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 five billion years.
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 Tuesday.
Measuring the Mysterious ‘Dark’ Force (December 17, 2008)
Times Topics: Dark Energy (Astronomy)
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 a telephone news conference on Tuesday and in two papers that will appear in The Astrophysical Journal.
This stifling of growth, Dr. Vikhlinin said, is the “unmistakable signature” of 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. 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. 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, Dr. Vikhlinin said, the new data strengthen the suspicion — but do not prove — that dark energy is a weird antigravity called the cosmological constant that was hypothesized and then abandoned by Albert Einstein as a “blunder” almost a century ago. If that is true, the universe is fated to empty itself out eventually, and all but the Milky Way’s closest neighbors will eventually be out of sight.
Other astronomers hailed the work as 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.
The fact that two different methods have given similar results for dark energy is a triumph of Einstein’s general theory of relativity, the last word on gravity for the last century, astronomers said. “This was a test that general relativity could have failed,” said William Forman of Harvard, a member of Dr. Vikhlinin’s team.
Some dark energy explanations that involve modifying Einsteinian gravity may be on the verge of extinction as a result.
Adam Riess of Johns Hopkins and the Space Telescope Science Institute, 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.”
Clusters of galaxies, which can contain thousands of individual galaxies, are perfect laboratories 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.
Dr. Vikhlinin and his colleagues used Chandra to observe 86 clusters that had been found in a survey by an earlier satellite, Rosat. One was a set of 37 about five billion light-years away; 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.
Comparing their data to models of cosmic evolution, Dr. Vikhlinin found that the most massive clusters are only about a fifth as plentiful today as they would be in a universe without dark energy. The clusters, he said, “are still growing, but very slowly.”
Einstein speculated that empty space was imbued with an antigravitational energy — his cosmological constant to explain why the universe was stable and did not collapse under its own gravity. But later Edwin Hubble 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.
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 argue, is to presume that our 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.”
The new cluster results, combined with other measurements of supernovas and of cosmic microwaves left over from the Big Bang, provide the tightest constraints yet on dark energy, Dr. Vikhlinin said. Its virulence, as measured by a parameter called w, is within 5 percent of the cosmological constant.
But that still leaves a lot of wiggle room for other theories, and some astronomers worry that the measurements will never get accurate enough to prove or disprove the cosmological constant.
David Spergel of Princeton said astronomers had to soldier on into the darkness. The universe has surprised us before, he said, and “I think we will continue to be surprised in the future.”