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In cosmology, the minimum density of matter that would ensure that the universe could not expand for ever. The observed expansion can be reversed by gravity only if the density is sufficiently high. The critical density is defined as the density that would ensure that both the deceleration and the velocity of expansion would become zero at the same time. Its value is between 10-29 and 2 × 10-29 g/cm3, about ten times larger than the density inferred from visible matter, such as stars and galaxies. For philosophical and aesthetic reasons many cosmologists like to imagine that the universe is closed. If it is, the amount of so-called missing mass is considerable. Dark matter in the form of particles other than baryons - such as neutrinos - would be required in order to match the actual density to the critical density. See also: closed universe, expanding universe, Big Bang. |