Viscosity-induced crossing of the phantom divide in the dark cosmic fluid

Abstract

Choosing various natural forms for the equation-of-state parameter w and the bulk viscosity ζ, we discuss how it is possible for a dark energy fluid to slide from the quintessence region across the divide w = −1 into the phantom region, and thus into a Big Rip future singularity. Different analytic forms for ζ, as powers of the scalar expansion, are suggested and compared with experiments.