Note on bulk viscosity as an alternative to dark energy

TL;DR

This paper critically examines bulk viscosity as an alternative explanation for cosmic acceleration, demonstrating that physical constraints and energy conditions limit its effectiveness in modeling dark energy.

Contribution

It analyzes two physical scenarios for bulk viscosity origin, revealing fundamental limitations and challenges in using bulk viscosity to explain dark energy within general relativity.

Findings

Bulk viscosity models violate classical energy conditions.

Proper pressure remains non-negative, limiting negative effective pressure.

Achieving cosmic acceleration requires physically questionable assumptions.

Abstract

Bulk viscosity, which characterizes the irreversible dissipative resistance of a fluid to volume changes, has been proposed as a potential mechanism for explaining both early- and late-time accelerated expansion of the Universe. In this work, we investigate two distinct physical scenarios for the origin of bulk viscosity: (1) nonminimal interactions between two fluids, and (2) elastic collisions in an ideal gas. In both cases, we demonstrate that while the associated energy-momentum exchange can significantly influence fluid dynamics, overall energy-momentum conservation precludes such exchange from having any direct gravitational effect in the context of General Relativity. In case (1), we show that the standard bulk viscous energy-momentum tensor can be obtained for the two-fluid system only at the cost of the violation of all classical energy conditions: null, weak, dominant, and strong. In case (2), we consider a single fluid composed of point particles undergoing instantaneous, energy- and momentum-conserving collisions, and find that the proper pressure remains strictly non-negative, with the equation-of-state parameter confined to the interval $[0,1/3]$. In both scenarios, achieving a sufficiently negative effective pressure to drive cosmic acceleration requires assumptions that compromise the physical viability of the model. Our results highlight some of the key physical challenges involved in modeling dark energy through bulk viscous effects.