Viscosity-Induced Crossing of the Phantom Barrier

Iver Brevik

doi:10.3390/e17096318

Abstract

We show explicitly, by using astrophysical data plus reasonable assumptions for the bulk viscosity in the cosmic fluid, how the magnitude of this viscosity may be high enough to drive the fluid from its position in the quintessence region at present time t = 0 across the barrier w = −1 into the phantom region in the late universe. The phantom barrier is accordingly not a sharp mathematical divide, but rather a fuzzy concept. We also calculate the limiting forms of various thermodynamical quantities, including the rate of entropy production, for a dark energy fluid near the future Big Rip singularity.

Highlights

Recent years have seen an increased interest in viscous cosmology
Because of the assumption about spatial isotropy in cosmology, the shear coefficient is usually omitted
In view of the mentioned smallness of |α| it becomes natural to ask: if the cosmic fluid starts from present time t = 0 in the quintessence region (α > 0), has it any possibility to slide through the phantom barrier into the phantom era and thereafter inevitably be drawn into the future singularity? This problem was analyzed already in 2005 [22] with the following result: if the magnitude of the bulk viscosity is large enough, and if the viscosity varies with the energy in a way that is natural physically, such a transition is possible

Summary

Introduction

Recent years have seen an increased interest in viscous cosmology. This contrasts the traditional approach, in which the cosmic fluid has usually be taken to be ideal (nonviscous). In view of the mentioned smallness of |α| it becomes natural to ask: if the cosmic fluid starts from present time t = 0 in the quintessence region (α > 0), has it any possibility to slide through the phantom barrier into the phantom era and thereafter inevitably be drawn into the future singularity? This leads us to the important conclusion that the the estimates for the present viscosity given in Equation (23) are roughly high enough to drive the fluid through the phantom barrier, even in the most extreme quintessential case w = wmax Another option would be to assume that the bulk viscosity varies with θ as ζ(ρ) = τ2 θ2 = τ2 · 24πGρ,. We find that the future development of the universe is critically dependent on the value of the equation-of-state parameter, assumed constant in the present model.

Further Comparisons

Summary