Dark goo: Bulk viscosity as an alternative to dark energy

TL;DR

This paper proposes a microscopic model where bulk viscosity from self-interacting particles can explain the universe's accelerated expansion, mimicking dark energy without fine-tuning.

Contribution

It introduces a new microscopic model linking bulk viscosity to dark energy phenomena, with analysis of background and perturbation behavior showing viability.

Findings

Bulk viscosity can produce negative effective pressure similar to dark energy.

Linear perturbations remain well-behaved in the model.

No fine-tuning required for parameters to match observations.

Abstract

We present a simple (microscopic) model in which bulk viscosity plays a role in explaining the present acceleration of the universe. The effect of bulk viscosity on the Friedmann equations is to turn the pressure into an "effective" pressure containing the bulk viscosity. For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic a dark energy equation of state. Our microscopic model includes self-interacting spin-zero particles (for which the bulk viscosity is known) that are added to the usual energy content of the universe. We study both background equations and linear perturbations in this model. We show that a dark energy behavior is obtained for reasonable values of the two parameters of the model (i.e. the mass and coupling of the spin-zero particles) and that linear perturbations are well-behaved. There is no apparent fine tuning involved. We also discuss the conditions under which hydrodynamics holds, in particular that the spin-zero particles must be in local equilibrium today for viscous effects to be important.