Gravitational lens
Gravitational lens

In 1915 the physicist Albert Einstein proposed his general theory of relativity to explain how gravity works. Since then, the theory has passed a series of high-precision tests within the solar system, but there had not been precise validation at large astronomical scales.

It has been known since 1929 that 
the universe is expanding, but in 1998 two teams of astronomers demonstrated that the universe is currently expanding even faster than in the past. This surprising discovery, which won the Nobel Prize in 2011, can not be explained unless the universe is composed primarily of an exotic component called dark energy. However, this interpretation is based on the fact that general relativity is also the correct theory on cosmological scales. Testing the properties of gravity over long distances is important to validate our cosmological model.

As it is, an international team of astronomers has performed the most accurate gravity test outside of our own solar system. 

By combining data taken with NASA’s Hubble Space Telescope and the Very Large Telescope of the European Southern Observatory (ESO), their results show that gravity in this galaxy behaves as predicted by Albert Einstein’s general theory of relativity, confirming the validity of the galactic-scale theory. 

Astronomers, led by Thomas Collett of the Institute of Cosmology and Gravitation at the University of Portsmouth (England), used a nearby galaxy as a gravitational lens to perform an accurate test of gravity on astronomical longitude scales.

General Relativity predicts that massive objects deform space-time, this means that when light passes near another galaxy the path of light is diverted, if two galaxies are aligned along our line of sight this can give Instead of a phenomenon, called a strong gravitational lens, where we see multiple images of the background galaxy, if we know the mass of the galaxy in the foreground, then the amount of separation between the multiple images tells us if General Relativity is the correct theory of gravity on galactic scales, “explains Collett in the journal Science, which publishes the study.

A few hundred strong gravitational lenses are known, but most are too distant to accurately measure their mass, so they can not be used to evaluate accurately to validate the theory of general relativity. However, the galaxy ESO325-G004 is among the closest lenses, 500 million light-years from Earth.

“We used data from the Very Large Telescope in Chile to measure how fast the stars moved in E325, this allows us to infer how much mass there must be in E325 to keep these stars in orbit, and then we compare this mass with the gravitational lens image separations. strong that we observed with the Hubble Space Telescope and the result was just what the Theory of General Relativity predicts with a precision of 9% .This is the most accurate extrasolar test of the general relativity to date, of a single galaxy “, clarifies Collett.

As Bob Nichol, Director of the Institute of Cosmology and Gravitation says, “it is so satisfying to use the best telescopes in the world to challenge Einstein, only to discover how right he was” …

We will have to be alert for the next experiment that tries to challenge Einstein!

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