E_G method was proposed by Prof. Pengjie Zhang and his collaborators in 2007 (Zhang, et al. 2007, PRL). To put it simply, E_G is the ratio of the galaxy lensing effect and the galaxy redshift distortion effect. E_G circumvents uncertainties in the galaxy bias and matter fluctuations, probes the nature of gravity directly and is capable of testing general relativity at cosmological scales robustly. The method has become one of the representative methods of cosmological test of General Relativity (refer to the review paper of Ferreira & Starkman, 2009, Science, 326, 812; the commentary article of Tony Tyson on the same issue of Nature, and the headline news report on Physics World http://physicsworld.com/cws/article/news/41948).

　  The work of Reyes et al. is the first successful measurement of E_G and is hence of great importance. They find that E_G=0.39 ± 0.06 at low redshift (~0.3), consistent with the prediction of General Relativity. At the same time, this result rules out the TeVeS modified gravity model. Ongoing surveys such as LAMOST and BOSS, and proposed missions such as BigBOSS will significantly improve the E_G measurement, allowing for testing General Relativity with 1% level accuracy, at redshift 0-2 and scale of ten millions to hundred millions light years.