On a roll. In a 12-sided universe, objects framed in one pentagonal face appear in its counterpart.

The Universe, Hemmed In?

Most cosmologists think the universe is infinite in all directions. But now scientists in France and the United States argue in this week's Nature that the universe might be finite and, in fact, 12-sided. Unsurprisingly, the hypothesis is already being challenged.

The impetus for the idea is a puzzle that comes from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. In February, the WMAP satellite produced an incredibly detailed picture of the cosmic microwave background--the ubiquitous cold light that the universe gave off in its infancy. The sizes of hot and cold spots in the microwave background revealed the age and the composition of the cosmos; however, on the very largest scales, there weren't as many temperature fluctuations as expected--which might mean that the universe is finite, rather than infinite.

In the current issue of Nature, physicists and mathematicians have shown that a dodecahedral universe in a slightly curved space can also explain the anomaly. (Opposite faces of the dodecahedron are associated; they are actually the same thing, so a spaceship zooming out one side of the universe winds up flying right back in the other.) "You really get a good fit" to WMAP data with a dodecahedron, says Jeffrey Weeks, a mathematician based in Canton, New York, who co-authored the paper.

"The nice thing about their result is that it makes very testable predictions," says Neil Cornish, a physicist at Montana State University in Bozeman. Nevertheless, says Cornish, other data already seem to belie the dodecahedral hypothesis. "We're already ruling it out with high confidence," he says. His team's analysis of the WMAP data, which will be submitted to Physical Review Letters shortly, shows no sign of duplicate features in the sky that would be the hallmark of a finite, periodic universe (Science, 22 June 2001, p. 2237).

Even if the dodecahedral universe falls apart, as seems likely, physicists will still have to explain the puzzling lack of large-scale structure in the WMAP data. Cosmologists hope the anomaly will tell us whether we're bounded in a nutshell or whether we can count ourselves kings of infinite space.

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The Wilkinson Microwave Anisotropy Probe