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Shift happens

Nature 477, 567–569 (2011)

Over nearly a century, general relativity has withstood a variety of experimental challenges. Now Radosław Wojtak and colleagues have trawled the archival data of the Sloan Digital Sky Survey to put relativity through its paces on cosmological scales — and independently of the prevailing ΛCDM cosmological model.

They have searched for evidence of gravitational redshift of the light from galaxies in clusters: the galaxies move in the gravitational potential well of their clusters (which are typically of about 1014 solar masses), and the light they emit appears redshifted, according to general relativity, in proportion to the difference in gravitational potential between the clusters and the observer.

At 99% confidence level, Wojtak et al. do see a redshift, and it is consistent with general relativity — and also consistent with the so-called f(R) model, a modified version of relativity that can explain the accelerating expansion of the Universe without invoking dark energy. Their results don't, however, tally with the 'TeVeS' model (a generalization of modified Newtonian dynamics), which dispenses with dark matter.

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Wright, A. Shift happens. Nature Phys 7, 743 (2011). https://doi.org/10.1038/nphys2123

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