We present a discussion of the effects induced by bulk viscosity on the very
early Universe stability. The viscosity coefficient is assumed to be related to
the energy density ρ via a power-law of the form ζ=ζ0ρs
(where ζ0,s=const.) and the behavior of the density contrast in
analyzed.
In particular, we study both Einstein and hydrodynamic equations up to first
and second order in time in the so-called quasi-isotropic collapsing picture
near the cosmological singularity. As a result, we get a power-law solution
existing only in correspondence to a restricted domain of ζ0. The
particular case of pure isotropic FRW dynamics is then analyzed and we show how
the asymptotic approach to the initial singularity admits an unstable
collapsing picture.Comment: 4 pages, no figur