Redshift evolution of ﹩\Delta _{\psi }/ ( \overline{x}_{\mathrm{H}\,}\Delta _{\delta \delta }) ﹩ in the ﹩\zeta =12﹩ (thin lines) and ﹩\zeta =40﹩ (thick lines) models. The curves are for ﹩\overline{x}_{\mathrm{H}\,}=0.96﹩ (dotted), ﹩\overline{x}_{\mathrm{H}\,}=0.8﹩ (short‐dashed), ﹩\overline{x}_{\mathrm{H}\,}=0.5﹩ (long‐dashed), and ﹩\overline{x}_{\mathrm{H}\,}=0.26﹩ (solid). The redshifts in the two models differ.