  We consider a modified form of gravity in which the action contains a power
alpha of the scalar curvature. It is shown how the presence of a bulk viscosity
in a spatially flat universe may drive the cosmic fluid into the phantom region
(w<-1) and thus into a Big Rip singularity, even if it lies in the quintessence
region (w>-1) in the non-viscous case. The condition for this to occur is that
the bulk viscosity contains the power (2 alpha-1) of the scalar expansion. Two
specific examples are discussed in detail. The present paper is a
generalization of the recent investigation dealing with barrier crossing in
Einstein's gravity: I. Brevik and O. Gorbunova, Gen. Relativ. Grav. 37 (2005)
2039.
