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A test of general relativity at the galaxy's center suggests Einstein's theory still holds

July 25, 2019

In a detailed study of a star orbiting the supermassive black hole at the center of our Galaxy, researchers report that Einstein's theory of general relativity (GR) accurately describes the behavior of light struggling to escape the gravity around this massive structure. The researchers' analysis involved detecting an effect known as "gravitational redshift" in the light emitted by a star closely orbiting the supermassive black hole, as the star was at its closest point to the blackhole in its 16-year orbit. While a similar analysis was conducted by the GRAVITY collaboration last year, here, Tuan Do, Andrea Ghez and colleagues report novel spectra data and expanded analyses. Even as general relativity has been tested in relatively weak gravitational fields, such as those on Earth and in the Solar System, it had not - before last year - been tested around a black hole as big as the one at our Galaxy's center, known as Sagittarius A*. Observations of the stars rapidly orbiting this supermassive black hole would create a method for GR to be evaluated in an extreme gravitational environment. Do, Ghez and colleagues analyzed new observations of the star S0-2 as it made its closest approach to the enormous black hole in 2018. These data were combined with measurements Ghez and her team have made over the last 24 years. The analysis revealed the gravitational redshift, which occurs when light is stretched to longer wavelengths by the gravitational field around the black hole, thereby shifting it towards the red part of the spectrum. The results are consistent with general relativity and substantially favor the theory over Newtonian gravity, which cannot account for the observed redshift. The findings are a "transformational change in our understanding about not only the existence of supermassive blackholes but the physics and astrophysics of black holes," says Ghez in a related video.
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American Association for the Advancement of Science

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