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Imprints of wave patterns support Big Bang theory
Friday, May 25, 2001 By Byron Spice, Science Editor, Post-Gazette
In a spurt of cosmological serendipity that has played out over the past four weeks, independent teams of astrophysicists have produced the equivalent of "before" and "after" snapshots of the universe.
These snapshots aren't anything to look at. Rather, they are statistical portraits of how matter and energy are distributed in the universe. But they are beautiful to a cosmologist's eye, providing confirmation of the theory that the universe began with the Big Bang 14 billion years ago and has been expanding ever since.
The patterns of energy distribution in the early universe, revealed by astronomers at a scientific meeting four weeks ago, are consistent with the distribution of visible matter that now can be seen in the heavens, as is being reported today in the journal Science by astrophysicists at Carnegie Mellon University and the University of Maine.
"We're seeing exactly what we expected to see," said an exultant Christopher Miller, a postdoctoral researcher at Carnegie Mellon and the lead author of today's Science article. "For a lot of us, it's enough proof that we understand the evolution of the universe from 300,000 years [after the Big Bang] on out."
In particular, scientists are seeing the faint, but unmistakable imprint of waves that they had theorized must have been present following the Big Bang. These waves -- so similar to sound waves that they are called acoustic oscillations -- no longer reverberate through the universe. But today's galaxies, galactic clusters and superclusters continue to conform to those wave patterns.
For most of the first 300,000 years following the Big Bang, physicists have speculated, the universe was something of a puddle -- a mass of protons, electrons and photons so hot that they couldn't clump together and form atoms. These particles would have behaved as if they were part of a liquid.
This soup also would have contained clumps of the mysterious material known as dark matter; most of the matter in the universe consists of dark matter. These clumps would have exerted a gravitational pull on the other particles, Miller explained, though another force, called photon pressure, would push the particles away from the clumps. This pushing and pulling eventually would have generated acoustic waves in the soup.
About 300,000 years after the Big Bang, this puddle finally cooled enough to allow the electrons and protons to combine as atoms and the universe would have begun to expand. The photons -- light particles -- also were released, creating an afterglow called the cosmic microwave background.
The acoustic oscillations would have ended once the universe was no longer in its liquid-like state. But those wave patterns would have been preserved at that moment of cooling, both as energy patterns within the cosmic microwave background and in the distribution of matter in the expanding universe.
Four weeks ago, at the American Physical Society meeting in Washington, D.C., three groups of researchers -- two using balloon-borne detectors and one using a special instrument at the South Pole -- announced that they had indeed seen these acoustic oscillations in the faint microwave glow of the early universe.
Miller and his colleagues, meanwhile, were examining whether the visible matter in the universe also carried the imprint of these waves, as predicted. To accomplish that task, they analyzed two large "catalogues" that list the positions of galaxy clusters and one catalogue of galaxies. By calculating the precise distance of more than 1,000 clusters and tens of thousands of galaxies, they were able to show that these structures conformed with the acoustic oscillations.
"It's not like I can go out and point to something and say, well, there's [a ripple] right there," said Robert Nichol, a CMU astrophysicist and Miller's co-author. The density fluctuations are small -- about 1 percent -- and stretched out over vast expanses. To prove the waves exist requires averaging the density of matter over many different volumes. That's why it required combining data from three galactic catalogues.
Also in the past few weeks, astronomers surveying the skies with a telescope in eastern Australia said that they also found the same evidence of acoustic oscillations, though they have not yet formally published their results.
"It's very, very good that they see the same thing we do," Miller said.
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