Did the universe contain enough matter to slow the expansion so much that one day it would stretch as far as it could, stop, and reverse itself, like the trajectory of a tossed ball returning to Earth? In such a universe, space would be finite, curving back on itself, like a globe. Or did the universe contain so little matter that the expansion would never stop but go on and on, like a rocket leaving Earth’s atmosphere? In this kind of universe, space would be infinite, curving away from itself, like a saddle. Or did the universe contain just enough matter to slow the expansion so that it
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