Viscosity in Isotropic Cosmological Backgrounds in General Relativity and Starobinsky Gravity

TL;DR

This paper investigates the impact of shear viscosity on isotropic cosmological backgrounds in Einstein and $f(R)$ gravity, concluding that shear viscosity does not influence background dynamics or luminosity distances, and critically examines claims of its effects in Starobinsky gravity.

Contribution

It provides a general analysis showing shear viscosity's non-effect on background evolution in isotropic cosmologies and refutes recent claims of its influence in Starobinsky gravity.

Findings

Shear viscosity does not affect background dynamics in isotropic cosmologies.

Claims of shear viscosity altering Hubble evolution in Starobinsky gravity are inconsistent with conservation laws.

Entropy evolution in models mimicking bulk viscosity conflicts with thermodynamic principles.

Abstract

We present a general analysis of the role of shear viscosity in cosmological backgrounds, focusing on isotropic space-time in both Einstein and $f(R)$ gravity. By computing the divergence of the stress-energy tensor in a general class of isotropic (but not necessarily homogeneous) geometries, we show that shear viscosity does not contribute to the background dynamics when the fluid is comoving. This result holds in both the Jordan and Einstein frames, and implies that shear viscosity cannot affect the electromagnetic luminosity distance which is determined by the background light-like geodesics. As an application of our results, we critically examine recent claims that shear viscosity can alter the Hubble evolution and the electromagnetic luminosity distance in Starobinsky gravity. We demonstrate that the continuity equation used in that work is at odds both with the covariant conservation of the stress-energy tensor and the local second law of thermodynamics. We further show that even in models where such modifications could mimic bulk viscosity, the resulting entropy evolution is inconsistent with standard thermodynamic expectations.