Is cosmic acceleration changing?
If it's possible to make a mystery more mysterious, then a study of dark energy by astronomer Bradley Schaefer of Louisiana State University in Baton Rouge may have done so. Earlier this month, at the American Astronomical Society's 207th meeting in Washington, Schaefer reported that distant gamma-ray bursts indicate dark energy's repulsive force is changing fast.
While cosmologists cast doubt on Schaefer's experimental method, the study underscores just how little scientists know about dark energy.
"Cosmic acceleration is the biggest mystery in all of science," says University of Chicago cosmologist Michael Turner. Astronomers discovered dark energy 8 years ago, when analyses of distant supernova explosions showed the universe's expansion is speeding up instead of being slowed by gravity. "We have to get our heads and telescopes around that," Turner says.
Astronomers use type 1a supernovae — exploding stars that reach the same intrinsic brightness — for measuring dark-energy changes. Knowing the distance to at least one of these supernovae lets scientists use the entire class as a "standard candle." They can measure the distance to any type 1a supernova just by comparing its apparent brightness to the one with the known distance.
The redshift of light in a supernova's spectrum provides a measure of how much space has expanded since the star detonated. Astronomers can plot the universe's expansion history — and any change in dark energy's acceleration — by plotting data from hundreds of supernovae.
The SuperNova Legacy Survey (SNLS) uses just this strategy. In November 2005, the group published a study reporting that dark energy acts like Albert Einstein's famous fudge factor, the cosmological constant, which he added to general relativity in order to balance gravity's attractive force. However, some scientists criticized the analysis, arguing that the SNLS team did not allow for the possibility that dark energy's density itself is changing.
Schaefer's study suggests this value is not only changing, but changing fast. While dark energy is repulsive today, it appears to have been attractive, like gravity, in the early universe. "The cosmological constant apparently isn't constant," he says.
"It would be absolutely extraordinary if correct," Turner says.
Instead of the well-established standard candle of type Ia supernovae, Schaefer used gamma-ray bursts (GRBs). Space probes like NASA's Swift can spot extremely distant GRBs, which are 1,000 times brighter than supernovae, and give scientists vital clues about early cosmic expansion.
Schaefer studied 52 GRBs located up to 12.8 billion light-years away. The most distant, he says, are much brighter than would be expected if dark energy were constant. But he admits the study has substantial room for error and that more observations are needed to confirm his result.
"We don't know much about bursts, so there is a risk this is misleading," says Harvard University's Robert Kirshner. "It is a blunt tool to measure a delicate effect."
"What he has may be a new ruler for measuring something, and that's great," says Ray Carlberg, a cosmologist at the University of Toronto and part of an international team studying dark energy, in the Baltimore Sun. "But whether that ruler is made out of rubber and won't really work or whether it gives an accurate measurement remains to be seen."
Adam Reiss, an astronomer at Baltimore's Space Telescope Science Institute, and codiscoverer of dark energy, voiced greater skepticism about Schaefer's results. "Stay tuned, come back a few weeks after his paper is refereed, and you'll see he's wrong," he says. No GRBs have exploded nearby, he says, so there's no reliable baseline for determining the distance of events at the edge of the cosmos.
Schaefer hopes analysis of a new set of GRBs will reproduce his result, but cosmologists would prefer confirmation using different techniques.
"Other methods will catch up, and then we'll know whether it is a statistical fluke," Turner says.