The theory of general relativity, under which space itself can curve, allows for the universe to take one of three forms: flat like a sheet of paper, closed like a sphere, or open like a saddle. This astronomical geometry is no trivial matter — the fate of the cosmos depends on it.
As Princeton University cosmologist David Spergel puts it, “The shape of the universe tells us about its past and its future." Whether the universe will expand forever or eventually collapse, and whether it’s finite or infinite — all are questions that tie back to its shape.
For a matter that bears on such grand questions, its components are remarkably simple. The ultimate structure of the universe depends on just two factors: its density and its rate of expansion.
