In cosmology, general relativity is often assumed to be valid out to arbitrarily large scales, but whether this is true or not has been difficult to test directly. Now, RadoslŁaw Wojtak and colleagues of the Niels Bohr Institute in Copenhagen have managed to use archived data from galaxies in clusters to make a direct test of general relativity out to scales of 1–10 megaparsecs. Working from a new galaxy cluster catalogue from the Sloan Digital Sky Survey they used data on 7800 galaxy clusters and 120,000 galaxy redshifts.

The new technique may be extended in the future, but for now it means that the standard gravitational redshift has been tested over scales spanning 22 orders of magnitude. Still viable are models such as f(R) gravity, which can be used to explain dark energy. However, the tensor–vector–scalar (TeVeS) theory, a relativistic version of modified Newtonian dynamics (MOND) that eliminates the need for dark matter, seems to be excluded.