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Best Long-range Test of General Relativity.

ASTRONOMERS HAVE CONDUCTED the best galaxy-scale test of general relativity yet, and it rules out some (but not all) theories of modified gravity. These theories provide the main alternative to the existence of dark matter.

General relativity--which describes gravity as the curvature that mass induces on spacetime--has passed extensive tests within the confines of our solar system. But fewer tests exist on scales of thousands or millions of light-years. On larger scales, theories of modified gravity predict that gravity behaves differently than it does in our solar system. While astronomers have conducted some tests on galactic scales, none of them has put strong limits on modified gravity.

Now, a study led by Thomas Collett (University of Portsmouth, UK) in the June 22nd Science has provided such a test. The team investigated Hubble Space Telescope and Very Large Telescope observations of a so-called Einstein ring, where a nearby galaxy's gravity has bent light from a distant star-forming galaxy into a blue circle around itself.

Collett and colleagues first calculated the mass of the intervening galaxy by measuring the movements of stars within it. Then they measured the mass the intervening galaxy would have to have in order to bend the background galaxy's light into a ring. The mass inferred by spacetime curvature matches the mass as measured by the stars' motions--exactly as general relativity predicts.

Unlike other lensing tests of relativity, Collett's team relied less on assumptions about the nature of the intervening galaxy. So this test is relatively free of systematic uncertainties that have plagued previous studies, says Lucas Lombriser (University of Geneva), who was not involved in the study. He calls the measurements "the most robust test of gravity of this type and on these length scales to date."

This study shows that gravity, as described by relativity, behaves as expected on scales less than 6,500 lightyears. Additional tests will be needed to test modified gravity more generally.

Caption: The nearby ESO 325-G004 (large elliptical galaxy in foreground) acts as a cosmic lens to distort the light from a more distant galaxy (inset). The so-called Einstein ring becomes visible after subtracting the lensing galaxy's light.

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Title Annotation:COSMOLOGY
Author:Young, Monica
Publication:Sky & Telescope
Date:Oct 1, 2018
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