MAGNETIC-FIELDS FROM PHASE-TRANSITIONS - ART. NO. 103505

The generation of primordial magnetic fields from cosmological phase t ransitions is discussed, paying particular attention to the electrowea k transition and to the various definitions of the ''average'' field t hat have been put forward. It is emphasized that only the volume avera ge has dynamical significance as a seed for galactic dynamos. On rathe r general grounds of causality and energy conservation, it is shown th at, in the absence of MHD effects that transfer power in the magnetic field from small to large scales, processes occurring at the electrowe ak transition cannot generate fields stronger than 10(-20) G on a scal e of 0.5 Mpc. However, it is implausible that this upper bound could e ver be reached, as it would require all the energy in the Universe to be turned into a magnetic field coherent at the horizon scale. Non-lin ear MHD effects seem therefore to be necessary if the electroweak tran sition is to create a primordial seed field. [S0556-2821(98)10720-8].