Big bang nucleosynthesis constraints on primordial magnetic fields.

We reanalyze the e ect of magnetic elds in BBN, incorporating several features which were omitted in previous analyses. We nd that the e ects of coherent magnetic elds on the weak interaction rates and the electron thermodynamic functions ( e, Pe, and d e=dT ) are unimportant in comparison to the contribution of the magnetic eld energy density in BBN. In consequence the e ect of including magnetic elds in BBN is well approximated numerically by treating the additional energy density as e ective neutrino number. A conservative upper bound on the primordial magnetic eld, parameterized as = 2eBrms=(T 2 ), is 2 ( B < 0:27 ). This bound can be stronger than the conventional bound coming from the Faraday rotation measures of distant quasars if the cosmological magnetic eld is generated by a causal mechanism.