A Big Crunch–fated universe is called a “closed” universe, because two parallel laser-cannon beams would eventually bend toward each other—it’s the same kind of thing that happens to lines of longitude on a globe. What’s happening in the cosmic case is that there’s so much matter in a closed universe that all of space is curved inward. A perfectly balanced universe is “flat” because the beams would just stay parallel forever, in much the same way two parallel lines would stay parallel on a flat sheet of paper. A universe with way more expansion than gravity is called an “open” universe, and in
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