By X-Raying Galaxies, Researchers Offer New Evidence of Rapidly Expanding Universe

May 19, 2004


Observations of giant clouds of galaxies far out in space and time have revealed new evidence that some mysterious force began to push the cosmos apart six billion years ago, astronomers said yesterday.

The results constitute striking confirmation of one of the weirdest discoveries of modern science: that the expansion of the universe seems to be accelerating, the galaxies flying apart faster and faster with time, under the influence of some antigravitational force. The work, astronomers said, opens up a powerful new way of investigating the nature of this "dark energy" and its effect on the destiny of the cosmos.

The astronomers used an orbiting X-ray satellite called Chandra to observe hot gases in the distant galactic clusters. By analyzing the X-rays emitted by those gases, they could calculate the distance from Earth and the speed of each of the clusters and thus trace the history of the expansion of the universe over the last 10 billion years, they said.

"The universe is accelerating," said Dr. Steve Allen of Cambridge University in England, leader of the international team that did the work. "We have found strong new evidence for dark energy."

They announced their results at a news conference at NASA headquarters in Washington. A paper describing the work has been submitted to the journal Monthly Notices of the Royal Astronomical Society.

Other astronomers hailed the X-ray cluster method as a potential complement to other ways of investigating dark energy but said they would withhold judgment about this particular calculation until they could study the details. Most of the previous studies, including those that led to the discovery of dark energy, used exploding stars known as Type 1a supernovas as cosmic distance markers.

Dr. Adam Riess of the Space Telescope Science Institute in Baltimore, an original discoverer of dark energy, hailed the work as another sign of the new age of "precision cosmology.''

Dr. Riess said in an e-mail message: "Cosmologists are all from Missouri, the Show-Me State. It appears that X-ray clusters have been added as a new tool in our surveyor's tool kit. All tests point to a strange form of gravity we call dark energy. Some love it, some hate it; it appears we have to deal with it."

Dr. Martin Rees, a cosmologist at Cambridge who was not part of the team, called the results "neat work and a promising method," which, he noted, involved "very straightforward assumptions and simple physics."

Another cosmologist who was not part of the team, Dr. Michael Turner of the University of Chicago, said: "We can now be quite confident that the expansion of the universe is speeding up. It's not a fluke, it's not going away."

Dark energy has confounded experts and everybody else since two competing groups of astronomers discovered six years ago that the expansion of the universe was not slowing down due to cosmic gravity, as had been presumed, but was speeding up.

At the news conference, Dr. Andrew Fabian of Cambridge, a team member, compared the phenomenon to tossing an apple in the air and watching it go up faster and faster rather than falling back down. "It requires new physics beyond everyday experience, even the experience of an astronomer," Dr. Fabian said.

In recent years theorists have filled the journals with ever more fanciful explanations of what might be causing this behavior.

One possibility, first suggested and then rejected by Einstein, is that space itself has a repulsive force. But according to modern particle physics theory, this cosmological constant, as this force is known, should be about 1060 times bigger than what astronomers have measured, causing theorists seek other explanations. Among them have been extra unseen dimensions to space, interactions with other, parallel universes and as-yet-undiscovered particles or forces.

Or perhaps, some theorists say, Einstein's theory of gravity, general relativity, which has been the backbone of cosmology for nearly a century, needs modification.

Astronomers hope that some answers will come if they can find out whether the density of dark energy - estimated to make up 75 percent of the universe - is changing with time.

If dark energy were constant, it would mean that Einstein's cosmological constant is in effect, and that most of the galaxies would move away too fast to be seen a mere 100 billion years from now.

If dark energy is increasing, it could mean the universe could end in a "big rip," in which even atoms would be torn apart. On the other hand, the dark energy could decrease and even turn into an attractive force, drawing the universe to an end in a "big crunch."

The new results are consistent with Einstein's cosmological constant but also allow for the possibility that the dark energy could be changing, echoing recent results from the supernova surveys.

"The nice thing is that this is a completely independent method based on very simple physics," Dr. Allen said. "It's the physics of hot gas and the physics of gravity."

Clusters of galaxies are the largest objects in the universe, containing thousands of galaxies and trillions of stars. But in a big cluster, the stars themselves are greatly outweighed by intergalactic gas, which has been condensed and heated to 100 million degrees or so by the cluster's immense gravity.

The X-rays that are spit out by this gas can be seen far across the universe. From their brightness astronomers can gauge the amount of gas in the cluster, and from the temperature of the gas, they can estimate the total mass in the galaxy cluster. Most of that mass is mysterious dark matter, which has been detected only by its gravitational effects on the luminous parts of the universe.

The astronomers made what they said was the simple assumption that clusters were a fair sample of the universe as a whole and that the cosmic ratio of dark matter to ordinary matter applied in each individual cluster. That allowed them to calculate distances to 26 clusters, from 1 billion to 10 billion light-years away, and thus measure how fast the universe was expanding when the light left those far-away galaxy clouds, confirming the cosmic acceleration.

"It's nice our results agree with previous experiments," Dr. Allen said. "It lets you feel rather more secure that everything is as it should be in those experiments."