Vector Particle Creation and Annihilation in a Friedmann Expansion

A vector field is considered in a Friedmann metric. At the level of classical field theory, one can show that the field comprises a spin-one part and a spin-zero part, whose mass has to be zero, and which is not the usual spin-zero part of vector fields in flat space-time. At the quantum level, because of the Friedmann expansion, the particle number varies. This is fully analogous to a well-known phenomenon already thoroughly studied for scalar and spinor fields. However, because of the presence of the new spin-zero part, in the initial stages of the universal expansion the number of particles must decrease, necessarily implying the existence of a large number of vector quanta since the Planck time. Later on, the expansion will cause particle creation in the same way as in the scalar and spinor theories.