As the temperature increases above 1 MeV, a relativistic plasma becomes almost a perfect conductor. There are two complementary effects associated with large-scale magnetic fields in curved backgrounds:
large-scale magnetic fields may evolve over a rigid background space-time determined, for instance, by the dynamics of barotropic fluids and, in this case the energy density of the magnetic field must always be smaller than the energy density of the fluid sources;
magnetic fields may be so intense to modify the structure of the space-time and, in this case, their energy density is comparable with the energy density of the other sources of the geometry.
The first effect will be examined in the present Section, while the discussion of the second effect, which is more speculative, will be confined to Section 9.