Spin temperature distribution for both models at various redshifts. For each snapshot, the panels show the number of simulation voxels whose spin temperature falls within a given bin, N, as a function of the spin temperature. The spin temperature values corresponding to three overdensities, Δ_b = 1, 10, 50, are highlighted in red, orange and blue. The bottom-right panels display the evolution of the mean spin temperature with redshift together with its standard deviation.