Constraints on dissipative unified dark matter

TL;DR

This paper investigates viscous dark fluid models as a unified dark matter and energy candidate, finding they fit some data but face significant challenges from cosmic microwave background and structure formation observations.

Contribution

It analyzes bulk viscosity models with a specific density dependence, assessing their viability against cosmological data and identifying key observational constraints.

Findings

Viscous dark fluids can mimic ΛCDM background evolution.

Such models produce large ISW effects inconsistent with observations.

They also suppress small-scale structure growth, conflicting with data.

Abstract

Modern cosmology suggests that the Universe contains two dark components -- dark matter and dark energy -- both unkown in laboratory physics and both lacking direct evidence. Alternatively, a unified dark sector, described by a single fluid, has been proposed. Dissipation is a common phenomenon in nature and it thus seems natural to consider models dominated by a viscous dark fluid. We focus on the study of bulk viscosity, as isotropy and homogeneity at large scales implies the suppression of shear viscosity, heat flow and diffusion. The generic ansatz $\xi \propto \rho^{\nu}$ for the coefficient of bulk viscosity ($\rho$ denotes the mass/energy density), which for $\nu = - 1/2$ mimics the $\Lambda$CDM background evolution, offers excellent fits to supernova and H(z) data. We show that viscous dark fluids suffer from large contributions to the integrated Sachs-Wolfe effect (generalising a previous study by Li & Barrow) and a suppression of structure growth at small-scales (as seen from a generalized Meszaros equation). Based on recent observations, we conclude that viscous dark fluid models (with $\xi \propto \rho^{\nu}$ and neglecting baryons) are strongly challenged.