Confirmation of general relativity on large scales from weak lensing and galaxy velocities

Modern cosmology relies on the assumption that general relativity is a valid theory of gravity at cosmological scales, though until now no test of the proposition has been possible. One such test was recently proposed, though, based on a determination of a quantity EG that combines measures of large-scale gravitational lensing, galaxy clustering, and the growth rate of structure as predicted by the standard model. Reyes et al. report a measurement of EG = 0.39±0.06 based on data from a sample of more than 70,000 distant galaxies, which is consistent with the value of 0.4 predicted by general relativity. Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Now, at a length scale of tens of megaparsecs, the quantity EG, which combines measures of large-scale gravitational lensing, galaxy clustering, and the growth rate of structure, has been measured to be 0.39±0.06, in agreement with the general relativistic prediction of about 0.4. Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Such a test has recently been proposed1, 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 structure2 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 ≈ 0.4. The measured value excludes a model1 within the tensor–vector–scalar gravity theory4,5, which modifies both Newtonian and Einstein gravity. However, the relatively large uncertainty still permits models within f( ) theory6, which is an extension of general relativity. A fivefold decrease in uncertainty is needed to rule out these models.