The theory of relativity – both special relativity and general relativity – has now been tested successfully so many ways ever since its early days that further tests seem almost redundant. (That little mix-up with superluminal neutrinos to the contrary notwithstanding – since it’s at most a higher order correction, but more likely an error in data interpretation.) But science never stops testing if there’s some creative new way to do so. And that’s what’s involved here.
One of the predictions of general relativity is that light photons lose energy when climbing out of a strong gravitational field. In that case, the light should be redshifted, in a way that doesn’t involve either a Doppler effect or the expansion of the universe.
It takes a very strong gravitational field to make this gravitational redshift apparent. Even a single galaxy isn’t large enough. But a cluster of galaxies will do, although the effect is almost masked by relative motion of galaxies in the cluster. However, in a cluster there’s a fairly straightforward way to check, since the shift should be larger with photons from the center of the cluster than from the periphery, as the gravitational field is stronger at the center.
Wojtak and his colleagues knew that measuring gravitational redshifting within a single galaxy cluster would be difficult because the effect is very small and needs to be teased apart from the redshifting caused by the orbital velocity of individual galaxies within the cluster and the redshifting caused by the expansion of the universe. The researchers approached the problem by averaging data collected from 8000 galaxy clusters by the Sloan Digital Sky Survey. The hope was to detect gravitational redshift “by studying the properties of the redshift distribution of galaxies in clusters rather than by looking at redshifts of individual galaxies separately,” Wojtak explains.
Sure enough, the researchers found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster, as predicted by general relativity. “We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge,” Wojtak says.
In addition to further confirming relativity, this research may also rule out some alternative theories of gravity that have been proposed to avoid the hypotheses of dark matter and dark energy, which are needed with standard relativity to explain many astronomical observations – and do so very well.