Abstract
The cosmic expansion history tests the dynamics of the global evolution of the universe and its energy density contents, while the cosmic growth history tests the evolution of the inhomogeneous part of the energy density. Precision comparison of the two histories can distinguish the nature of the physics responsible for the accelerating cosmic expansion: an additional smooth component---dark energy---or a modification of the gravitational field equations. With the aid of a new fitting formula for linear perturbation growth accurate to 0.05%--0.2%, we separate out the growth dependence on the expansion history and introduce a new growth index parameter $\ensuremath{\gamma}$ that quantifies the gravitational modification.
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