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).

单词	mean density of matter
释义	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).
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