Dissipation of dark matter

TL;DR

This paper investigates the effects of bulk viscosity in cold dark matter, constraining its magnitude using cosmological data and analyzing its impact on small-scale structure formation.

Contribution

It introduces a model of viscous dark matter, constrains its viscosity with observational data, and examines its influence on galaxy formation.

Findings

Viscous dark matter viscosity is constrained to be less than approximately 10^7 Pa·s from cosmological data.

Small-scale structure formation limits viscosity to below about 10^{-3} Pa·s to allow dwarf galaxy formation.

Viscous dark matter models are consistent with current observations within these viscosity bounds.

Abstract

Fluids often display dissipative properties. We explore dissipation in the form of bulk viscosity in the cold dark matter fluid. We constrain this model using current data from supernovae, baryon acoustic oscillations and the cosmic microwave background. Considering the isotropic and homogeneous background only, viscous dark matter is allowed to have a bulk viscosity $\lesssim 10^7$ Pa$\cdot$s, also consistent with the expected integrated Sachs-Wolfe effect (which plagues some models with bulk viscosity). We further investigate the small-scale formation of viscous dark matter halos, which turns out to place significantly stronger constraints on the dark matter viscosity. The existence of dwarf galaxies is guaranteed only for much smaller values of the dark matter viscosity, $\lesssim 10^{-3}$ Pa$\cdot$s.