Curvature scalar instability in f(R) gravity

An instability in the presence of matter in theories of gravity which include a 1/R correction in the gravitational action has been found by Dolgov and Kawasaki. In the present paper this instability is discussed for f® gravity in general. We focus on the Palatini formalism of the theory and it is shown that no such instability occurs in this version of f® gravity. The reasons for the appearance of the instability in the metric but not in the Palatini formalism are fully investigated.