Propagation of light pseudoscalar particles in a cosmological background

The interaction between light pseudoscalar particles, having properties much like those of the invisible axion, and the cosmic-microwave-background electromagnetic field is considered in a simple cosmic evolutionary scenario. The quantum theory of this interaction is hence canonically developed and applied to study seed, possibly of cosmological origin, extragalactic radiation fields. After looking at the issue of anisotropies in the microwave cosmic-background radiation spectrum, the main arguments setting limits on the couplings of the right, popular, invisible axion are briefly reviewed. By allowing for a minimal extension of the Big Bang Standard Cosmology, namely by a primordial magnetic field, we argue that a hypothetical, very light pseudoscalar particle, not too different from the recently proposed alternate version of the invisible axion, could give rise to very peculiar observational effects in the microwave cosmic-background radiation sector. In a quite simplified version of such a model, the most important effect which emerges is the issue of a highly polarized, wavelength-dependent quadrupolar-like anisotropy. This prediction could be feeling with the claimed, at present wellestablished observation of a quadrupolar component in the cosmic microwave background radiation by COBE satellite.