Four years ago, cosmologists astonished their colleagues by announcing that the universe appears to be expanding at ever-increasing speed--and that a mysterious antigravity force must be doing the pushing. Since then, other scientists searched in vain for evidence that the unexpected acceleration might be an illusion. Now an international consortium of astronomers has confirmed the original finding with a completely different approach.
The original finding and the new study reflect the two different strategies scientists use to map the structure and geometry of far-flung corners of the universe. One is to take a "standard candle"--an object of known brightness--and then calculate its velocity by measuring how much the redshift "stretches" its light as it traverses the universe. The 1998 announcements that the universe's expansion is speeding up (Science, 30 January 1998, p. 651) relied on this technique, using exploding stars dubbed type IA supernovae as standard candles. Distant supernovae recede more slowly than expected, researchers found, suggesting a cosmic acceleration.
The new work, by a 27-person team from 14 institutions worldwide, takes a very different approach that exploits the lumpiness of the universe. The team started relatively close to home, by calculating the variations in clustering within a huge swarm of nearby galaxies. These variations can be traced all the way back to the ripples in the afterglow of the big bang, the cosmic microwave radiation, says study leader George Efstathiou of the Institute of Astronomy in Cambridge, United Kingdom. By comparing the modern variations in clustering to the ancient ripples in the cosmic microwave radiation, the team calculated how much matter must be sprinkled through the cosmos to transform the primordial ripples into multigalactic clumps as the universe aged.
The results, published in the 21 February issue of the Monthly Notices of the Royal Astronomical Society, add weight to the idea of an accelerating universe. First, they confirm earlier findings that the universe is flat, but they indicate that there is only a third as much matter as is needed to create that flatness. This means that the remaining two thirds is what cosmologists call dark energy. "Dark energy has this strange property that it's essentially repulsive," explains Max Tegmark, a cosmologist at University of Pennsylvania in Philadelphia. "It pushes everything away and makes the universe accelerate faster and faster."
The case that the universe is accelerating is "compelling," says Tegmark, "but it certainly hasn't been established beyond any reasonable doubt." That should come with future generations of cosmic microwave background measurements, he says.