Viscous Cold Dark Matter in agreement with observations

Abstract

We discuss bulk viscous cosmological models. Since the bulk viscous pressure is negative, viable viscous cosmological scenarios with late time accelerated expansion can in principle be constructed. After discussing some alternative models based on bulk viscous effects we will focus on a model very similar to the standard $\Lambda$CDM. We argue that a $\Lambda${\rm v}CDM model, where we assign a very small (albeit perceptible) bulk viscosity to dark matter is in agreement with available cosmological observations. Hence, we work with the concept of viscous Cold Dark Matter ({\rm v}CDM). At the level of the perturbations, the growth of {\rm v}CDM structures is slightly suppressed when compared with the standard CDM ones. Having in mind that the small scale problems of the $\Lambda$CDM model are related to an excess of clustering, our proposal seems to indicate a possible direction for solving the serious drawbacks of the CDM paradigm within the standard cosmological model.Comment: 16 pages, 2 figures, to the proceedings of "49th Winter School of Theoretical Physics Cosmology and non-equilibrium statistical mechanics", L{\ka}dek-Zdr\'oj, Poland, February 10-16, 201