We investigate the out-of-equilibrium dynamics of viscous fluids in a spatially flat Friedmann-Lemaître-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 equilibrium energy density $\rho$ can evolve to an asymptotic future solution in which the Hubble parameter approaches a constant while $\rho \to 0$, even in the absence of a cosmological constant (i.e., $\mathrm{\Lambda }=0$). Thus, while viscous effects in this model drive an accelerated expansion of the universe, the equilibrium energy density 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.

• Received 3 November 2022
• Accepted 18 December 2022

DOI:https://doi.org/10.1103/PhysRevD.107.023512