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

Recently Cheng, Olinto, Schramm, and Truran reexamined the constraints on the strength of primordial B fields from big bang nucleosynthesis (BBN). Their bottom line agreed with that of an earlier recent paper o n the subject by Kernan, Starkman, and Vachaspati, both in its final l imit on the B field during BBN and in its conclusion that, for allowed values of the B field, the dominant factor for BBN is the increased e xpansion rate at a given temperature caused by the energy density of t he magnetic field, B-2/8 pi. However, their conclusion that weak inter action rates increased with an increasing B field at these low field v alues contradicted the earlier results of Kernan, Starkman, and Vachas pati. In this Comment we point out that the Taylor series expansion of the weak interaction rate about B=0 used by Cheng et al. is not well defined, while the Euler-McLaurin expansion of Kernan, Starkman, and V achaspati is well behaved and reliable. Using the Euler-McLaurin expan sion we find that the weak interaction rates decrease rather than incr ease with an increasing B field at small values of the B field.