Modified equation of state, scalar field, and bulk viscosity in Friedmann universe

A generalized dynamical equation for the scale factor of the universe is proposed to describe the cosmological evolution, of which the ACDM model is a special case. It also provides a general example to show the equivalence of the modified equation of state (EOS) and a scalar field model. In the mathematical aspect, the EOS, the scalar field potential V( phi), and the scale factor a(t) all have possessed analytical solutions. Such features are due to a simple form invariance of the equation inherited which determines the Hubble parameter. From the physical point of view, this dynamical equation can be regarded as the ACDM model with bulk viscosity, an existence content in the universe. We employ the SNe la data with the parameter A measured from the SDSS data and the shift parameter R measured from WMAP data to constrain the parameters in our model. The result is that the contribution of the bulk viscosity, accumulated as an effective dark energy responsible for the current cosmic accelerating expansion, is made approximately ten percent to that of the cosmological constant. (c) 2006 Published by Elsevier B.V.