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© Garry Wade/Stone/GettyToday we dismiss flat-Earthers as ignorant, yet we may be making an almost identical mistake - not about our planet, but about the entire universe.
For centuries the ancients believed the Earth was flat. Evidence to the contrary was either ignored or effortlessly integrated into the dominant world view. Today we dismiss flat-Earthers as ignorant, yet we may be making an almost identical mistake - not about our planet, but about the entire universe.

When it comes to the universe, "flatness" refers to the fate of light beams travelling large distances parallel to each other. If the universe is "flat", the beams will always remain parallel. Matter, energy and dark energy all produce curvature in space-time, however. If the universe's space-time is positively curved, like the surface of a sphere, parallel beams would come together. In a negatively curved, saddle-shaped universe, parallel beams would diverge.

Thanks in part to the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, which revealed the density of matter and dark energy in the early universe, most astronomers are confident that the universe is flat. But that view is now being questioned by Joseph Silk at the University of Oxford and colleagues, who say it's possible that the WMAP observations have been misinterpreted.

In a paper accepted for publication in Monthly Notices of the Royal Astronomical Society, they took data from WMAP and other cosmology experiments and analysed it using Bayes's theorem, which can be used to show how the certainty attached to a particular conclusion is affected by different starting assumptions.

Using modern astronomers' assumptions, which presuppose a flat universe, they calculated the probability that the universe was in one of three states: flat, positively curved or negatively curved. This produced a 98 per cent probability that the universe is indeed flat. When they reran the calculation starting from a more open-minded position, however, the probability changed to 67 per cent, making a flat universe far less of a certainty than astronomers generally conclude.

"It's a reasonable assumption that the universe isn't entirely flat," Silk says, adding that the calculation reveals how strongly astronomers' prejudices can affect their conclusions. David Spergel of Princeton University, the spokesman for WMAP, agrees. "They've developed a statistically rigorous way of examining the question," he says.

Silk says astronomers need to achieve a 99.9999 per cent level of confidence on the flat universe, high enough that the case starts to look compelling no matter what the starting assumptions are. It's possible, however, that no measurements will ever be able to get to that level of accuracy.