Thu, 12 Jan 2006 12:00 CST
Dark energy the mysterious force that drives the acceleration of the universe changes over time, controversial new calculations suggest. If true, the work rules out Einstein's notion of a "cosmological constant" and suggests dark energy, which now repels space, once drew it together.
Astronomers invoked the concept of dark energy to explain supernovae observations in the late 1990s that the universe is not only expanding but accelerating. The supernovae appeared dimmer and therefore more distant than expected, given their red shift, which measures how much their light has been stretched by the expansion of space.
But it is still not clear what dark energy is. Theories range from a "vacuum" energy of space itself which is fixed in value an idea Einstein proposed as the "cosmological constant" to more exotic possibilities, such as quintessence a type of energy field that can vary over space and time.
So far, supernova studies have supported the cosmological constant one recent study of 70 supernovae reported that the strength of repulsion given by dark energy could not have changed by more than about 20% over the past eight billion years.Reliable data
But supernovae are too dim to be seen over the largest cosmic distances. So some astronomers argue that gamma-ray bursts (GRBs) violent, fleeting explosions that accompany the deaths of some massive stars are better signposts. At about 100 times brighter than supernovae, they can be seen at much greater distances.
Now, astronomer Bradley Schaefer of Louisiana State University in Baton Rouge, US, has used observations of 52 GRBs to suggest that dark energy has changed over time.
In the largest GRB study of its kind, Schaefer found that 12 of the most distant GRBs lying nearly 13 billion light years away were all brighter than expected, suggesting the universe was expanding at a slower rate than it is today.
He says the fact that all 12 were brighter than would be predicted by a cosmological constant increases his confidence in the data. "It's like if you flip a coin and get 12 heads in a row," Schaefer told New Scientist. View several graphs of his results, here.
He says that rather than pushing space apart, dark energy appears to have changed over time and was in fact drawing space together in the early universe. What that means for the fate of the universe is not clear, but it seems to open a Pandora's box of outlandish possibilities for dark energy, he says: "With quintessence, you can do anything you want."Standard candles
But other researchers are yet to be convinced. Type 1a supernovae all explode with the same intrinsic energy, making them ideal "standard candles" to measure distance. But GRBs explode with a variety of energies. So Schaefer used five observed properties of the bursts such as how their brightness changes over time to estimate their intrinsic brightness, and thus their distance.
But GRB expert Dale Frail of the National Radio Astronomy Observatory in New Mexico, US, says GRBs vary too widely and are not understood well enough to make such inferences. "I think it's premature to start using GRBs as standard candles," he told New Scientist.
Robert Kirshner, an astronomer at Harvard University in Cambridge, Massachusetts, US, and a pioneer of supernova dark energy studies, agrees: "You've got kind of a blunt tool to measure a very delicate effect."
Schaefer acknowledges the research is preliminary, but says the analysis will improve as more GRBs are discovered and studied.
The research was presented on Wednesday at a meeting of the American Astronomical Society in Washington DC, US.