  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 \gamma that quantifies the
gravitational modification.
