Pair creation in cosmology when electromagnetic fields are present

The amount of particle pairs created in a 3-flat Robertson-Walker Universe with an expansion law for the early Universe is calculated exactly when a homogeneous electromagnetic field is present. Under some restrictions a time-dependent particle creation rate is found. Finally it is shown that the low-frequency part of the cosmological 2.7K background radiation can be identified with the stationary electromagnetic field discussed before. According to this a large amount of particles of the order of the number of particles in the Universe should be created out of the vacuum in the immediate neighbourhood of the 'big bang'.