We can describe the geometry of the universe in terms of the sum of the angles of a triangle drawn on its surface. The "curvature" of space-time is equated with whether the universe is open or closed:
| Density greater than the critical density | positive curvature | closed universe |
| Density equal to the critical density | flat | critical universe |
| Density less than the critical density | negative curvature | open universe |
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