PROPAGATION OF LIGHT PSEUDOSCALAR PARTICLES IN A COSMOLOGICAL BACKGROUND

The interaction between light pseudoscalar particles, having propertie s much like those of the invisible axion, and the cosmic-microwave-bac kground electromagnetic field is considered in a simple cosmic evoluti onary scenario. The quantum theory of this interaction is hence canoni cally developed and applied to study seed, possibly of cosmological or igin, extragalactic radiation fields. After looking at the issue of an isotropies 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 extens ion of the Big Bang Standard Cosmology, namely by a primordial magneti c field, we argue that a hypothetical, very light pseudoscalar particl e, not too different from the recently proposed alternate version of t he invisible axion, could give rise to very peculiar observational eff ects in the microwave cosmic-background radiation sector. In a quite s implified version of such a model, the most important effect which eme rges is the issue of a highly polarized, wavelength-dependent quadrupo lar-like anisotropy. This prediction could be feeling with the claimed , at present well-established observation of a quadrupolar component i n the cosmic microwave background radiation by COBE satellite.