Astronomy for the advanced

Part of Einstein's general theory of relativity has finally been proved - by accident

Astronomers have proved an effect predicted by Albert Einstein's general relativity theory 100 years ago - completely by accident.

They captured the moment a far off star exploded, not once but four times.

The exploding star, or supernova, was directly behind a cluster of huge galaxies whose enormous mass warped space-time, creating a cosmic magnifying glass.

Light passing through the distorted region from the supernova produced multiple images.

Einstein's theory explains how massive objects bend space-time to create what we feel as the force of gravity.

Dr Brad Tucker, from The Australian National University, said: "It's perfectly set up, you couldn't have designed a better experiment. You can test some of the biggest questions about Einstein's theory of relativity all at once."

It can be envisaged by imagining a football pressing down the centre of a rubber sheet. A pea rolled into the depression would spiral inwards towards the ball.

A large cluster galaxy (centre of the box) has split light from an exploding supernova in a magnified background galaxy into four yellow images (arrows) (Z. Levay at NASA's Space Telescope Science Institute/ESA)

Astronomers have looked for the kind of "gravitational lensing" event described in Science for the past 20 years.

Dr Patrick Kelly, from the University of California at Berkeley, US, made the discovery while searching for distant galaxies.

"It really threw me for a loop when I spotted the four images surrounding the galaxy - it was a complete surprise," he said.

The find not only acts as a test of general relativity, but also provides information about the strength of gravity and the amount of dark matter and dark energy in the universe.

Light from a supernova is deflected by the gravity of a large collection of galaxies and an elliptical galaxy, which act like a "cosmic magnifying glass" producing four magnified images (FrontierSN team/ESA/GLASS/Nasa)

Dark matter is an invisible substance that cannot be seen but binds galaxies together. It makes up most of the matter in the universe but its nature is unknown.

Even more mysterious, dark energy produces an "anti-gravity" force that is causing galaxies to fly apart at an accelerating rate.

Because the lensing effect also magnifies the supernova, it offers scientists a window into the distant past by uncovering light that has spent billions of years crossing the universe.

"It's a relic of a simpler time, when the universe was still slowing down and dark energy was not doing crazy stuff," said Dr Tucker.

The observation forms part of a special report on general relativity in the journal Science marking the centenary of the theory published in December 1915.