Cosmological constraints on

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 the best fit to the data is a non-null leading order correction to the Einstein gravity. However, the current data exhibits no significant trend toward such corrections compared to the concordance ΛCDM model. Our results show that the oft-considered $1/R$ models are not compatible with the data. The results demonstrate that background expansion alone can act as a good discriminator between modified gravity models when multiple data sets are used