The gravitational growth index formalism provides a model independent way to look for deviations from general relativity by testing dark energy physics distinct from its effects on the cosmic expansion history. Here we extend the approach to incorporate an early time parameter $g_{\star }$ in addition to the growth index in describing the growth of large-scale structure. We illustrate its utility for models with modified gravity at high redshift, early acceleration, or early dark energy. Future data will have the capability to constrain the dark energy equation of state, the growth index $\gamma$, and $g_{\star }$ simultaneously, with no degradation in the equation of state determination.

• Received 22 January 2009

DOI:https://doi.org/10.1103/PhysRevD.79.063519