Recombinationaikaa

Recombinationaikaa (rekombinointiaika) refers to the period in the early universe when free electrons combined with protons to form neutral hydrogen. This process made the cosmos transparent to photons, allowing matter and radiation to decouple and giving rise to the cosmic microwave background (CMB). The epoch occurred roughly 380 000 years after the Big Bang, at a redshift of about z ≈ 1100 and a temperature near 3000 kelvin.

Rekombinationaika varioi over a range of redshifts, roughly from z ≈ 1600 to z ≈ 1100, and it was not instantaneous. In the hot, dense early universe ionization equilibrium kept a sizable fraction of electrons free, as described by the Saha equation. As expansion cooled the gas, recombination could proceed faster than ionization, and electrons increasingly bound with protons to form neutral hydrogen. Non-equilibrium effects and the presence of high-energy photons slow the completion of recombination. Modern models use Peebles-type equations and more detailed numerical codes (RECFAST, HyRec, CosmoRec) to describe the process accurately.

The culmination of recombinationaikaa is the decoupling of photons, rendering the universe optically thin to Thomson scattering. The resulting last scattering surface imprints the temperature and polarization anisotropies observed in the CMB. The ionization history influences the amplitude and positions of acoustic peaks, as well as the damping tail and polarization signals, making precise recombination modelling essential for cosmological parameter estimation.

After recombination, the universe entered a period known as the dark ages, before the formation of the first stars and subsequent reionization.