KINETIC VERSUS THERMAL-FIELD-THEORY APPROACH TO COSMOLOGICAL PERTURBATIONS

A closed set of equations for the evolution of linear perturbations of homogeneous, isotropic cosmological models can be obtained in various ways. The simplest approach is to assume a macroscopic equation of st ate, e.g., that of a perfect fluid. For a more refined description of the early Universe, a microscopic treatment is required. The purpose o f this paper is to compare the approach based on classical kinetic the ory to the more recent thermal-field-theory approach. It is shown that in the high-temperature limit the latter describes cosmological pertu rbations supported by collisionless, massless matter, wherein it is eq uivalent to the kinetic theory approach. The dependence of the perturb ations in a system of a collisionless gas and a perfect fluid on the i nitial data is discussed in some detail. All singular and regular solu tions are found analytically.