Evolution of cooling and heating components of the gas temperature as a function of redshift, in the homogeneous expanding universe scenario. All the components are described in terms of energy density per second. Ψ_adia arises from the expansion of the universe. As it is negative (cooling component), we display it as an absolute value. Ψ_com is due to Compton coupling between matter and radiation. ${\mathrm{\Psi }}_{\mathrm{mol}}={\mathrm{\Psi }}_{{\mathrm{H}}_2}+{\mathrm{\Psi }}_{\mathrm{HD}}$ corresponds to the thermal contribution of molecules H₂ and HD.