No evidence for modifications of gravity from galaxy motions on cosmological scales

Current tests of general relativity (GR) remain confined to the scale of stellar systems or the strong gravity regime. A departure from GR on cosmological scales has been advocated(1) as an alternative to the cosmological constant Lambda (ref.( 2)) to account for the observed cosmic expansion history(3-4). However, such models yield distinct values for the linear growth rate of density perturbations and consequently for the associated galaxy peculiar velocity field. Measurements of the resulting anisotropy of galaxy clustering(5,6) have thus been proposed as a powerful probe of the validity of GR on cosmological scales(7), but despite substantial efforts(8,9), they suffer from systematic errors comparable to statistical uncertainties(10). Here, we present the results of a forward-modelling approach that fully exploits the sensitivity of the galaxy velocity field to modifications of GR. We use state-of-the-art high-resolution N-body simulations of a standard GR (Lambda cold dark matter (CDM)) model(11) and a compelling f(R) model(12)-one of GR's simplest variants, in which the Ricci scalar curvature, R, in the Einstein-Hilbert action is replaced by an arbitrary function of R-to build simulated catalogues of stellar-massselected galaxies through a robust match to the Sloan Digital Sky Survey(13). We find that f(R) fails to reproduce the observed redshift-space clustering on scales of similar to 1-10 Mpch(-1), where h is the dimensionless Hubble parameter. Instead, the standard Lambda CDM GR model agrees impressively well with the data. This result provides strong confirmation, on cosmological scales, of the robustness of Einstein's general theory of relativity.