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A possible solution to the dark energy problem is that Einstein's theory ofgeneral relativity is modified. A suite of models have been proposed that, ingeneral, are unable to predict the correct amount of large scale structure inthe distribution of galaxies or anisotropies in the Cosmic MicrowaveBackground. It has been argued, however, that it should be possible toconstrain a general class of theories of modified gravity by focusing onproperties such as the growing mode, gravitational slip and the effective, timevarying Newton's constant. We show that assuming certain physical requirementssuch as stability, metricity and gauge invariance, it is possible to come upwith consistency conditions between these various parameters. In this paper wefocus on theories which have, at most, 2nd derivatives in the metric variablesand find restrictions that shed light on current and future experimentalconstraints without having to resort to a (as yet unknown) complete theory ofmodified gravity. We claim that future measurements of the growth of structureon small scales (i.e. from 1-200 h^{-1} Mpc) may lead to tight constraints onboth dark energy and modified theories of gravity.

The linear growth rate of structure in Parametrized Post Friedmannian Universes