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Omega far from universal Scientists disagree about whether the universe will continue to expand into infinity or someday reverse its course Monday, February 16, 1998 By Byron Spice, Science Editor, Post-Gazette
Reports out of Washington, D.C., last month made it seem as if cosmologists at an American Astronomical Society meeting were speaking with one voice: Prepare for a long, cold ride into the future.
It looks as if the universe, which has been expanding for billions of years, will just keep on expanding into infinity as the stars burn themselves out. Several lines of evidence, they agreed, seemed to be converging on that conclusion.
Not so fast, advises Bob Nichol, a cosmologist at Carnegie Mellon University.
Nichol's research, also presented at the Washington meeting and published on an Internet journal last Friday, suggests the fate of the universe remains an open question.
He and his collaborators say their observations of X-ray-emitting clusters of galaxies show the universe contains enough mass to halt the expansion that began with the Big Bang. Billions of years from now, they say, the pull of gravity should be just strong enough to stop the expansion.
Then again, Nichol admits he may be wrong.
One international study, headed by Saul Perlmutter of Lawrence Berkeley Laboratory in California, studied a series of exploding stars, called supernovae, to calculate how the universe's expansion rate had changed over time. They not only found no sign that the expansion was slowing, but picked up hints that something might somehow be nudging the expansion even faster.
But Perlmutter says it will take another couple years of work to prove his findings are correct.
"I think all of us would agree that we are at the beginning of the end" of efforts to determine the universe's fate by calculating its density, Nichol said. Scientists reduce this density measurement to a single, almost magical number called omega.
"I think we're entering the Decade of Omega."
If omega equals 1, then the universe has just enough mass to stop its expansion. If omega is less than 1, it will expand forever; more than 1, the expansion would actually reverse and result in a Big Crunch.
New telescopes will make observations that should enable everyone to agree on a value of omega within 10 years, he said. For instance, Carnegie Mellon scientists have just installed their Viper telescope at the South Pole, where it will study microwaves emitted by the cold remnants of the Big Bang. This should provide hints about how mass was distributed following the Big Bang and provide constraints on the size of omega.
This spring, the National Aeronautics and Space Administration will launch the Advanced X-ray Astrophysics Facility, an orbiting X-ray telescope. Later, NASA plans to launch an orbiting version of Viper called the Microwave Anisotropy Probe and the European Space Agency will fly its version, called Planck. Together, the satellites should be able to calculate omega with 99 percent accuracy once their results start coming in, perhaps in five years.
In their recent study, Nichol and Daniel Reichart, a doctoral student at the University of Chicago, tried to calculate omega by studying X-ray-emitting galactic clusters. The data came from a couple space-based telescopes, the Roentgen Satellite known as ROSAT and the Einstein X-ray Observatory.
"It's a very simple experiment," Nichol said. First, they started with an existing survey that had found about 200 galactic clusters that are nearby, within a sphere of about 300 million light-years. Then they looked further into space - in essence, back in time - to a point where the universe was perhaps half its present age. There, Nichol and Reichart found just 50 or 60 clusters in a volume of space much larger than that used for the nearby survey.
This difference in cluster density is significant, Nichol said. If the universe is very dense, cosmologists expect gravity to be constantly pulling and tugging at galaxies to create new galactic clusters. If matter is sparsely distributed, however, they expect that galactic clusters formed early in the universe's evolution and then stopped.
So the discovery that the number and density of galactic clusters has increased over a period of billions of years suggests that clusters are still being formed and that the universe is dense. Omega, Nichol said, thus would at least equal 1.
"That's simply wrong," counters Neta Bahcall, an astrophysicist at Princeton University. If Nichol didn't see more clusters in the distant, or more ancient, universe, it may simply be that the telescopes missed them.
That's like looking for ponies in a city, finding none and concluding that the Earth has no ponies, she said.
Bahcall and her colleagues also have studied massive galactic clusters, but came to an opposite conclusion about omega. Like Nichol, she expects that clusters would form over a long period of time if the universe is dense and would form early in a less-dense universe.
It might seem counter-intuitive, she admits. But just as a bump that would be tiny on an elephant's skin would be considered huge on a butterfly wing, variations in the distribution of matter in a lightweight universe would have more immediate effects than variations in a heavyweight universe.
Bahcall's group looked further back in time than Nichol, back to a point where no clusters should have formed if the universe had an omega of 1 or more. As they reported at the Washington meeting, they found several clusters. These clusters, they argued, prove that omega must be less than 1; they contended it's a lot less than 1.
Very powerful stuff, Nichol admitted. But it's not quite the iron-clad argument Bahcall suggests. While Nichol simply counted galactic clusters, Bahcall's group based its findings on calculations of mass.
"How we measure mass is a very tricky thing," and small errors can quickly multiply, Nichol explained. Also, Bahcall studied only a subset of the total volume of space that Nichol studied.
Perlmutter suspects the results of Nichol and of Bahcall are so contradictory that scientists will be able to sort out which one is right in short order. But it may take longer to sort out other discrepancies among cosmologists.
"It's a very unusual time in scientific history," Perlmutter said. "It's the first time that you have data coming in" on the value of omega and no one is yet sure which findings are to be believed.
"Science takes a long time to get the right answer," Nichol said. "It wasn't long ago - and I'm talking five years here - that we didn't have any idea what omega is. I think it's premature to say we know the answer."
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