An Etymological Dictionary of Astronomy and Astrophysics

One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent.

1. For matter:
   Ω_m = 8πGρ_m/(3H²), where G is the → gravitational constant, ρ_m is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ω_m = ρ_m/ρ_crit, where ρ_crit is the → critical density.
   

2. For radiation: Ω_r = 8πGρ_r/(3H²), where ρ_r is the radiation equivalent of matter density.
   This parameter is also expressed as Ω_r = ρ_r/ρ_crit.
   

3. For the → cosmological constant: Ω_Λ = Λc²/(3H²). Similarly, Ω_Λ = ρ_Λ/ρ_crit, where &rho_Λ = Λc²/(8πG)
   is sometimes referred to as the density of → dark energy.
   

4. For the → curvature of space-timeΩ_k = -kc²/(R²H²), where k is the → curvature constant and R the → cosmic scale factor.
   

Note that: Ω_m + Ω_r + Ω_Λ + Ω_k = 1, and Ω_total = Ω_m + Ω_r + Ω_Λ = 1 - Ω_k.

See also: → density; → parameter.