critical density

In cosmology, the minimum average density that the universe must have in order for it to stop expanding at some point in the future.

The precise value depends on Hubble's constant and so is not fixed, but it is between approximately 10−29 and 2 × 10−29g/cm3, equivalent to a few hydrogen atoms per cubic metre. The density parameter (symbol Ω) is the ratio of the actual density to the critical density. If Ω < 1, the universe is open and will expand forever. If Ω > 1, the universe is closed and the expansion will eventually halt, to be followed by a contraction (Big Crunch). Current estimates from visible matter in the universe indicate that Ω is about 0.01, well below critical density, but unseen dark matter may be sufficient to raise Ω to somewhere between 0.1 and 2.