Study of the Quasi-isotropic Solution near the Cosmological Singularity in Presence of Bulk-viscosity Received Day Month Year Revised Day Month Year Communicated by Managing Editor

We analyze the dynamical behavior of a quasi-isotropic Universe in the presence of a cosmological fluid endowed with bulk viscosity. We express the viscosity coefficient as a power-law of the fluid energy density: ζ = ζ0 ǫs. Then we fix s = 1/2 as the only case in which viscosity plays a significant role in the singularity physics but does not dominate the Universe dynamics (as requested by its microscopic perturbative origin). The parameter ζ0 is left free to define the intensity of the viscous effects. Following the spirit of the work by E.M. Lifshitz and I.M. Khalatnikov on the quasiisotropic solution, we analyze both Einstein and hydrodynamic equations up to first and second order in time. As a result, we get a power-law solution existing only in correspondence to a restricted domain of ζ0.