  A single parameter, the gravitational growth index \gamma, succeeds in
characterizing the growth of density perturbations in the linear regime
separately from the effects of the cosmic expansion. The parameter is
restricted to a very narrow range for models of dark energy obeying the laws of
general relativity but takes on distinctly different values in models of
beyond-Einstein gravity. In analogy to the parameterized post-Newtonian (PPN)
formalism for testing gravity, we extend and motivate the gravitational growth
index, or Minimal Modified Gravity, approach to parameterizing beyond-Einstein
cosmology. Using a simple analytic formalism, we show how the growth index
parameter applies to early dark energy, time-varying gravity, DGP braneworld
gravity, and scalar-tensor gravity.
