Impacts of Viscous Matter and Radiation on the Temporal Evolution of the Universe

We propose that the cosmic background is simply characterized by an equation of state of a relativistic fluid consisting of homogeneously and isotropically distributed single particle and single photon. Other components such as dark matter and dark energy (cosmological constant) are not taken into consideration. We assume that the expansion of the Universe is isotropic. Accordingly, the shear viscosity coefficient vanishes. We have derived expressions for the energy density and the bulk viscosity and found that both quantities largely increase with the inclusion of the photon. As for their temporal evolution, there is a rapid decrease in the early stages of the Universe. This seems to saturate at large co-moving time. ///////// We also found that the scale factor in non-viscous and viscous cosmic backgrounds is almost non-distinguish able in the early Universe. The latter suggest going beyond the standard cosmological model (SCM). The non-viscous cosmic background seems to have a scale factor larger than that of the viscous one. The result obtained for the temporal evolution of the Hubble parameter confirms the scale factor results. Namely, the distinguish ability between non-viscous and viscous cosmic backgrounds is the largest in the early Universe. (author)