Exploding star tests famous theory
Astronomers have captured the moment a far off star exploded - not once but four times.
In so doing they confirmed an effect predicted by Albert Einstein's general theory of relativity 100 years ago.
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 from the supernova passing through the distorted region produced multiple images.
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."
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.
The theory explains how massive objects bend space-time to create what we feel as the force of gravity.
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.
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.
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.