Distances and lookback time in FLRW cosmology
In an expanding FLRW model with present scale factor one, the line-of-sight comoving distance is D_C=c∫₀ᶻ dz′/H(z′), and the lookback time is t_L=∫₀ᶻ dz′/[(1+z′)H(z′)].
Key knowledge
In an expanding FLRW model with present scale factor one, the line-of-sight comoving distance is D_C=c∫₀ᶻ dz′/H(z′), and the lookback time is t_L=∫₀ᶻ dz′/[(1+z′)H(z′)]. Spatial curvature converts D_C to transverse comoving distance D_M through a sine, identity or hyperbolic sine. These definitions refer to different geometric or temporal observables.
Values and equations
D_C=c∫₀ᶻ dz′/H(z′); t_L=∫₀ᶻ dz′/[(1+z′)H(z′)]; D_H=c/H₀
- D_C,D_H
- line-of-sight comoving and Hubble distances m
- t_L
- lookback time s
- H(z),H₀
- expansion rate at redshift z and today s⁻¹
- z,z′
- cosmological redshift and integration variable 1
- c
- speed of light m s⁻¹
D_M = D_H sinh(√Ω_k0 D_C/D_H)/√Ω_k0 if Ω_k0>0; D_C if Ω_k0=0; D_H sin(√|Ω_k0| D_C/D_H)/√|Ω_k0| if Ω_k0<0
- D_M,D_C,D_H
- transverse comoving, line-of-sight comoving and Hubble distances m
- Ω_k0
- present curvature density parameter with the stated convention 1
Conditions and scope
- Cosmological redshift z, a consistent expanding background with H(z)>0 and convergent integrals; peculiar velocities and local gravitational shifts require separate treatment.
- D_H=c/H₀ and Ω_k0=−kc²/(H₀²R₀²), with k=+1 for closed spatial curvature and k=−1 for open curvature. No numerical cosmological fit or inference that c/H is the universe's age or particle horizon is made.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read the line-of-sight and transverse distance integrals and lookback-time expression, with their curvature convention. Excluded the source's inconsistent prose shorthand for da/dz, dated parameter preferences and ambiguous uses of proper distance.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
Entry details
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