Linear effects of perturbed recombination

Perturbations in the ionization fraction after recombination affect the Compton cooling of density perturbations. Once the gas temperature starts to decouple from the cosmic microwave background (CMB) temperature, ionization fraction perturbations can have a significant influence on the subsequent gas temperature perturbation evolution. This directly affects the 21 cm spin temperature of the gas, and also modifies the small-scale baryon perturbation evolution via the difference in baryon pressure. The effect on the gas temperature perturbations can be significant on all scales, and galactic-scale baryon perturbations are modified at the percent level at redshifts z > or approx. 100 where numerical simulations are typically started