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In three dimensions there are 10 finite closed flat 3-manifolds, of which 6 are orientable and 4 are non-orientable. The most familiar is the [[Torus#The n-dimensional torus|3-Torus]].
===Spherical universe===
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One of the endeavors in the analysis of data from the [[WMAP|Wilkinson Microwave Anisotropy Probe (WMAP)]] is to detect multiple "back-to-back" images of the distant universe in the cosmic microwave background radiation. Assuming the light has enough time since its origin to travel around a bounded universe, multiple images may be observed. Current results and analysis do not rule out a bounded global geometry (i.e. a closed universe), but they do confirm that the spatial curvature is small, just as the spatial curvature of the surface of the [[Earth]] is small compared to a horizon of a thousand [[kilometer]]s or so. If the universe is bounded this does not imply anything about the sign{{Fact|date=October 2007}} or zeroness of the curvature.
Based on analyses of the WMAP data, cosmologists during 2004-2006 focused on the
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