A precise verification of the general theory of relativity outside the Milky Way

The galaxy ESO325-G004 and the Einstein ring (Image ESO, ESA/Hubble, NASA)
The galaxy ESO325-G004 and the Einstein ring (Image ESO, ESA/Hubble, NASA)

An article published in the journal “Science” describes the most precise verification of Albert Einstein’s general theory of relativity outside the Milky Way. A team of researchers led by Thomas Collett of the Institute of Cosmology and Gravitation at the British University of Portsmouth used data collected by the Hubble Space Telescope and ESO’s VLT to observe a gravitational lensing effect, one of the relativistic predictions, created by the galaxy ESO325-G004. The two instruments provided separate data that, compared, confirmed the correctness of the theory.

Part of the Abell S0740 cluster, the galaxy ESO325-G004, or simply E325, is about 500 million light years away from Earth and is one of the closest gravitational lenses. So far, hundreds of galaxies have been found that show an effect of that kind, with the distortion of light coming from other, more distant galaxies that appear behind them, because of their gravity. E325 was chosen because other gravitational lenses are too far away to be able to measure their mass with enough precision, a crucial factor in this research.

The MUSE (Multi Unit Spectroscopic Explorer) instrument installed on the VLT (Very Large Telescope) in Chile was used to measure how fast the stars move within the E325 galaxy. This allowed to calculate how much mass there must be in that galaxy to keep its stars in orbit.

The researchers also used the Hubble space telescope to observe what’s called an Einstein ring, one of the possible results of a gravitational lensing effect. Looking at the ring generated by the distortion caused by the galaxy E325 it was possible to measure how much light gets distorted by its mass because space-time itself gets distorted.

In simple words, the researchers calculated the mass of the E325 galaxy and the distortion that generated on the image of another galaxy behind it. By comparing these two ways of calculate the overall gravity of E325, it was possible to evaluate the correctness of the general theory of relativity’s predictions, which was verified with an uncertainty of 9%, very little when dealing with galaxies of which the nearest one is 500 million light years away.

The theory of relativity has been tested many times but this is the most accurate verification outside the Milky Way. This is important to make sure that our cosmological models are correct as they’re based on the fact that gravity works according to the formulas proposed by Albert Einstein even on galactic scales. Alternative models have been proposed but this research clearly indicates that the models based on the general theory of relativity are correct.

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