density parameter

The term density parameter (Ω) is used in cosmology for the ratio of some average universe-wide mass/energy density to the universe's critical density, which is the density that would result in a flat universe, a universe that expanding just barely enough that it will expand forever (analogous to escape velocity being the lowest velocity that will carry something away from a body such as a planet). Ω₀ (and sometimes Ω by itself) indicates the density parameter for the universe's current total density, the 0 subscript indicating zero lookback time (like the 0 subscript of H₀). The (total) density parameter is the sum of sub-parameters for different components of the universe (according to the cosmological model being discussed):

• Ω_M - matter density.
• Ω_R - radiation density.
• Ω_Λ - dark energy density.

Matter density may be broken down as:

• Ω_B - baryonic matter density.
• Ω_D (or Ω_DARK or similar) - dark matter density.

These individual density parameters shift as time passes and did so significantly in the early universe. The total density parameter for a universe at the critical density (Ω_C) would be 1, and observation suggests the universe is at this density or very close.