In the framework of Einstein-Maxwell-Vlasov-axion model we
analyzed the dispersion relations for the perturbations in an
initially isotropic and homogeneous axionically active
ultrarelativistic plasma, which expands in the de Sitter-type
cosmological background, and classify the longitudinal and
transversal electromagnetic modes. We show that for the special
choice of the guiding model parameters the transversal
electromagnetic waves in the axionically active plasma can
propagate with the phase velocity less than speed of light in
vacuum, thus displaying a possibility for a new type of resonant
particle-wave interactions.
The presence of the pseudoscalar (axion) field $\phi$ provides the
plasma to become a gyrotropic medium, which displays the
phenomenon of optical activity. The frequencies of transversal
electromagnetic waves are shown to depend not only on the
wavelength, but also on the gyration coefficient $p=\dot\phi$, and
this dependence has a critical character. To be more precise, when
$p\neq0$, the dispersion equations admit some new branches of
solutions in addition to the standard ones. If to consider the
transversal electromagnetic wave propagation in terms of left- and
right-hand rotating components, one can state, that one of the
waves (say, with left-hand rotation) can have arbitrary
wavelength, while the second wave can possess the wave number less
than critical one; in this sense we deal with some kind of mode
suppression caused by the axion-photon interactions.
This work was supported by the Russian Foundation for Basic
Research (Grant No. 14-02-00598) and by the
Program of Competitive Growth of Kazan Federal University (Project
No. 0615/006.15.02302.034)