# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 1 Search : density parameter
 density parameter   پارامون ِ چگالی   pârâmun-e cagâliFr.: paramètre de densité   One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent. 1) For matter: Ωm = 8πGρm/(3H2), where G is the → gravitational constant, ρm is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ωm = ρm/ρcrit, where ρcrit is the → critical density. 2) For radiation: Ωr = 8πGρr/(3H2), where ρr is the radiation equivalent of matter density. This parameter is also expressed as Ωr = ρr/ρcrit. 3) For the → cosmological constant: ΩΛ = Λc2/(3H2). Similarly, ΩΛ = ρΛ/ρcrit, where &rhoΛ = Λc2/(8πG) is sometimes referred to as the density of → dark energy. 4) For the → curvature of space-timeΩk = -kc2/(R2H2), where k is the → curvature constant and R the → cosmic scale factor. Note that: Ωm + Ωr + ΩΛ + Ωk = 1, and Ωtotal = Ωm + Ωr + ΩΛ = 1 - Ωk.→ density; → parameter.