Cosmological consequences of first-order general-relativistic viscous fluid dynamics

TL;DR

This paper explores how a causal, first-order viscous fluid can cause accelerated cosmic expansion without a cosmological constant, with the viscous effects driving acceleration while the fluid density diminishes.

Contribution

It introduces a new framework for relativistic viscous fluids in cosmology, demonstrating that viscosity alone can lead to accelerated expansion without dark energy.

Findings

Viscous fluids can cause universe acceleration with zero fluid density.

The model is consistent with Einstein's equations and causality.

Accelerated expansion emerges naturally from viscous effects.

Abstract

We investigate the out-of-equilibrium dynamics of viscous fluids in a spatially flat Friedmann-Lema\^itre-Robertson-Walker cosmology using the most general causal and stable viscous energy-momentum tensor defined at first order in spacetime derivatives. In this new framework a pressureless viscous fluid having density $\rho$ can evolve to an asymptotic future solution in which the Hubble parameter approaches a constant while $\rho \rightarrow 0$, even in the absence of a cosmological constant (i.e., $\Lambda = 0$). Thus, while viscous effects in this model drive an accelerated expansion of the universe, the density of the viscous component itself vanishes, leaving behind only the acceleration. This behavior emerges as a consequence of causality in first-order theories of relativistic fluid dynamics and it is fully consistent with Einstein's equations.