mean density of matter

mean density of matter

Symbol: 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

ω = 8πG ρ₀ /(3H ₀ ²)

It is related to the deceleration parameter, q ₀, by

ω = 2q ₀

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).

Collins Dictionary of Astronomy © Market House Books Ltd, 2006