Is dark energy an illusion? Perhaps, says Edward Kolb, a physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois. In mid-March, Kolb and three Italian collaborators posted a provocative paper arguing that dark energy--the mysterious antigravity force that makes the universe expand ever faster--is actually a byproduct of enormous ripples in the fabric of spacetime. Kolb's paper created ripples of its own, and now two theorists from Princeton University argue that Kolb's team made an accounting error that invalidates the result.
Kolb's paper, which appeared on the arXiv preprint server [www.arxiv.org], suggested that dark energy--whose effects have been observed by supernova hunters and other astronomers--is not really an energy or a substance. Instead, Kolb says, enormous "perturbations" or ripples in spacetime much larger than the observable universe cause the accelerating expansion of the universe. These ripples, which were caused by the rapid period of inflation just after the big bang, would mimic the fluidlike substance scientists now call dark energy.
But not so fast, say Princeton physicists Uroš Seljak and Chris Hirata. In a paper also posted on the archive, the scientists launched a two-pronged attack on the Kolb hypothesis. First, using a powerful equation derived from those of general relativity, the two derive a "no-go" theorem that says that huge ripples can't make the universe expand faster and faster. "The equation shows they cannot lead to acceleration," says Seljak. "You cannot have acceleration with only ordinary matter" in the universe; there has to be dark energy.
Next, Seljak and Hirata attack Kolb's mathematics. They argue that in the intricate mathematical calculations leading to the result, Kolb and colleagues inadvertently left out some crucial terms that exactly cancel the effect that they are claiming. "They have been fooled into thinking that there's no cancellation," Seljak says. "These things happen. It's not an easy calculation."
Some physicists, such as Edmund Bertschinger of the Massachusetts Institute of Technology, say Seljak and Hirata have put the matter to rest. "This is definitive in my mind," he says. Kolb, however, holds firm. "I think the no-go theorems eventually will go," he says, adding that he believes Seljak and Hirata have themselves made subtle errors that invalidate their criticisms. "But their work is sharpening our thinking, and we are writing another paper."