Cosmic circles. A simulated pattern of cosmic microwave background radiation in a finite universe sports matching circles of hot and cold spots, outlined in black.

Soccer Ball Universe Out of Bounds

A new analysis of the cosmic microwave background radiation has found no evidence that the universe is shaped like a soccer ball--a controversial idea proposed last fall. Although this doesn't totally rule out the possibility that the universe is small and finite, it does make it significantly less likely.

The overall shape of the universe hinges on whether it is finite or infinite. Most astronomers think it's infinite. But the question drew new life last year, when the Wilkinson Microwave Anisotropy Probe (WMAP) brought back the most detailed measurements yet of the microwave radiation left over from the big bang. The results confirmed that, although cosmic microwaves have many hot and cold spots on small scales, their temperature doesn't fluctuate much over large scales, raising the possibility that a finite universe could be limiting their wavelengths.

In October a group pointed out that the results seemed consistent with a relatively small, finite universe shaped like a soccer ball (ScienceNOW, 8 October). If the universe is finite, light emanating from a point will eventually return to the same point from many directions, like Pac-Man going off the edge of a video game screen and coming back the other side. This wraparound effect would create repeated images of the universe like a hall of mirrors, leading to signals in the microwave data that researchers could identify.

An intensive computer search for such patterns has turned up nothing so far, report cosmologists in an upcoming issue of Physical Review Letters. The group looked for pairs of equally wide circles, centered in opposite sides of the sky, having identical patterns of hot and cold spots along their circumferences. The search was painstaking because they had to check circles of all sizes and centers, and account for the patterns being rotated relative to each other. They argue that if the universe has any of a number of "simple," smoothly curving finite shapes, including a dodecahedron or soccer ball shape, it must be least 10 times bigger than cosmologists thought, says co-author Neil Cornish of Montana State University in Bozeman.

"They did great work," says theoretical physicist Janna Levin of Columbia University in New York City. "This test had to be performed." But the universe could be finite in an infinite number of ways and signs of finiteness would be easy to miss for statistical reasons, she notes, so it's nearly impossible to rule out every option. "It's such a subtle, difficult thing. I think people are going to sit back and think about what would be another convincing test" to rule out even more possible shapes.

Related Sites
Neil Cornish's Web site
Wilkinson Microwave Anisotropy Probe