COSMOLOGICAL PLASMA MODES

Scalar perturbations of a four-fluid cosmological model are investigat ed for the case that two of the fluid components are charged. The norm al modes of a mixture of ionized hydrogen, radiation, and dark matter before hydrogen recombination are determined analytically for several limiting cases. The corresponding modes in an electron-positron plasma are also studied. Well-known growing and decaying modes of fluid cosm ologies are recovered. Additionally, plasma oscillations both on scale s larger and smaller than the Hubble horizon are found. All modes are gauge-invariantly characterized in terms of thermodynamical variables on comoving hypersurfaces. While the plasma oscillations decay and do not essentially influence the gravitational instability scenario, the latter is modified by the possibility of growing baryonic energy densi ty perturbations as soon as the dark matter becomes dynamically domina ting. At the decoupling time of matter and radiation all the matter, i ncluding the baryonic one, may be more clumpy than the radiation. The picture of baryons falling into the dark matter potential wells after decoupling turns out to be superfluous.