The mean density of matter that is required for gravity to halt the expansion of the Universe, equivalent to about 10–29 g/cm3. If the cosmological constant is zero, a universe with a density below the critical density will expand for ever, whereas a universe with a density greater than the critical density will eventually collapse. A universe with exactly the critical density is described by the Einstein–de Sitter model, which lies on the dividing line between these two extremes. The average density of material that can be directly observed in our Universe is less than 1% of the critical value. However, various observations suggest that most of the matter in the Universe is dark matter, which cannot be seen directly but which can be detected through its gravitational effect. Even when the dark matter is added in, however, the average density of the Universe is still only about 30% of the critical density. It therefore seems likely that the Universe will expand for ever, especially because recent observations imply that the cosmological constant is not zero, which will cause the expansion to accelerate.
Subjects: Astronomy and Astrophysics.