Distant supernova shows Universe defies gravity

Astronomers have confirmed one of the most profound discoveries ever made about the nature of the Universe: it is expanding at a faster and faster rate.

The Hubble Space Telescope (HST) has seen a burst of light from the most distant supernova ever detected and its brightness indicates that a mysterious force is opposing gravity and pushing galaxies away from each other.

The supernova, a stellar explosion, is at a great distance, 10 billion light-years, from Earth. It appears brighter than it should be and the only way to explain this is with the existence of a mysterious form of "dark energy" that pervades the Universe.

The concept of dark energy, a repulsive force that pushes galaxies away from each other at an ever-increasing speed, was first proposed, then discarded, by Albert Einstein early in the last century.

Dark energy

The Hubble observations reinforce the startling idea that the Universe only recently began speeding up, a discovery made about three years ago, when the unusually dim light of several distant supernovae suggested the Universe is expanding more quickly now than in the past.

Following that discovery, astronomers advanced several explanations for the speeding-up effect, including the so-called "dark energy" idea.

Now, the new observations with Hubble of an even more distant supernova refute all other alternatives.

"The supernova appears to be one of a special class of explosions that allows astronomers to understand how the Universe's expansion has changed over time, much as the way a parent follows a child's growth spurts by marking a doorway," said Adam Riess of the Space Telescope Science Institute.

"It shows us the Universe is behaving like a driver who slows down approaching a red stoplight and then hits the accelerator when the light turns green," he added.

'High gear'

That the record-breaking supernova appears unusually bright is a consequence of the Universe slowing down in the past (when the supernova exploded) and accelerating only recently.

The ancient stellar detonation was detected by digitally subtracting a before and after image of a faint, yellowish galaxy seen in the now famous Hubble "deep field" image.

"Long ago, when the light left this distant supernova, the Universe appears to have been slowing down due to the mutual tug of all the mass in the Universe," said Riess.

"Billions of years later, when the light left more recent supernovae, the Universe had begun accelerating, stretching the expanse between galaxies and making objects in them appear dimmer."

"Hubble's ability to find titanic stellar explosions at these extreme distances is what it takes to confirm this theory that the Universe must have been slowing down before it switched into high gear," said Dr Anne Kinney, director of the American space agency's (Nasa) Origins programme.

"While we don't know what dark energy is, we are certain that understanding it will provide crucial clues in the quest to unify the forces and particles in the Universe, and that the route to this understanding involves telescopes, not particle accelerators," said astrophysicist Michael Turner of the University of Chicago.