NASA/CXC/SAO/A.Vikhlinin et al
A composite image of the galaxy cluster Abell 85, located about 740 million light years from Earth. Observations of it helped astronomers trace how dark energy has stifled the growth of galaxies over the last seven billion years.
By DENNIS OVERBYE
Published: December 16, 2008
The same mystery force that is speeding up the expansion of the universe is also stunting the growth of the objects inside it, astronomers said on Tuesday.
After bulking up rapidly in the first 10 billion years of cosmic time, clusters of galaxies, the cloudlike swarms that are the largest conglomerations of matter in the universe, have grown anemically or not at all during the last five billion years, like sullen teenagers who suddenly refuse to eat.
“This result could be explained as arrested development of the universe,” said Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics, who led a multinational team using NASA’s Chandra X-ray Observatory to weigh galaxy clusters from far across space. The group reported the results in two papers that will appear in the Astrophysical Journal.
The culprit, he said, appears to be an antigravitational force that astronomers have labeled “dark energy.” It was discovered 10 years ago by astronomers who were using exploding stars called supernovas as distance markers to chart the expansion of the universe. In a puzzle that is still reverberating, they found that instead of slowing down because of cosmic gravity, as common sense would suggest, the expansion of the universe was actually speeding up, with galaxies zooming apart faster and faster.
Dr. Vikhlinin’s results dovetail eerily with the supernova results, suggesting that dark energy emerged as a dominant force in the universe about seven billion to five billion years ago. Clusters grow by gravity, according to cosmological theory, starting as small dimples in the heat and fizz of the Big Bang and then drawing in surrounding material over the eons. Dark energy would work against gravity and try to push the matter falling in back out, stalling growth.
Together with earlier observations of supernovas and other effects, Dr. Vikhlinin said, the new data strengthen the suspicion — but do not prove — that dark energy is the result of a weird antigravity called the cosmological constant that was hypothesized and then abandoned by Albert Einstein as a “blunder” almost a century ago.
Many other theories are still in contention, but some that involve modifying Einstein’s general theory of relativity, which has been the last work on gravity for almost a century, might be on the verge of extinction, astronomers said.
“If this was a fox hunt and dark energy was the fox, I think they have closed off another escape route. But there is still a lot of terrain left for the fox, and we’ve seen little more than a glimmer of fur,” said Adam Riess, of Johns Hopkins and the Space Telescope Science Institute, and one of the original discoverers of dark energy.
Other astronomers hailed the work as opening a new avenue in the investigation of what is happening and will happen to the cosmos.
“To date, only one technique — supernovae — has detected dark energy without folding in other observations,” said Michael Turner of the University of Chicago. “This would be the second, stand-alone, detection of dark energy and the validation of an important technique for probing dark energy.”
To explain why the universe was stable and did not collapse under the collective gravity of its contents, Einstein speculated that empty space, what physicists call the vacuum, was imbued with an antigravitational energy — his cosmological constant. Einstein’s constant did not even work mathematically, and later it was discovered that the universe was not stable — it was expanding.
Modern quantum mechanics predicts that empty space should indeed be imbued with this strange energy, but the possibility that the dark energy might actually be Einstein’s cosmological constant has thrown physics into philosophical turmoil. “The discovery of dark energy has greatly changed how we think about the laws of nature,” Edward Witten, a theorist at the Institute for Advanced Study in Princeton, N.J., said recently.
According to the calculations, the cosmological constant should be 1060 times bigger than what astronomers have measured; in such a universe, stars, planets and of course ourselves could not exist. The only way out, some physicists and cosmologists have argued, is to presume that our own universe is only one of as many as 10500 parallel universes, in which the laws of physics happen to be conducive to our existence. But many others bitterly disagree.
As a result, many astronomers and physicists are desperate for evidence of another explanation. Dr. Riess said of the cosmological constant, “The biggest thing we could learn is by ruling that out.”
Last month, NASA and the Department of Energy signed a memorandum of agreement to build a dark energy observatory satellite that would be launched in the next decade.
Clusters of galaxies are the largest conglomerations of matter in the universe, and so, next to the universe itself, they are a perfect laboratory for studying gravity and its enemy, dark energy, on a grand scale. They are also easy to find. They are filled with gas so scorchingly hot that it emits X-rays, which can be seen by satellite observatories like Chandra and Rosat, which was built in Germany and launched by NASA, from billions of light-years away.
Starting in 2005, Dr. Vikhlinin and his colleagues used Chandra to observe 86 clusters that had previously been found in a survey by Rosat. One was a set of 37 about five billion light-years away, while 49 others were about half a billion light-years or closer. Their masses, determined from the extent of the X-ray images and their spectra, ranged from 100 trillion Suns to a quintillion Suns.
The span between the two sets amounts to about a third of the age of the universe. Dr. Vikhlinin and his colleagues used theoretical models to calculate how the numbers of clusters with different masses would change during that span under different conditions, including no dark energy.
The models without dark energy would not fit, they concluded. The most massive clusters, they found, are only about a fifth as plentiful today as they would be in a universe without dark energy, Dr. Vikhlinin said. The clusters, he said, “are still growing, but very slowly.”
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