Two-Fluid Viscous Modified Gravity on a RS Brane

Abstract

Singularities in the dark energy late universe are discussed, under the assumption that the Lagrangian contains the Einstein term R plus a modified gravity term R^\alpha, where \alpha is a constant. The 4D fluid is taken to be viscous and composed of two components, one Einstein component where the bulk viscosity is proportional to the scalar expansion \theta, and another modified component where the bulk viscosity is proportional to the power \theta^{2\alpha-1}. Under these conditions it is known from earlier that the bulk viscosity can drive the fluid from the quintessence region (w > -1) into the phantom region (w<-1), where w is the thermodynamical parameter [I. Brevik, Gen. Rel. Grav. 38, 1317 (2006)]. We combine this 4D theory with the 5D Randall-Sundrum II theory in which there is a single spatially flat brane situated at y=0. We find that the Big Rip singularity, which occurs in 4D theory if \alpha >1/2, carries over to the 5D metric in the bulk, |y|>0. The present investigation generalizes that of an earlier paper [I. Brevik, arXiv:0807.1797; to appear in Eur. Phys. J. C] in which only a one-component modified fluid was present.