Can Einstein’s biggest theory be proven on the grandest of scales?

22 Jun 2018

This image shows the galaxy cluster MACS J2135-010217 (centre), which is gravitationally lensing the distant galaxy SMM J2135-0102. Image: NASA/ESA Hubble Space Telescope

Scientists using some of the world’s most powerful telescopes have helped confirm Einstein’s theory of general relativity on an unprecedented, galaxy-sized scale.

One of Albert Einstein’s most famous theories is the one describing general relativity, which predicts that objects deform spacetime around them, causing any light that passes by to be deflected.

This results in the observations of a phenomenon called the Einstein ring, a gravitational lensing effect when a star in the foreground passes exactly between us and a background star, creating a perfect circle of light.

Well now, a team of researchers from the University of Portsmouth has taken this theory and tested it on a galactic scale, resulting in the most precise test of the theory ever conducted outside of the Milky Way.

In a paper published online, the team revealed it had used the MUSE instrument on the Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) to measure the mass of the galaxy dubbed ESO 325-G004.

It did this by measuring the movement of stars within this nearby elliptical galaxy. Then, using the Hubble Space Telescope, the team was able to observe an Einstein ring resulting from light from a distant galaxy being distorted by the intervening ESO 325-G004.

By observing the ring, scientists are able to measure how light and spacetime is being distorted by the galactic mass.

Einstein ring

An image of the nearby galaxy ESO 325-G004, created using data collected by the NASA/ESA Hubble Space Telescope and the MUSE instrument on the VLT. The inset shows the Einstein ring resulting from the distortion of light from a more distant source. Image: ESO, ESA/Hubble, NASA

‘The universe is an amazing place’

What makes this research so significant is that, while a few hundred strong gravitational lenses are known, the majority are too far away to be able to measure their mass accurately, but this one is just a ‘mere’ 450m light years from Earth.

Until now, Einstein’s theory had been tested with significant accuracy on solar system scales, but there had so far been no precise tests on larger astronomical scales until now, showing an uncertainty of just 9pc.

Speaking of what these findings mean for science, the researchers said they may have major implications for models of gravity alternative to general relativity – the same used to explain the accelerated expansion of the universe, which predicts that the effects of gravity on the curvature of spacetime are ‘scale-dependent’.

In essence, this means that gravity should behave differently on an astronomical scale than how it might appear on a solar system scale.

“The universe is an amazing place, providing such lenses which we can use as our laboratories,” said researcher Bob Nichol.

“It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was.”

Colm Gorey was a senior journalist with Silicon Republic