Density fluctuations in an expanding viscous and conducting universe. I

The effect of viscosity and thermal conductivity on density fluctuations in a hot Einstein-de Sitter expanding universe is investigated in the frame of classical hydrodynamics in the gaseous period after decoupling. Initial conditions are taken as general and density fluctuations evolve according to a superposition of two acoustic modes and one conductive mode, which are all exponentially damped when irreversible effects become important. Isochoric initial perturbations are shown to yield a transfer function which has a maximum at a mass typical of a globular cluster.