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 forever, whereas a universe with Ω‎ greater than 1 will eventually collapse. Current observations imply that the density parameter is 0.268. The Universe will therefore expand forever. The observations also imply that the Universe contains something like a cosmological constant, with a density parameter of 0.714, which is causing this expansion to accelerate (see Cosmology).