  The metric of a perturbed Robertson-Walker spacetime is characterized by
three functions: a scale-factor giving the expansion history and two potentials
which generalize the single potential of Newtonian gravity. The Newtonian
potential induces peculiar velocities and, from these, the growth of matter
fluctuations. Massless particles respond equally to the Newtonian potential and
to a curvature potential. The difference of the two potentials, called the
gravitational slip, is predicted to be very small in general relativity but can
be substantial in modified gravity theories. The two potentials can be
measured, and gravity tested on cosmological scales, by combining weak
gravitational lensing or the Integrated Sachs-Wolfe effect with galaxy peculiar
velocities or clustering.
