PRIMORDIAL MAGNETIC-FIELDS FROM COSMOLOGICAL FIRST-ORDER PHASE-TRANSITIONS

We give an improved estimate of primordial magnetic fields generated d uring cosmological first order phase transitions. We examine the charg e distribution at the nucleated bubble wall and its dynamics. We consi der instabilities on the bubble walls developing during the phase tran sition. It is found that damping of these instabilities due to viscosi ty and heat conductivity caused by particle diffusion can be important in the QCD phase transition, but is probably negligible in the electr oweak transition. We show how such instabilities together with the sur face charge densities on bubble walls excite magnetic fields within a certain range of wavelengths. We discuss how these magnetic seed field s may be amplified by magneto hydrodynamics effects in the turbulent f luid. The strength and spectrum of the primordial magnetic field at th e present time for the cases where this mechanism was operative during the electroweak or the QCD phase transition are estimated. On a 10 Mp c comoving scale, field strengths of the order 10(-29) G for electrowe ak and 10(-20) G for QCD, could be attained for reasonable phase trans ition parameters.