Dying star reveals universe's dark energy

By ANNE BESTON science reporter

When supernova 1997ff died in a titanic cosmic explosion 10 billion years ago, the universe was in its infancy.

Now the brilliant flash from this dying star is providing more evidence that mysterious "repulsive dark energy" is accelerating the expansion of the cosmos.

The supernova, which is the furthest exploding star ever seen from Earth, was discovered by the Hubble Space Telescope, launched in 1990.

Supernova 1997ff appeared roughly twice as bright as astronomers had expected, providing more evidence that "repulsive dark energy" is shoving objects in the cosmos away from each other at an increasing rate, causing the universe to expand.

Dr Ian Griffin, a former director of the Auckland Observatory and Stardome Planetarium, who now works at the Hubble Space Telescope Science Institute in the United States, said the discovery of the supernova was like opening a new door in the search to understand the cosmos.

"It's being described as one of the biggest discoveries of the last few years - astronomers are very excited about it," said Dr Griffin, who is the Herald's astronomy columnist, from his home in Baltimore, Maryland.

Dying stars such as 1997ff - about the size of Earth but 400,000 times heavier - can be used to measure distances in space because they always explode with the same absolute brightness.

Astronomers can compute their distance by measuring their apparent brightness: the dimmer the supernova, the more distant it must be.

Combined with measurements of the supernova's "redshift," or how fast it was receding from Earth, astronomers can try to understand how the expansion rate of the universe has changed over time.

The supernova was discovered when Hubble was peering into the Hubble Deep Field, a far-distant region of space, in 1997. Scientists spotted the dying phases of the star by chance from hundreds of images of the field.

Some scientists have always believed that if astronomers looked deep into time and space, objects would appear brighter than expected because gravity was restraining galaxies from flying apart.

"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 one of the astronomers involved in the discovery, Adam Riess of the Space Telescope Science Institute in Baltimore.

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

Observations of several distant supernovae by two teams of astronomers in 1998 led to the prediction that the universe received the "green light" to accelerate when it was about half its present age.

After the "big bang," which formed the cosmos about 15 billion years ago, gravity is thought to have slowed expansion down.

But gravity was eventually beaten by the repulsive force of "dark energy" that is thought to make up at least two-thirds of the cosmos.

Astronomers involved in the discovery of supernova 1997ff say the latest findings put "a stake through the heart" of other theories.

One of those was that dust in the universe caused objects to appear brighter or dimmer.

Dr Griffin said the only thing scientists had successfully done with dark energy was to give it a name.

It may take decades of further studies to begin to understand it.