  The accelerating expansion of the Universe at recent epochs has called into
question the validity of general relativity on cosmological scales. One probe
of gravity is a comparison of expansion history of the Universe with the
history of structure growth via gravitational instability: general relativity
predicts a specific relation between these two observables. Here we show that
the mean pairwise streaming velocity of galaxy clusters provides a useful
method of constraining this relation. Galaxy cluster velocities can be measured
via the kinetic Sunyaev-Zeldovich distortion of the cosmic microwave background
radiation; future surveys can provide large enough catalogs of cluster
velocities to discriminate between general relativity and other proposed
gravitational theories.
