All observations in astronomy（天文学） are based on light emitted from stars and galaxies¹ and, according to the general theory of relativity, the light will be affected² by gravity. At the same time all interpretations³ in astronomy are based on the correctness of the theory of relatively⁴, but it has never before been possible to test Einstein's theory of gravity on scales larger than the solar system. Now astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how the light is affected by gravity on its way out of galaxy⁵ clusters. The observations confirm the theoretical predictions. The results have been published in the scientific journal, Nature.

Observations of large distances in the universe are based on measurements of the redshift, which is a phenomenon where the wavelength⁶ of the light from distant galaxies is shifted more and more towards the red with greater distance. The redshift indicates how much the universe has expanded from when the light left until it was measured on Earth. Furthermore, according to Einstein's general theory of relativity, the light and thus the redshift is also affected by the gravity from large masses like galaxy clusters and causes a gravitational redshift of the light. But the gravitational influence of light has never before been measured on a cosmological（宇宙论的） scale.

"It is really wonderful. We live in an era with the technological⁷ ability to actually measure such phenomena⁸ as cosmological gravitational redshift," says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.