Using a data set of over 8,000 galaxy clusters from the Sloan Digital Sky Survey, astronomers have been able to test the behaviour of photons as predicted by Einstein’s theory of general relativity. Radek Wojtak of the Niels Bohr Institute at the University of Copenaghen and collaborators focused on the redshift properties of light coming from this large sample.
According to general relativity, light will lose energy as it is escapes from a gravitational field and the stronger the field the greater the energy loss experienced by photons. This loss of energy will change of wavelength of the photon, a process known as gravitational redshift. So photons escaping from the center of a galaxy cluster (where gravity is strongest) should lose more energy than photons escaping from the edge of the cluster (where gravity is weaker). Therefore the light from the cluster center will have a longer wavelength (and be redder) than light coming from the cluster edges. (Gravitational redshift shouldn’t be confused with cosmological redshift, which is an effect due to the expansion of the Universe.)
The new results of Wojtak et al. is the first significant result of gravitational redshifts from galaxy clusters. The researchers have measured the difference in gravitational redshift of galaxies near the cluster centres and those at the cluster periphery. The total cluster mass was used to tell of the cluster’s gravitational potential. The gravitational redshift from typical galaxy clusters correspond to a velocity of about 10 km/s. The work shows that photons from cluster centres were redshifted in proportion to the distance from the center of the clusters, which agrees with the predictions of the general relativity. The results were published in this week’s edition of the journal Nature.
So far, the effects of gravity on light have only been tested in our Solar System by measuring the the redshift of sunlight due to the gravitational effects of Mercury. These results are particularly timing as last week physicists at CERN announced the results of a neutrino experiment which measured particles travelling 6 km/s faster than the speed of light, which – if confirmed – violates Einstein’s theory of special relativity.
For more information, see
- Cosmology: Gravity tested on large scales, Nature News & Views by Gary Wegner [Swinburne login required]
- Gravitational redshift of galaxies in clusters as predicted by general relativity, Wojtak et al. (2011), Nature, 477, p567
[Post by Francesco Pignatale & Sarah Maddison]