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 \zeta, 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 \zeta, as powers of the scalar expansion, are suggested and compared with experiments.