Within standard Big Bang cosmology essentially only three futures are available for the physical universe. The universe is either open, which means it will continue to expand at an ever increasing rate; or it is flat, which means it will only expand at a rate just sufficient to avoid collapse; or it is closed, which means the universe will expand to a maximum size and then collapse in upon itself. The total mass-density of the universe determines which scenario will be realized. At a critical mass-density, the universe is flat. If the mass-density is higher than the critical level, the universe is closed. Certain astronomical measurements suggest that the universe is very nearly flat, and yet estimates of mass-density are far below the critical level. This has led scientists to suspect that there is a great deal of matter as yet undetected. Whether there is enough of this unseen dark matter to cause the recollapse of the universe is a still unresolved question.
See also Big Bang Theory; Big Crunch Theory; Cosmology, Physical Aspects
"Closed Universe." Encyclopedia of Science and Religion. . Encyclopedia.com. (August 22, 2017). http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/closed-universe
"Closed Universe." Encyclopedia of Science and Religion. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/closed-universe
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