Abstract
A thermodynamically oriented gauge-invariant description of scalar cosmological perturbations around a flat Friedman universe is given. All basic variables have their physical meaning on comoving hypersurfaces, i.e. on hypersurfaces orthogonal to the matter worldlines. Several well known growth and decay rates of cosmological perturbation modes are rediscovered in a transparent way. The formalism is extended to multicomponent systems and applied to a mixture of radiation and non-relativistic particles for perturbations both smaller and larger than the particle horizon. Thermodynamical perturbation relations in the Minkowski spacetime turn out to be valid in the expanding universe, provided all perturbed quantities are replaced by their gauge-invariant counterparts.
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