By Adrian Cho
For the past 20 years, physicists have known that the expansion of the universe is accelerating, as if some bizarre “dark energy” is blowing up space like a balloon. In fact, cosmologists’ well-tested standard model assumes that 69% of the content of the universe is dark energy. However, there may be no need for the mysterious stuff, a team of theorists claims. Instead, the researchers argue, the universe’s acceleration could be driven by variations, or inhomogeneities, in its density. If so, then one of the biggest mysteries in physics could be explained away with nothing other than Albert Einstein’s familiar general theory of relativity. Other researchers are skeptical, however.
“If it’s right, somebody is going to have to take back Nobel prizes” awarded in 2011 for the discovery of the accelerating expansion of the universe, says Nick Kaiser, a cosmologist at the University of Hawaii in Honolulu. Tom Giblin, a computational cosmologist at Kenyon College in Gambier, Ohio, who has worked on a similar analysis, says, “I would love if inhomogeneities explained dark energy.” However, he says, “I don’t see any evidence from our simulations to expect it to be as big an effect as they see here.”
At issue is the way cosmologists calculate how the universe evolved over the past 13.8 billion years. Roughly speaking, they rely on two equations. One describes how matter coalesces into galaxies and clusters of galaxies. The other, known as the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, comes out of Einstein’s theory of gravity, or general relativity, and scientists use it to calculate how much the universe has expanded at any time. At each step in time in a simulation, the cosmologists’ program uses the FLRW metric to calculate the “scale factor,” which specifies how much the universe has grown. The program then uses the scale factor as an input to calculate how the formation of galaxies and clusters advances in that step.
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