Following the most precise test of gravity outside the Milky Way, an international team of astronomers has confirmed the validity of Albert Einstein’s general theory of relativity in another galaxy.

Albert Einstein first proposed his general theory of relativity in 1915 to unravel the way gravity works. And although the theory has passed a series of high precision tests within the solar system, there have been no tests of relativity theory on galactic scales, that is – outside our solar system.

In 1929, Edwin Hubble proved that the universe is expanding. And in 1998, its accelerated expansion was discovered. This surprising discovery that the Universe is expanding faster now than it was in the past – can be explained only because our universe is filled entirely with a positive vacuum energy, called “dark energy“. However, this understanding is influenced by general theory of relativity being the definite theory of gravity on cosmic scales. According to researchers, evaluating the long distance properties of gravity is important to ensure the cosmological measurements.

Einstein proved right in another galaxy

For the study, Dr Thomas Collett of the Institute of Cosmology and Gravitation at the University of Portsmouth and his team incorporated data from NASA’s Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope to prove gravity in this galaxy behaves as conjectured by general theory of relativity.

To begin with, a team of astronomers, led by Collett made a nearby act as gravitational lens. This helped them make a precise test of gravity on astronomical length scales.

“General Relativity predicts that massive objects deform space-time, this means that when light passes near another galaxy the light’s path is deflected,” Collett explained. “If two galaxies are aligned along our line of sight this can give rise to a phenomenon, called strong gravitational lensing, where we see multiple images of the background galaxy. If we know the mass of the foreground galaxy, then the amount of separation between the multiple images tells us if General Relativity is the correct theory of gravity on galactic scales.”

There are only a few strong gravitational lenses that we know of yet, and most are too distant to precisely measure their mass. So, they won’t be of any help to accurately test General Relativity theory. However, the galaxy ESO325-G004 is amongst the closest at 500 million light years from Earth, so the team used that to function as gravitational lens.

“We used data from the Very Large Telescope in Chile to measure how fast the stars were moving in E325 — this let us infer how much mass there must be in E325 to hold these stars in orbit,” Collett continued. “We then compared this mass to the strong lensing image separations that we observed with the Hubble Space telescope and the result was just what GR predicts with 9 per cent precision. This is the most precise extrasolar test of GR to date, from just one galaxy.”

“The Universe is an amazing place providing such lenses which we can then use as our laboratories,” said Professor Bob Nichol, Director of the Institute of Cosmology and Gravitation. “It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was.”

The study titled “A precise extragalactic test of General Relativity” has been published in the journal Science.

Source: University of Portsmouth

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