mean density of matter
mean density of matterSymbol: ρ0. The factor that determines the dynamical behavior of the Universe, i.e. whether it is open or closed. The density parameter, ω, is a dimensionless quantity given by
It is related to the deceleration parameter, q 0, by
If the Universe is a continuously expanding open system, ω must be ≤1; if it exceeds unity the Universe is closed and must eventually collapse. The critical density for which ω is unity is 5 × 10–27 kg m–3 for a Hubble constant H 0 equal to 55 km s–1 megaparsec–1. Various estimates involving the mass to luminosity ratios of galaxies lead to values for ω of about 0.01 due to galaxies. This is considerably less than the baryon density parameter, the difference being due to the intergalactic medium. The missing-mass problem for clusters of galaxies indicates the presence of dark matter and suggests that the true value is about 0.3. A value of unity is not precluded by these observations; many theoretical cosmologists believe that ω = 1 precisely, because any deviation from unity in the early Universe would have been immensely amplified in the proposed inflationary phase (see inflationary Universe).