Dark Energy and Viscous Cosmology

TL;DR

This work investigates how bulk viscosity in a flat FRW dark-energy cosmology can drive a late-time Big Rip and enable crossing of the phantom barrier at w = -1. By formulating a general viscous evolution with p = w(ρ)ρ and ζ = ζ(ρ), and focusing on the physically natural case ζ ∝ θ, the authors show that the effective equation of state can transition from quintessence to phantom, with the critical condition given by α+24πGτ>0 in the constant-w scenario. The analysis reveals that viscosity lowers the effective pressure, facilitating phantom crossing and finite-time singularities even when the nonviscous fluid is in the quintessence regime; the key criterion and modified dynamics are extended to more general f(ρ) forms, including f(ρ)=Aρ^β, where the large-density behavior dictates whether viscosity dominates. Overall, the paper highlights a robust mechanism by which irreversible processes in the cosmic fluid, encoded in bulk viscosity, can qualitatively alter the ultimate fate of the universe and motivates further exploration of quantum- and viscosity-driven cosmologies.

Abstract

Singularities in the dark energy universe are discussed, assuming that there is a bulk viscosity in the cosmic fluid. In particular, it is shown how the physically natural assumption of letting the bulk viscosity be proportional to the scalar expansion in a spatially flat FRW universe can drive the fluid into the phantom region (w < -1), even if lies in the quintessence region (w > -1) in the non-viscous case.