Magnetic fields in the early Universe

This review concerns the origin and the possible effects of magnetic fields in the early Universe. We start by providing the reader with a short overv iew of the current state of the art of observations of cosmic magnetic fiel ds. We then illustrate the arguments in favor of a primordial origin of mag netic fields in the galaxies and in the clusters of galaxies. We argue that the most promising way to test this hypothesis is to look for possible imp rints of magnetic fields on the temperature and polarization anisotropies o f the cosmic microwave background radiation (CMBR). With this purpose in mi nd, we provide a review of the most relevant effects of magnetic fields on the CMBR. A long chapter of this review is dedicated to particle-physics-in spired models which predict the generation of magnetic fields during the ea rly Universe evolution. Although it is still unclear if any of these models can really explain the origin of galactic and intergalactic magnetic field s, we show that interesting effects may arise anyhow. Among these effects, we discuss the consequences of strong magnetic fields on the big-bang nucle osynthesis, on the masses and couplings of the matter constituents, on the electroweak phase transition, and on the baryon and lepton number violating sphaleron processes. Several intriguing common aspects, and possible inter play, of magnetogenesis and baryogenesis are also discussed. (C) 2001 Elsev ier Science B.V. All rights reserved.