MAGNETIC-FIELDS AND LARGE-SCALE STRUCTURE IN A HOT UNIVERSE .1. GENERAL EQUATIONS

We consider that no mean magnetic field exists during this epoch, but that there is a mean magnetic energy associated with large-scale magne tic inhomogeneities. We study the evolution of these inhomogeneities a nd their influence on the large scale density structure, by introducin g linear perturbations in Maxwell equations, the conservation of momen tum-energy equation, and in Einstein field equations. The primordial m agnetic held structure is time independent in the linear approximation , only being diluted by the general expansion, so that BR2 is conserve d in comoving coordinates. Magnetic fields have a strong influence on the formation of large-scale structure. Firstly, relatively low fields are able to generate density structures even if they were inexistent at earlier times. Second, magnetic fields act anisotropically more rec ently, modifying the evolution of individual density clouds. Magnetic flux tubes have a tendency to concentrate photons in filamentary patte rns.