Note on bulk viscosity as an alternative to dark energy

TL;DR

This work critically examines bulk viscosity as an alternative to dark energy by exploring two physically motivated origins: nonminimal interactions between two fluids and nonminimal interactions among constituents of a single fluid. It shows that, within GR, energy–momentum exchange between fluids or among particles does not produce a direct gravitational effect, and that achieving cosmic acceleration would require violations of energy conditions or pressures that are inconsistent with microscopic constraints. In both scenarios, the models fail to yield the sustained negative pressure needed for acceleration without compromising physical viability. The results highlight fundamental challenges for bulk-viscosity-based explanations of dark energy and suggest that such approaches require more complex microphysics or modified gravity frameworks to be viable.

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.