Bulk viscous cosmology

We propose a scenario in which the dark components of the Universe are manifestations of a single bulk viscous fluid. Using dynamical system methods, a qualitative study of the homogeneous, isotropic background scenario is performed in order to determine the phase space of all possible solutions. The specific model which we investigate shares similarities with a generalized Chaplygin gas in the background but is characterized by nonadiabatic pressure perturbations. This model is tested against supernova type Ia and matter power spectrum data. Different from other unified descriptions of dark matter and dark energy, the matter power spectrum is well behaved, i.e., there are no instabilities or oscillations on small perturbation scales. The model is competitive in comparison with the currently most popular proposals for the description of the cosmological dark sector.

Figure 1

Phase diagrams for $\nu <1/2$ (upper panel) and for $1/2<\nu <3/2$ (lower panel). For $\nu >3/2$ the directions of the arrows in the lower panel have to be reversed.

Figure 2

The two-dimensional plots of the probability density function (PDF) as a function for the parameter $B$, the baryon density, and the Hubble constant. The joint PDF peak is shown by the large dot, the confidence regions of $1\sigma$ (68, 27%) by the red dotted line, the $2\sigma$ (95, 45%) in the blue dashed line, and the $3\sigma$ (99, 73%) in the green dashed-dotted line.

Figure 3

The one-dimensional plots of the PDF as a function for the parameter $B$, the baryonic density, the Hubble constant, and the deceleration parameter. The joint PDF peak is shown by the large dot, the confidence regions of $1\sigma$ (68, 27%) by the red dotted line, the $2\sigma$ (95, 45%) in the blue dashed line, and the $3\sigma$ (99, 73%) in the green dashed-dotted line.

Figure 4

Predicted power spectrum for the dark viscous fluid compared with the 2dFGRS observational data for $q_0=-0.3$ (left) and $q_0=-0.4$ (right). The initial conditions using the BBKS transfer function correspond to wrong ratios (see text) of dark matter and dark energy: ${\Omega }_{dm0}=0.49$ and ${\Omega }_{\Lambda 0}=0.51$ (left) and ${\Omega }_{dm0}=0.52$ and ${\Omega }_{\Lambda 0}=0.48$ (right).

Figure 5

Predicted power spectrum for the dark viscous fluid compared with the 2dFGRS observational data for $q_0=-0.5$ (left) and $q_0=-0.6$ (right). The initial conditions using the BBKS transfer function correspond to wrong ratios of dark matter and dark energy: ${\Omega }_{dm0}=0.44$ and ${\Omega }_{\Lambda 0}=0.56$ (left) and ${\Omega }_{dm0}=0.52$ and ${\Omega }_{\Lambda 0}=0.48$ (right).