Fluid dynamics in higher order gravity

We examine the kinematic and dynamic properties of fluid spacetimes in higher order gravity. In particular we extend the general equations of Ehlers and Ellis governing relativistic fluid dynamics from general relativity to the higher order theory. We find exact results for the evolution of shear in Bianchi spacetimes with isotropic surfaces, thus generalising the general relativity results. Furthermore we show that the vanishing of vorticity, shear and acceleration does not imply FRW geometry in R + αR² gravity without the further assumption of a barotropic equation of state, p = p(p), p′(p) {superset of with not equal to} 0. In particular, this result means that the Ehlers-Geren-Sachs theorem on cosmic background radiation also holds in the higher order theory.