  Analytic properties of physical quantities in the cosmic fluid such as energy
density \rho(t) and Hubble parameter H(t) are investigated near the future
singularity (Big Rip). Both 4D and 5D cosmologies are considered (the
Randall-Sundrum II model in the 5D case), and the fluid is assumed to possess a
bulk viscosity \zeta. We consider both Einstein gravity and modified gravity,
where in the latter case the Lagrangian contains a term R^\alpha with \alpha a
constant. If \zeta is proportional to the power (2\alpha-1) of the scalar
expansion, the fluid can pass from the quintessence region into the phantom
region as a consequence of the viscosity. A property worth noticing is that the
4D singularity on the brane becomes carried over to the bulk region.
