The content of f(R) gravity
  We analyze the propagating degrees of freedom in gravity models where the scalar curvature in the action is replaced by a generic function $f(R)$ of the curvature. That these gravity models are equivalent to Einstein's gravity with an extra scalar field had previously been shown by applying a conformal transformation. We confirm this result by calculating the particle propagators. This provides further evidence of the unability of these models to explain the accelerated expansion of the Universe without contradicting solar system experiments. 
Alvaro Nunez and Slava Solganik