  In General Relativity, the average velocity field of dark matter around
galaxy clusters is uniquely determined by the mass profile. The latter can be
measured through weak lensing. We propose a new method of measuring the
velocity field (phase space density) by stacking redshifts of surrounding
galaxies from a spectroscopic sample. In combination with lensing, this yields
a direct test of gravity on scales of 1-30 Mpc. Using N-body simulations, we
show that this method can improve upon current constraints on f(R) and DGP
model parameters by several orders of magnitude when applied to upcoming
imaging and redshift surveys.
