Christopher M. Hirata developed the HyRec calculation of cosmological recombination with Yacine Ali-Haïmoud. Their method couples atomic level populations, the free electron fraction and radiative transfer, including two-photon transitions and diffusion near the Lyman-alpha line.[ref]
The scientific need is predictive accuracy: the free electron population sets the Thomson scattering history that shapes Cosmic microwave background anisotropy. Equilibrium ionization alone cannot capture the entire decoupling process.
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