Solar System constraints to general<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>f</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>R</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>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 parameterized post-Newtonian (PPN) parameter in the equivalent scalar-tensor theory is $\ensuremath{\gamma}=1/2$, 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 $\ensuremath{\gamma}=1/2$. 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 $\ensuremath{\gamma}=1/2$.

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