REVISITING NUCLEOSYNTHESIS CONSTRAINTS ON PRIMORDIAL MAGNETIC-FIELDS

In view of several conflicting results, we reanalyze the effects of ma gnetic fields on the primordial nucleosynthesis. In the case the magne tic field is homogeneous over a horizon volume, we show that the main effects of the magnetic field are given by the contribution of its ene rgy density to the Universe expansion rate and the effect of the field on the electrons quantum statistics. Although, in order to get an upp er limit on the field strength, the weight of the former effect is num erically larger, the latter cannot be neglected. Including both effect s in the PN code we get the upper limit B less than or equal to 1 x 10 (11) Gauss at the temperature T = 10(9) K, We generalize the considera tions to cases when instead the magnetic is inhomogeneous on the horiz on length, We show that in these cases only the effect of the magnetic field on the electron statistics is relevant. If the coherence length of the magnetic field at the end of the PN is in the range 10 much le ss than L(0) much less than 10(11) cm our upper limit is B less than o r equal to 1 x 10(12) Gauss.