During the latter half of the 20th century, cosmologists narrowed the universe’s fate to three possibilities, and they all depend on its density. If the density were high enough, then gravity would eventually slow the expansion of the universe and cause it to re-collapse in a “Big Crunch.” On the opposite side of the spectrum, a low density would allow the the universe to expand forever. Everything would gradually dim, cool, and spread out in a fate known as the “Big Freeze.” But if the density were just right, then the universe’s expansion would very, very gradually slow down, coming to a complete stop only after an infinite amount of time. This third picture is known as a “flat” universe, and would also end in a Big Freeze.

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