The Hubble Space Telescope, peering 10 billion years back in time to when the universe was in its adolescence, has spotted the most distant exploding star ever observed.
Researchers said the discovery bolsters the controversial theory that mysterious "dark energy" is accelerating the expansion of the cosmos.
Invisible and poorly understood, dark energy might account for as much as two-thirds of space. Proposed a century ago by Einstein, it may counteract more familiar forces such as gravity.
The supernova, barely discernible with the most powerful instruments, provides clues to dark energy. While dim, the dying star gleams brighter and moves differently than it would if the universe had expanded at a steady rate since the beginning of time.
The Hubble finding is prompting researchers to rethink how the universe works.
"(Dark energy) is every bit as amazing as black holes," said
University of Chicago cosmologist Michael Turner in a briefing at NASA's Washington headquarters. "It controls the density of nature. It's the key to understanding how all of nature's particles and forces fit together."
"The discovery that the universe is speeding up will be viewed as one of the most important discoveries in all of science in the past 25 years," Turner said.
A supernova is an exploding star, a cosmic flashbulb. One occurs each second somewhere in space, and that single star beams brighter than the billions of stars in its galaxy combined.
But astronomers have to search mightily for supernovae, making the discovery of a supernova a dramatic event.
Supernova 1997ff exploded more than 10 billion light years from Earth, or 1.5 times farther than any previously recorded exploding star, when the universe was a quarter of its current age.
Only recently did the flash reach Hubble. In that way, telescopes act as time machines allowing astronomers to observe events in the early history of the universe as they are happening.
Ten billion years ago, the universe was only about 4 billion years old. Supernova 1997ff probably was a white dwarf star about the size of the sun, but extremely dense.
It sucked matter from a companion star until it reached critical mass and temperature. The carbon ignited in a runaway thermonuclear explosion of "astonishing brightness," said co-discoverer Peter Nugent of Lawrence Berkeley National Laboratory.
In 1995 and 1997, Hubble examined a tiny part of the northern Hemisphere sky. One object was visible only in the later images - meaning it was so distant its light didn't reach Earth until then.
That was supernova 1997ff.
Later images by infrared instruments allowed researchers to track the supernova's motion and other characteristics. They reached firm conclusions about the supernova several months ago.
Cosmologists believe the universe probably was born in an explosion known as the Big Bang 12 billion to 15 billion years ago.
Gravity put the brakes n the immediate expansion. Then, they suspect, the universe started expanding more rapidly again 4 billion to 8 billion years ago. The reason is unclear; possibly dark energy gained the upper hand.
If astronomers were to look deep into time and space, theorists have predicted objects would appear brighter than expected because gravity was restraining galaxies from flying apart.
Sure enough, the relative brightness and motion of 1997ff might be the first tantalizing clues the theorists were right.
"This supernova shows us the universe is behaving like a driver who slows down approaching a red stoplight, then accelerates when the light turns green," said Adam Reiss of the Space Telescope Science Institute in Baltimore.
Alternative theories suggest that dust in the universe was not as dense 10 billion years ago, making the dying star appear brighter than expected. Scientists agreed more observations are needed.
By Joesph Verrengia
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