A perplexing study suggesting that the early universe was so rarefied it could barely contain the amount of ordinary matter astronomers observe today appears to have bitten the dust. A new analysis of the amount of deuterium made in the big bang may have dispelled this puzzle and opened the way for a universe that contains extra, unseen ordinary matter, as many astronomers suspect.
Astronomer Craig Hogan of the University of Washington, Seattle, and his then-graduate student Martin Rugers had analyzed a gas cloud so far away that it likely contains material fresh out of the big bang. Their conclusion, published in the 1 March 1996 issue of Astrophysical Journal Letters, was that the cloud holds about one deuterium atom for every 5000 hydrogen atoms. That was a startlingly high value, because the more of this hydrogen isotope emerged from the big bang, the lower the universe's total density of other ordinary matter must be.
But now a team led by David Tytler of the University of California, San Diego, has obtained "clearly superior" data on the same cloud and failed to find the clues that had led Hogan and Rugers to their conclusion, Hogan announced recently at the 18th Texas Symposium on Relativistic Astrophysics. "What we thought was a smoking gun of ... [high] deuterium is not there," Hogan says.
Both teams took their measurements at the 10-meter Keck Telescope on Mauna Kea, in Hawaii. Its light-gathering power allows astronomers to record the "shadows" cast by gas clouds billions of light-years away in the light of brilliant quasars at even greater distances. How much light the clouds absorb, and at which wavelengths, holds clues to their composition. But deuterium and the far more abundant hydrogen in the clouds have such similar spectral signatures that disentangling the two is a delicate, not to say risky, task.
"The case for high deuterium is basically gone," says Limin Lu, an astronomer at Caltech who has made similar measurements and who heard talks by Tytler and Hogan at the symposium. That may remove a puzzling discrepancy with values nearly 10 times lower, which Tytler and colleagues had measured in two other gas clouds. But Hogan's concession isn't the last word, because some astronomers maintain that Tytler's own measurements are not airtight.