Confirmation of general relativity on large scales from weak lensing and
  galaxy velocities
  Although general relativity underlies modern cosmology, its applicability on
cosmological length scales has yet to be stringently tested. Such a test has
recently been proposed, using a quantity, EG, that combines measures of
large-scale gravitational lensing, galaxy clustering and structure growth rate.
The combination is insensitive to 'galaxy bias' (the difference between the
clustering of visible galaxies and invisible dark matter) and is thus robust to
the uncertainty in this parameter. Modified theories of gravity generally
predict values of EG different from the general relativistic prediction
because, in these theories, the 'gravitational slip' (the difference between
the two potentials that describe perturbations in the gravitational metric) is
non-zero, which leads to changes in the growth of structure and the strength of
the gravitational lensing effect3. Here we report that EG = 0.39 +/- 0.06 on
length scales of tens of megaparsecs, in agreement with the general
relativistic prediction of EG $\approx$ 0.4. The measured value excludes a
model within the tensor-vector-scalar gravity theory, which modifies both
Newtonian and Einstein gravity. However, the relatively large uncertainty still
permits models within f(R) theory, which is an extension of general relativity.
A fivefold decrease in uncertainty is needed to rule out these models.
