WASHINGTON - Astronomers have found new evidence the expansion of the universe is speeding up, and at a rate consistent with a concept introduced by Albert Einstein nearly a century ago.
A research team used NASA's orbiting Chandra X-Ray Observatory to study 26 clusters of galaxies at distances between one billion and 10 billion light years from Earth. It found new support for the existence of "dark energy," a mysterious force that may be pushing the universe apart.
Study leader Steven Allen of the University of Cambridge told a NASA briefing yesterday that the dark energy appears to behave much like what Einstein dubbed the "cosmological constant," a fudge factor he included in his general theory of relativity in 1917 as a sort of antigravity to keep the universe from collapsing under its own weight.
Later, after astronomers found the universe to be expanding, Einstein called the cosmological constant his greatest blunder. But theorists have been taking a new look at it since 1998, when astronomers found evidence of a repulsive counterpart to gravity in studies of distant exploding stars called supernovae. Researchers also have found evidence for dark energy in close scrutiny of the microwave background radiation left over from the very early universe.
The Chandra study provides another line of evidence. The satellite can detect hot gas in galaxy clusters, allowing scientists to estimate the amount of normal matter and hidden, or dark, matter in a cluster. The distance to the cluster can be derived from that ratio, Allen said, and gives a yardstick for assessing how the universe has been growing over time. The Chandra team was able to show that the universe's expansion, which had been slowing, began to speed up about six billion years ago.
While the observed quickening is real, the nature of dark energy is unknown. Theorists also debate whether it could become stronger or weaker. If it were to strengthen, University of Chicago astrophysicist Michael S. Turner said, it could mean the universe eventually will tear itself apart in a violent "big rip." If it were to weaken, gravity would predominate and cause the cosmos to collapse in a "big crunch."
But if dark energy density is unchanging or changing slowly - as the available data now suggest - it would mean a more gentle expansion as galaxies move farther and farther apart. Within about 100 billion years, Turner said, only a few hundred galaxies would be visible from Earth. "It will be a very lonely place," he said.
But Turner cautioned that dark energy may not be the final answer. It is possible, he said, scientists simply do not understand all there is to know about gravity. It may be, he said, that "the universe is teaching us a lesson about how to go beyond Einstein's theory."
