In the previous chapter we have seen that, very strong magnetic fields could have been produced in the early Universe. Here we investigate the effects of such strong fields on bound states of quarks and on condensates created by spontaneous symmetry breaking. For example, we have already seen in Chap. 3 that strong magnetic fields can affect masses and decay rates of charged particles and modify the rate of weak processes. As we already discussed in Sec. 1.4 another crucial issue concerns the stability of strong magnetic fields. QED allows the existence of arbitrary large magnetic fields provided matter constituents have spin less than 1/2 .
This is so because the Lorentz force cannot perform any work on charged particles so that real particle-antiparticles free pairs cannot be produced. In Sec. 3.1 we have already seen that quantum corrections do not spoil this classical argument. We have also seen that although pair production can be catalyzed by strong magnetic fields at finite temperature and density, in this case it is the heat-bath that pays for the energy cost of the effect. This situation changes when, in the presence of very strong fields, QCD and electroweak corrections cannot be disregarded. We shall see that the QCD and electroweak field allow for the formation of condensates of charged particles with no energy cost. This may lead to screening of magnetic, or hypermagnetic, fields resembling the Meissner effect in superconductors.