Solar System constraints to general f(R) gravity
  It has been proposed that cosmic acceleration or inflation can be driven by replacing the Einstein-Hilbert action of general relativity with a function f(R) of the Ricci scalar R. Such f(R) gravity theories have been shown to be equivalent to scalar-tensor theories of gravity that are incompatible with Solar System tests of general relativity, as long as the scalar field propagates over Solar System scales. Specifically, the PPN parameter in the equivalent scalar-tensor theory is gamma=1/2, which is far outside the range allowed by observations. In response to a flurry of papers that questioned the equivalence of f(R) theory to scalar-tensor theories, it was recently shown explicitly, without resorting to the scalar-tensor equivalence, that the vacuum field equations for 1/R gravity around a spherically symmetric mass also yield gamma= 1/2. Here we generalize this analysis to f(R) gravity and enumerate the conditions that, when satisfied by the function f(R), lead to the prediction that gamma=1/2. 
Takeshi Chiba (Nihon University), Tristan L. Smith (Caltech), and   Adrienne L. Erickcek (Caltech)