Galaxy clusters back Einstein, leave Newton's descendants in the cold

Relativity is the reigning theory of gravity. In situations where we can measure it directly, such as binary neutron stars, its predictions match the real world with remarkable precision. And, when supplemented with inflation and dark matter, relativity nicely reproduces the large-scale structure of the Universe. But this reliance on other models like dark matter means that we don't have a direct, large-scale test of relativity. Now, scientists have measured the redshifting of light by galaxy clusters to give use the biggest test of relativity yet. Their results show that relativity passes muster, while modified forms of Newtownian gravity fall short.

Light emitted by distant objects rarely makes it to Earth at the same wavelength that it started out at. The fabric of the Universe is expanding, which causes a redshift. Most objects are also moving relative to the Earth, which adds a Doppler shift to the light. Finally, light that has to climb out of a large gravity well on its way to Earth also gets red-shifted.

In theory, it should be easy to account for the distance and Doppler shift; anything that's left over should be the effect of gravity. Unfortunately, even with something as massive as a galaxy cluster, the gravity-induced redshift is about two orders of magnitude smaller than a typical Doppler shift. On top of that, the motion of galaxies within clusters should be random relative to the Earth, creating a broad, Gaussian distribution of color shifts. Picking a gravitational signal out of that curve would require a large data set to help cut down on the statistical noise.