CONSTRAINTS ON THE STRENGTH OF PRIMORDIAL MAGNETIC-FIELDS FROM BIG-BANG NUCLEOSYNTHESIS REEXAMINED

In this paper, we reexamine in detail the effects of primordial magnet ic fields on big bang nucleosynthesis (BBN) including a discussion of the magnetic field geometry and the anomalous magnetic moment. The pre sence of magnetic fields affects BBN by (1) increasing the weak reacti on rates, (2) increasing the electron density due to changes to the el ectron phase space, and (3) increasing the expansion rate of the unive rse, due both to the magnetic field energy density and to the modified electron energy density. Of the effects considered, the increase in t he expansion rate due to the magnetic field energy is the most signifi cant for the interests of BBN. The allowed magnetic field intensity at the end of nucleosynthesis (0.01, MeV) is about 2 x 10(9) G and corre sponds to an upper limit on the magnetic field energy density of about 28% of the neutrino energy density (rho(B) less than or equal to 0.28 rho(nu)).