Recombination, last scattering and reionization
Atomic recombination binds many previously free electrons and lowers photon-scattering opacity.
Key knowledge
Atomic recombination binds many previously free electrons and lowers photon-scattering opacity. Photon last scattering describes the probabilistic final scattering along an observed path; it is related to recombination but is not an instantaneous atomic event. Later ultraviolet sources reionize intergalactic gas, creating free electrons that can scatter some CMB photons again. For homogeneous nonrelativistic Thomson scattering, optical depth from time t to now is τ(t)=∫ₜᵗ⁰ cσ_T nₑ(t′)dt′, with no-further-scattering probability e^(−τ).
Values and equations
τ(t)=∫ₜᵗ⁰ cσ_T nₑ(t′)dt′; P_no_scattering=e^(−τ)
- τ,P_no_scattering
- Thomson optical depth and conditional survival probability 1
- t,t′,t⁰
- emission/path epoch, integration time and present cosmic time s
- nₑ
- proper number density of free electrons m⁻³
- σ_T
- Thomson scattering cross section m²
- c
- vacuum light speed m s⁻¹
Conditions and scope
- Use proper free-electron density, cosmic proper time, low photon energy in the electron rest frame and the specified background/path. Recombination and the last-scattering distribution have finite widths; an equilibrium Saha ratio alone does not solve their kinetics.
- Ionization fraction, spatial inhomogeneity and source history require modelling. CMB polarization constrains integrated scattering without uniquely determining every reionization source or time profile; no current fitted epoch or cosmological parameter value is supplied.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read PDG recombination/last-scattering body, optical-depth equation29.4 and finite-thickness paragraph, and ESA's actual reionization mechanism. Wrote the positive time integral with explicit c in SI, avoiding the source's natural-unit/redshift-sign shorthand. The exponential survival factor is the disclosed optical-depth probability model. Excluded dated parameter/epoch estimates and universal absence of all residual electrons.
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
Permanent link to this entry ↗- Record identity
- uni-cosmos-recombination-scattering-history
- Revision
- core.1
- Structured reference record
- University core expansion