From Nature:
The doughnut is making a comeback – at least as a possible shape for our Universe.
The idea that the universe is finite and relatively small, rather than infinitely large, first became popular in 2003, when cosmologists noticed unexpected patterns in the cosmic microwave background (CMB) – the relic radiation left behind by the Big Bang. The CMB is made up of hot and cold spots that represent ripples in the density of the infant Universe, like waves in the sea. An infinite Universe should contain waves of all sizes, but cosmologists were surprised to find that longer wavelengths were missing from measurements of the CMB made by NASA’s Wilkinson Microwave Anisotropy Probe.
One explanation for the missing waves was that the universe is finite. “You can think of the Universe as a musical instrument – it cannot sustain vibrations that have a wavelength that is bigger than the length of the instrument itself,” explains Frank Steiner, a physicist at Ulm University in Germany. Cosmologists have suggested various ‘wrap-around’ shapes for the Universe: it might be shaped like a football or even a weird ‘doughnut’. In each case, the Universe would appear to be infinite, because you would never physically reach its edge – if you travelled far enough in any direction you would end up back where you started, just as if you were circumnavigating the globe.
More here.