Does Bulk Viscosity Create a Viable Unified Dark Matter Model?

Understand

We investigate in detail the possibility that a single imperfect fluid with bulk viscosity can replace the need for separate dark matter and dark energy in cosmological models.

• With suitable choices of model parameters, we show that the background cosmology in this model can mimic that of a LCDM Universe to high precision.
• However, as the cosmic expansion goes through the decelerating-accelerating transition, the density perturbations in this fluid are rapidly damped out.
• We show that,although this does not significantly affect structure formation in baryonic matter, it makes the gravitational potential decay rapidly at late times, leading to modifications in predictions of cosmological observables such as the CMB power spectrum and weak lensing.