| critical density | has definition The cosmic density of matter required to "close" the universe and so, eventually to halt cosmic expansion. Its value amounts to about ten hydrogen atoms per cubic meter of space. The observed density is so close to the critical value that the question of whether the universe is open or closed has not yet been resolved by observation. See open universe, closed universe. |  |
| has definition The density that just stops the expansion of space, after infinite cosmic time has elapsed. In the standard models, the critical density requires that the spatial geometry be flat. | |
| has definition The value of average cosmic mass density above which the universe is closed. The average mass density of the universe is obtained by measuring the mass in a very large volume of space, including many galaxies, and dividing by the size of the volume. The critical mass density is determined by the current rate of expansion of the universe. According to estimates of the current rate of expansion, the current critical mass density is about 10-29 grams per cubic centimeter. According to the best measurements, the average mass density of our universe appears to be about one tenth the critical mass density. (See closed universe; omega; open universe.) | |
| has definition If the cosmological constant is assumed to vanish, then the critical mass density is that density which puts the universe just on the border between eternal expansion (open universe) and eventual collapse (closed universe). | |
| is a kind of density | |
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