Only in a flat universe does space obey all the standard geometry-class rules-the angles of a triangle add up to exactly 180 degrees and parallel lines never meet.

Silk says astronomers need to achieve a 99.9999 per cent level of confidence on the flat universe, high enough that the case startsto look compelling no matter what the starting assumptions are.

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.