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

TL;DR

This study tests modifications of gravity on cosmological scales using galaxy velocity fields and finds no evidence supporting deviations from Einstein's general relativity, confirming GR's robustness.

Contribution

It introduces a forward-modelling approach with high-resolution simulations to test modified gravity theories against galaxy clustering data.

Findings

f(R) gravity model fails to match observed galaxy clustering

Standard GR model aligns well with observations

Results support the validity of Einstein's general relativity on large scales

Abstract

The recent discovery of gravitational waves marks the culmination of a sequence of successful tests of the general theory of relativity (GR) since its formulation in 1915. Yet these tests remain confined to the scale of stellar systems or the strong gravity regime. A departure from GR on larger, cosmological scales has been advocated by the proponents of modified gravity theories as an alternative to the Cosmological Constant to account for the observed cosmic expansion history. While indistinguishable in these terms by construction, such models on the other hand yield distinct values for the linear growth rate of density perturbations and, as a consequence, for the associated galaxy peculiar velocity field. Measurements of the resulting anisotropy of galaxy clustering, when spectroscopic redshifts are used to derive distances, have thus been proposed as a powerful probe of the validity of GR on cosmological scales. However, despite significant effort in modelling such redshift space distortions, systematic errors remain comparable to current statistical uncertainties. Here, we present the results of a different forward-modelling approach, which 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 and a compelling f(R) model, one of GR's simplest variants, to build simulated catalogues of stellar-mass-selected galaxies through a robust match to the Sloan Digital Sky Survey observations. We find that, well within the uncertainty of this technique, f(R) fails to reproduce the observed redshift-space clustering on scales 1-10 Mpc/h. Instead, the standard LCDM GR model agrees impressively well with the data. This result provides a strong confirmation, on cosmological scales, of the robustness of Einstein's general theory of relativity.