Characteristic Properties of Two Different Viscous Cosmology Models for the Future Universe

Understand

We analyze characteristic properties of two different cosmological models: (i) a one-component dark energy model where the bulk viscosity $\zeta$ is associated with the fluid as a whole, and (ii) a two-component model where $\zeta$ is associated with a dark matter component $\rho_{\rm m}$ only, the dark energy component considered inviscid.

• Shear viscosity is omitted.
• We assume throughout the simple equation of state $p=w\rho$, with $w$ a constant.
• In the one-component model we consider two possibilities, either to take $\zeta$ proportional to the scalar expansion (equivalent to the Hubble parameter), in which case the evolution becomes critically dependent on the value of the small constant $\alpha=1+w$ and the magnitude of $\zeta$.