Microwave Ripples Reveal a Flat Universe

PARIS--Astrophysicists may have the best indication yet that the cosmos contains the full complement of matter and energy. Observations of the big bang's faint afterglow, made by two microwave telescopes at the South Pole and announced here today at a conference called--despite the location--the 19th Texas Symposium on Relativistic Astrophysics and Cosmology, should reassure theorists that their favorite theory of how the big bang got started, called inflation, could be correct.

The ripples in the cosmic background radiation (CMB) record slight irregularities in the distribution of the early universe's matter. The apparent size of these waves reflects the geometry of the universe: If the ripples are most common at a size of about 1 degree on the sky, the universe is "flat," which means that if made only of matter, it should eventually stop expanding. But if the universe's empty space is stocked with a hypothetical type of energy (called the cosmological constant), expansion could even be speeding up--just as recent measurements of distant exploding stars suggest. Shape is crucial to astronomers, because a flat universe is a key prediction of the simplest version of the inflation theory.

Tentative hints of a flat universe had come from measurements taken by balloons launched near Saskatoon, Canada, and from the Cambridge Anisotropy Telescope in the United Kingdom. Now more details have come from South Pole experiments, called Python and Viper, which take advantage of the thin, dry air at the South Pole for a clear view of the CMB. Their findings, presented in talks this afternoon, reveal a peak in ripples of the telltale 1-degree scale. "It's a very nice observation," says Neta Bahcall of Princeton University. "It's very suggestive."