Comment on "Accelerating cosmological expansion from shear and bulk viscosity"

TL;DR

This paper critiques a recent claim that shear viscosity alone causes cosmic acceleration, arguing it only affects small scales within the Hubble radius and not the entire universe.

Contribution

It clarifies the limitations of shear viscosity's role in cosmic acceleration, emphasizing it cannot explain the observed acceleration on large cosmological scales.

Findings

Shear viscosity impacts small-scale cosmic dynamics.

Shear viscosity does not drive large-scale acceleration.

The original claim overstates shear viscosity's cosmological effects.

Abstract

In a recent Letter [Phys. Rev. Lett. 114 091301 (2105)] the cause of the acceleration of the present Universe has been identified with the shear viscosity of an imperfect relativistic fluid even in the absence of any bulk viscous contribution. The gist of this comment is that the shear viscosity, if anything, can only lead to an accelerated expansion over sufficiently small scales well inside the Hubble radius.