big bang model for the primordial 4He abundance remain unaltered by the spin procession and subsequent thermalization of right-handed, as well as left-handed, Dirac neutrinos in the early Universe.

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