| closed universe |
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A model of the universe in which the amount of matter present is sufficient to halt its expansion and start it contracting at some finite time far in the future. In all Big Bang models, the only mechanism by which the expansion of the universe currently observed may be slowed down is the mutual gravitational attraction of all the material in it. The universe is closed if the total amount of matter is great enough to reverse the expansion. Present observations show that there is insufficient luminous matter for closure. We do not know whether non-luminous matter, for example in the form of elementary particles such as neutrinos, might bring the density up to that required for closure. Hypothetical, additional but unseen matter that may exist is termed missing mass. See also: dark matter. |