Expansion history and f(R) modified gravity

We attempt to fit cosmological data using f(R) modified Lagrangians containing inverse powers of the Ricci scalar varied with respect to the metric. While we can confirm the a ~ t^1/2 behaviour at medium to high redshifts reported elsewhere, we are still able to fit the supernova, CMB and baryon oscillation data. The age of the universe is also compatible with globular clusters. We show, however, that this class of models can be ruled out by considering the thickness of the last scattering surface.