  Modified gravity theories may provide an alternative to dark energy to
explain cosmic acceleration. We argue that the observational program developed
to test dark energy needs to be augmented to capture new tests of gravity on
astrophysical scales. Several distinct signatures of gravity theories exist
outside the linear regime, especially owing to the screening mechanism that
operates inside halos like the Milky Way to ensure that gravity tests in the
solar system are satisfied. This opens up several decades in length scale and
new classes of galaxies at low-redshift that can be exploited by surveys. While
theoretical work on models of gravity is in the early stages, we can already
identify new regimes which cosmological surveys could target to test gravity.
These include: 1. A small scale component that focuses on the interior and
vicinity of galaxy and cluster halos. 2. Spectroscopy of low redshift galaxies,
especially galaxies smaller than the Milky Way, in environments that range from
voids to clusters. 3. A program of combining lensing and dynamical information,
from imaging and spectroscopic surveys respectively, on the same (or
statistically identical) sample of galaxies.
