2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/186535Perturbations in the ionization fraction after recombination affect the Compton cooling of density perturbations. Once the gas temperature starts to decouple from the CMB temperature, ionization fraction perturbations can have a significant influence on the subsequent gas temperature perturbation evolution. This directly affects the 21cm 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 >~ 100 where numerical simulations are typically started.5 pages; for more details of effect on 21cm see astro-ph/0702600; code available at http://camb.info/sources/Astrophysicstext