In principle, it should be easy to decide whether the universe will end in eternal night or singular noon, in the infinitely thin dispersal of its substance or in the re-creation of the flash of the Big Bang. If the present average density of the universe is greater than three protons for every thousand liters of space, then gravity will eventually prevail and the universe will collapse back toward its beginning. On the other hand, if the average density of the present universe is less than this critical value, the expansion will continue on forever.

(Chet Raymo, The Soul of the Night, p194)