More evidence for a flat cosmos.

According to inflation, a theory that seeks to explain the origin of structure in the universe, the cosmos underwent an episode of enormous expansion during the first fraction of a second of its existence. Like the surface of a balloon blown up to enormous proportions, any curvature to space-time was stretched out by the expansion (SN: 12/19&26/98, p. 392). In other words, the universe should be flat.

Astronomers have now found an additional hint that the universe indeed has zero curvature. The finding comes from studies of the cosmic microwave background, the whisper of radiation left over from the Big Bang.

Two telescopes at the South Pole have recorded variations in the intensity of the microwave background on different spatial scales. One instrument examined variations on scales of 1/4 to 3 1/2 degrees, the other on scales of 1 to 10 degrees. Inflation predicts that the variations should be greatest in patches of the sky a half a degree across--the size of the full moon--and the combination of data from the two telescopes matches the predicted pattern.

One of the instruments, Python, recently ended its 5-year tour of duty. The other, Viper, began taking measurements last February, and researchers have now finished analyzing data taken during its first few weeks of operation, says Jeffrey B. Peterson of Carnegie Mellon University in Pittsburgh. The Python and Viper measurements "tie together beautifully," he says. Peterson and Kimberly A. Coble of the University of Chicago reported the findings last month in Paris at the Texas Symposium on Relativistic Astrophysics.

"While the data are consistent with a flat universe and inflation, I do not think that the data are strong enough to rule out alternative models," notes David N. Spergel of Princeton University. Over the next year or two, new ground-based and balloon experiments that will scan a larger area of the sky and explore a greater range of wavelengths "will [more] accurately measure the microwave background fluctuations," he says.

Spergel is working on the Microwave Anisotropy Probe, a NASA satellite set for launch in 2000. It "will make the definitive measurements of the background fluctuations," he says.