Cosmographic constraints on a class of Palatini f(R) gravity

Modified gravity, known as f(R) gravity, has presently been applied to cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryons and dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form f(R) R - alpha/R-n within the Palatini approach. Although extensively discussed in recent literature, cosmological tests are inconclusive about the true signal of n. This is particularly important to define which kind of corrections (infrared or high-energy) to general relativity this class of theory represents. We shed some light on this question by examining the evolution of the deceleration parameter for these theories. We find that for a large range of alpha, these models can only have positive values for n, placing thus a broad restriction on this class of gravity.