پارامون ِ چگالی pârâmun-e cagâli
*Fr.: paramètre de densité*
One of the four terms that describe an arranged version of the
→ *Friedmann equation*s. They are all time dependent.
1) For matter:
Ω_{m} = 8π*G*ρ_{m}/(3*H*^{2}), 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} = 8π*G*ρ_{r}/(3*H*^{2}),
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} = Λ*c*^{2}/(3*H*^{2}).
Similarly, Ω_{Λ} = ρ_{Λ}/ρ_{crit},
where *&rho*_{Λ} = Λc^{2}/(8πG)
is sometimes referred to as the density of → *dark energy*.
4) For the → *curvature of space-time*Ω_{k} = -*kc*^{2}/(*R*^{2}*H*^{2}),
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}. → *density*; → *parameter*. |