Cosmological constraints on f ( R ) gravity theories within the Palatini approach

We investigate f (R) theories of gravity within the Palatini approach and show how one can determine the expansion history, H(a), for an arbitrary choice of f (R). As an example, we consider cosmological constraints on such theories arising from the supernova type Ia, large scale structure formation and cosmic microwave background observations. We find that best fit to the data is a non-null leading order correction to the Einstein gravity, but the current data exhibits no significant preference over the concordance ΛCDM model. Our results show that the often considered 1/R models are not compatible with the data. The results demonstrate that the background expansion alone can act as a good discriminator between modified gravity models when multiple data sets are used.