ELECTROWEAK ORIGIN OF COSMOLOGICAL MAGNETIC-FIELDS - ART. NO. 043501

Magnetic fields may have been generated in the electroweak phase trans ition through spontaneous symmetry breaking or through the subsequent dynamical evolution of semiclassical field configurations. Here I demo nstrate explicitly how magnetic fields emerge spontaneously in the pha se transition also when no gradients of the Higgs field are present. U sing a simple model, I show that no magnetic fields an generated, at l east initially, from classical two-bubble collisions in a first-order phase transition. An improved gauge-invariant definition of the electr omagnetic field is advocated which is more appropriate in the sense th at it never allows electrically neutral fields to serve as sources for the electromagnetic field. In particular, semiclassical configuration s of the Z field alone do not generate magnetic fields. The possible g eneration of magnetic fields in the decay of unstable Z strings is dis cussed.