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 鈥� not about our planet, but about the entire universe.
When it comes to the universe, 鈥渇latness鈥� refers to the fate of light beams travelling large distances parallel to each other. If the universe is 鈥渇lat鈥�, the beams will always remain parallel. Matter, energy and dark energy all produce curvature in space-time, however. If the universe鈥檚 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鈥檚 possible that the WMAP observations have been misinterpreted.
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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 , which can be used to show how the certainty attached to a particular conclusion is affected by different starting assumptions.
Using modern astronomers鈥� 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.
鈥淚t鈥檚 a reasonable assumption that the universe isn鈥檛 entirely flat,鈥� Silk says, adding that the calculation reveals how strongly astronomers鈥� prejudices can affect their conclusions. David Spergel of Princeton University, the spokesman for WMAP, agrees. 鈥淭hey鈥檝e developed a statistically rigorous way of examining the question,鈥� he says.
鈥淭he calculation reveals how strongly astronomers鈥� prejudices can affect their conclusions鈥�
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鈥檚 possible, however, that no measurements will ever be able to get to that level of accuracy.
