The ratio of the actual mean density of matter in the Universe to the critical density required for the Universe to collapse. In the absence of a cosmological constant, a universe with Ω less than 1 will expand for ever, whereas a universe with Ω greater than 1 will eventually collapse. Current observations imply that the density parameter is about 0.3. The Universe will therefore expand for ever. The observations also imply that the Universe contains something like a cosmological constant, which is causing this expansion to accelerate (see Cosmology).
Subjects: Astronomy and Astrophysics.