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Supernova sheds light on dark energy

The most distant supernova ever detected has turned one of the weirdest theories ever proposed into the leading explanation for how our universe works.
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The most distant supernova ever detected has turned one of the weirdest theories ever proposed into the leading explanation for how our universe works, scientists say. The exploding star was spotted 11 billion light-years away by comparing images from the Hubble Space Telescope taken years apart.

The stellar explosion shows up as only a few pixels on a computer-processed image, released by NASA on Monday. But a research team led by Adam Riess of the Baltimore-based Space Telescope Science Institute analyzed the character of that light to figure out how far away the source was, as well as how the supernova’s behavior matched up with theoretical predictions.

That behavior matches up best with a theory contending that most of the universe consists of “dark energy,” a mysterious repulsive quality that is pushing galaxies away from each other at an ever-increasing speed.

“This is very weird stuff,” said University of Chicago astrophysicist Michael Turner. “In fact, I think it’s fair to say that our main achievement in understanding dark energy to this date is to give it a name.”

Dark history
Dark energy actually has picked up several other names, including the cosmological constant, lambda, quintessence and repulsive gravity. The concept was first proposed, then discarded, by Albert Einstein as part of his theory of general relativity. Einstein once called it his “biggest blunder.” But cosmologists are increasingly coming around to the view that Einstein was right the first time, even though dark energy has not been detected directly.

In order for the prevailing theories about the nature of the universe to work out correctly, all the matter that we can see would have to account for only 5 percent of the cosmos, scientists say. They say dark matter, which does not radiate but holds together galaxies and clusters like “cosmic glue,” should make up another 30 percent, and the dark energy behind the expanding universe’s speed-up would account for the final 65 percent.

Dark energy came into the spotlight full force about three years ago, when the unusually dim light of several distant supernovae suggested that the universe is expanding more quickly than in the past. At the time, there were several explanations for the dimness of the supernovae. The theory about dark energy and the accelerating universe was one of the possibilities, but skeptics said the supernovae might also appear dimmer because there was more dust than expected between Earth and the faraway objects, or because supernova blasts were intrinsically dimmer in the early universe.

Turner said the newfound supernova “drives a stake through the heart” of those alternative theories, because the ultra-distant blast was actually brighter than expected. This is in tune with the dark energy theory, which says the expansion of the universe should have slowed down after the Big Bang due to the effect of gravity, and then should speed up as dark energy wins out over gravity’s grip.

Riess said the new supernova, dubbed SN 1997ff, suggests that the universe began speeding up relatively recently in cosmic terms — perhaps 4 billion to 8 billion years ago. Earth and the rest of the solar system are thought to have been formed a little more than 4 billion years ago.

“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,” Riess said in a statement. “This supernova 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.”

How the observation was made
The team of astronomers made the discovery by analyzing hundreds of images taken by Hubble in infrared and visible light to study how galaxies formed.

Fortuitously, one of those galaxies contained a supernova previously discovered by astronomers Ron Gilliland of the Space Telescope Science Institute and Mark Phillips of the Carnegie Institutions of Washington.

The researchers were able to get a better fix on the supernova by comparing the well-known Hubble Deep Field image, made in 1995, with follow-up images from 1997 and 1998. When the colors from the earlier image were digitally “subtracted” from the later images, the supernova showed up as a speck of light.

Then the researchers used supercomputers at the Lawrence Berkeley National to analyze the brightness and the redshift of that speck. They confirmed that the light started out from the supernova an estimated 11 billion years ago, when the universe was merely a quarter of its present age. That would make the supernova about 50 percent farther away than the next-most-distant contender, Riess said.

Riess and his colleagues noted that the supernova was brighter than astronomers would have expected at that distance — in contrast to closer, more recent supernovae, which were dimmer than expected. That was the key to their claims about a cosmic slowdown in the expansion of the universe, followed by a speed-up.

“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,” Riess said. “Billions of years later, when the light left more recent supernovas, the universe had begun accelerating, stretching the expanse between galaxies and making objects in them appear dimmer.”

Looking ahead
Turner acknowledged that much more data would have to be collected to verify the dark energy theory, but he said the tables have been turned on any alternate explanations for the data.

“Now the so-called ‘conventional’ explanations, because they’ve fallen on their face, are the ones that are starting to look contrived,” he said. “Fifty years out, when we look back, I think we’re going to say this was a discovery perhaps as fundamental as the discovery of the expansion of the universe.”

However, the supernova observations still do not address the big question: What is the nature of dark energy?

Turner said that the repulsive gravity may arise from the rise and fall of virtual subatomic particles in the “quantum vacuum” of empty space, or it may be the effect of higher planes beyond three-dimensional space. Or it may be something even weirder.

“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 accelerators,” Turner said.

Cosmologists say one huge step along that route would be the development of a new orbiting space telescope dubbed the Supernova Acceleration Probe, or SNAP.

Riess’ colleagues in the new research were Peter Nugent (Lawrence Berkeley National Laboratory), Brian Schmidt (Mount Stromlo Observatory) and John Tonry (Institute for Astronomy). The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

This report includes information from NASA.