Cosmic acceleration could be an effect of a “dark energy” fluid pervading the Universe. It could also be an indication that general relativity is not the correct theory of gravity at cosmological length scales. Unfortunately, observations of the cosmic acceleration alone provide insufficient data to allow us to distinguish between these two possibilities. Cosmic perturbation theory may be able to relieve this uncertainty. Without positing a specific alternative gravity theory, we parametrize departures from general relativity in terms of a redshift-dependent, scale-independent gravitational slip between the Newtonian and longitudinal potentials appearing in the perturbed Robertson-Walker metric. We calculate the effects of non-zero values of this new parameter on linear and quasi-linear order cosmological perturbation theory. We find that existing data sets are consistent with unmodified general relativity with a cosmological constant at the ∼10% level. We discuss the ability of future work, both in the form of more precise experiments and more detailed theoretical calculations, to either refine or refute this observation.