The serendipitous discovery of a distant supernova has confirmed that the universe is expanding at an accelerating rate because of the presence of a pervasive, mysterious "dark energy," astronomers reported Monday.

The supernova -- the most distant yet located, some 11 billion light years from the Earth -- shows that the universe was expanding at a decelerating rate early in its history, and only more recently started to accelerate as a poorly-understood repulsive force started to win out over gravity.

Using Hubble, astronomers pinpointed a blaze of light from the farthest supernova ever seen, called 1997ff. This panel of images shows the supernova's cosmic neighborhood; its home galaxy; and the dying star itself. Astronomers found this supernova in 1997 during a second look at the northern Hubble Deep Field [top panel]. The image shows the myriad of galaxies Hubble spied when it peered across more than 10 billion years of time and space. The white box marks the area where the supernova dwells. The photo at bottom left is a close-up view of that region. The white arrow points to the exploding star's home galaxy, a faint elliptical. The picture at bottom right shows the supernova itself, distinguished by the white dot in the center. Photo: NASA, Adam Riess (Space Telescope Science Institute, Baltimore, MD)

"This is a very significant development in our understanding of the universe," Michael Turner, a University of Chicago astrophysicist, said at a NASA press conference Monday afternoon where the discovery was announced.

Astronomers first raised the possibility of an accelerating universe a few years ago based on evidence from a class of supernova explosions known as Type Ia. Such supernovae always explode with the same absolute brightness, allowing astronomers to 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 the Earth, astronomers could understand how the expansion rate of the universe changed over time.

To their surprise, astronomers found several years ago that the expansion of the universe was not slowing down, as had been assumed, but was actually accelerating under the influence of an unknown force. "It was so startling that a lot of people were very skeptical about it and other possibilities were suggested," explained Adam Riess, the Space Telescope Science Institute (STScI) astronomer who led the current investigation. Those alternatives included clouds of dust between the supernovae and Earth and the possibility that Type Ia supernovae are not the "standard candles" they are assumed to be but in fact vary in brightness.

Testing these hypotheses required observing more distant supernovae, a challenge as even brilliant supernovae become dim and difficult to detect at great distances. A turn of good luck, though, helped Riess and colleagues find such a supernova. In late 1997 STScI astronomer Ron Gilliland used the Hubble Space Telescope to revisit a region of the sky known as the Hubble Deep Field studied two years earlier by the orbiting telescope. By comparing the original Hubble Deep Field images with his new observations, he discovered a bright pinpoint of light visible in 1997 that did not exist in 1995: a distant supernova, designated 1997ff.

That single data point was not enough, however, to determine the supernova's distance or redshift. However, Riess discovered last year that the same region of the sky was studied just a few weeks after Gilliland's observations with an infrared camera on Hubble. Those images, taken over the course of a month, allowed Riess and colleagues to measure the supernova's brightness and redshift, and confirm that it was associated with a massive elliptical galaxy visible in the Hubble images that was more than 10 billion light years away.

The redshift of the distant supernova did not match models based on dust or varying brightness of Type Ia supernovae. It did, though, closely match models based on an accelerating universe, a vindication of those earlier observations. "We now have great confidence that the universe is accelerating today," said Riess.

"This supernova has driven a stake through the heart of two conventional explanations that tried to avoid a cosmological speedup," added Turner.

This diagram shows how the universe slowed down and then revved up since the Big Bang. The concentric red circles denote that galaxies are migrating apart at a slower rate during the first half of the cosmos. Then a mysterious, dark force overcame gravity and began pushing galaxies apart at an ever-faster rate, signified by the green circles. Astronomers found evidence of the universe's deceleration when they observed the farthest supernova ever seen, which detonated so long ago that the universe was still slowing down. Photo: Ann Feild (STScI)

While astronomers are more confident that the universe is accelerating, they still have little idea why it is accelerating. Their best guess is that a mysterious energy force dubbed "dark energy" -- which may make up to two-thirds of the universe and whose gravity is repulsive, not attractive -- is overwhelming the gravitational force of normal matter and this causes the universe to expand at an ever-faster rate.

At the time supernova 1997ff exploded, the expansion of the universe was still decelerating from the Big Bang as gravity was the dominant force in a smaller universe. However, about four to eight billion years ago the universe expanded to the point where the repulsive force from the dark energy became greater than the weakening force of gravity, causing the expansion rate to accelerate.

Such a repulsive force is supported by Einstein's theory of General Relativity, which includes a "cosmological constant" factor originally introduced by Einstein to counteract the attractive force of gravity and create a static universe. That theory, though, gives little guidance on what could generate a repulsive force.

Turner said that this mysterious force is more like energy than matter because it is spread evenly between galaxies, hence the "dark energy" moniker. "More conservative" explanations for the dark energy, he said, include virtual particles in a quantum vacuum and the influence of hidden dimensions predicted by superstring theory. "This is very weird stuff," he noted.

To better understand dark energy and its influence on the expansion of the universe may require years or decades of additional studies with new instruments and telescopes that can peer even deeper into the universe than Hubble can. Turner believes it is the "number one problem" in both physics and astronomy today, and considers the discovery of the accelerating universe one of the biggest discoveries in astronomy since Edwin Hubble discovered the expansion of the universe.

"The discovery that the universe is speeding up and not slowing down," Turner said, "will be viewed as one of the most important discoveries in all of science in the past 25 years."