宇宙的形状象足球：Universe could be football-shaped

Finite cosmos may be smaller than we think.
9 October 2003

If you journeyed far enough in such a Universe, you would end up back where you started.

© GettyImages

The Universe could be shaped like a soccer ball, say mathematicians¹.

The idea is prompted by data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite. This sees back to when the Universe was about 380,000 years old, and reveals the all-pervading radiation left over from the Big Bang - the cosmic microwave background.

There are fluctuations in this background, like waves in the sea. They are the legacy of the small lumps in the early Universe that gave rise to stars and galaxies.

An infinite Universe would contain waves of all sizes. The WMAP did not see any very large waves. This points to space being finite - for the same reasons that you don't see breakers in your bathtub.

The best explanation for these observations is that the cosmos is a Poincaré dodecahedral space, says a team led by Jeffrey Weeks, an independent mathematician based in Canton, New York. Mathematical models of a spherical, solid Universe edged by 12 curved pentagons produce the patterns seen in the background radiation without any special fine-tuning. "It fits the data surprisingly well," says Weeks.

The dodecahedron is "a nice solution", agrees cosmologist Janna Levin of the University of Cambridge, UK. But other geometries could produce similar patterns in the microwave background, she warns. "It's going to be a surprise if the Universe has chosen such a beautiful platonic form," she says. "And I'd be surprised if the Universe was so small."

Most physicists assume that the Universe is infinite, explains Levin. But Einstein's theories actually say nothing about whether the Universe stops or not.

Bouncing back

A journey of 60 billion light years across a dodecahedral Universe would bring you right back to Earth. Like a circumnavigation of the globe, it would be a seamless ride: there would be no obvious point at which one 're-entered' the Universe.

The most distant objects would be visible in opposite directions, although they would be seen at different ages. Trying to spot the same galaxy in two different places "would be like trying to recognize the same person viewed at age 50 face-on, and at the age of 7 from the top of their head, in a crowd of billions," says Weeks.

Astrophysicist Neil Cornish, of Montana State University in Bozeman, is one of a team that is looking for these circles. The researchers will present their latest results at a cosmology conference beginning on Friday in Cleveland, Ohio.

So far, their search has drawn a blank. "There is a little room left for the small-Universe idea, but not much," Cornish says.

Weeks remains optimistic, however. He thinks that the telltale circles might be hiding in parts of the WMAP data that have yet to be analysed.

（From: Nature Science Update）