Dr. Thomas Collett, an astrophysicist and cosmologist at the University of Portsmouth, UK, and co-authors have made the most precise test of Albert Einstein’s gravity on large astronomical scales to date. By combining data from the NASA/ESA Hubble Space Telescope and ESO’s Very Large Telescope (VLT), the team’s results show that gravity in a giant elliptical galaxy called ESO 325-G004 behaves as predicted by Einstein’s general theory of relativity.
This Hubble image shows the diverse collection of galaxies in the galaxy cluster Abell S0740. The giant elliptical galaxy ESO 325-G004 looms large at the cluster’s center. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / J. Blakeslee, Washington State University.
In 1915, Albert Einstein proposed his general theory of relativity (GR) to explain how gravity works.
Since then GR has passed a series of tests within the Solar System, but there have been no highly precise tests of GR on large astronomical scales.
It has been known since 1929 that the Universe is expanding, but in 1998 astronomers showed that the Universe is expanding faster now than it was in the past.
This surprising discovery cannot be explained unless the Universe is mostly made of an exotic component called dark energy. However, this interpretation relies on GR being the correct theory of gravity on cosmological scales.
To make a precise test of gravity on astronomical length scales, Dr. Collett and colleagues used ESO 325-G004 — the brightest galaxy of the galaxy cluster Abell S0740, located more than 450 million light-years away in the constellation Centaurus — as a gravitational lens.
“GR predicts that massive objects deform space-time, this means that when light passes near another galaxy the light’s path is deflected,” Dr. Collett said.
“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 GR is the correct theory of gravity on galactic scales.”
This infographic compares the two methods used to measure the mass of ESO 325-G004. The first method used ESO’s Very Large Telescope to measure the velocities of stars in this galaxy. The second method used the NASA/ESA Hubble Space Telescope to observe an Einstein ring caused by light from a background galaxy being bent and distorted by ESO 325-G004. By comparing these two methods of measuring the strength of the gravity of ESO 325-G004, it was determined that Einstein’s general theory of relativity works on extragalactic scales — something that had not been previously tested. Image credit: NASA / ESA / Hubble / ESO.
Several strong gravitational lenses are known, but most are too distant to precisely measure their mass, so they can’t be used to accurately test GR.
However, the ESO 325-G004 galaxy is amongst the closest lenses.
“We used VLT data from to measure how fast the stars were moving in ESO 325-G004 — this let us infer how much mass there must be in this galaxy to hold these stars in orbit,” Dr. Collett said.
“We then compared this mass to the strong lensing image separations that we observed with Hubble and the result was just what GR predicts with 9% 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. It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was,” said co-author Professor Bob Nichol, Director of the Institute of Cosmology and Gravitation at the the University of Portsmouth.
The findings appear online today in the journal Science.
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Thomas E. Collett et al. 2018. A precise extragalactic test of General Relativity. Science 360 (6395): 1342-1346; doi: 10.1126/science.aao2469
