Friedmann cosmological evolution
Homogeneity and isotropy reduce gravitational expansion to two balances.
Key knowledge
For a homogeneous isotropic perfect-fluid universe, H = ȧ/a, H² = 8πGρ_tot/3 − kc²/a² and ä/a = −4πG(ρ_tot + 3p_tot/c²)/3, with ρ_tot a mass-equivalent density.
Values and equations
H = ȧ/a; H² = 8πGρ_tot/3 − kc²/a²; ä/a = −4πG(ρ_tot + 3p_tot/c²)/3
- H
- Hubble expansion rate s⁻¹
- a
- Length-valued scale factor m
- ρ_tot
- Total mass-equivalent energy density kg m⁻³
- p_tot
- Total pressure Pa
- k
- Normalized spatial curvature sign
- G
- Gravitational constant m³ kg⁻¹ s⁻²
- c
- Vacuum light speed m s⁻¹
- t
- Cosmic proper time used for dots s
Conditions and scope
- Cosmic proper time, scale factor a > 0 in length units and normalized k = −1,0,+1.
- Total density and pressure include any vacuum component; no additional Λ term is added here. Local inhomogeneity and quantum regimes require other treatment.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Equations 8.30–8.37: vacuum fluid, acceleration, expansion and H; source c = 1 restored with mass-equivalent density.
2026-10-10 · Classical FLRW background relations only; historical observational values in the 1997 notes are excluded.
Entry details
Permanent link to this entry ↗- Record identity
- physics-friedmann-equations
- Revision
- university-reference.1
- Structured reference record
- University reference