The picture shows the evolution of a thin representative slice of the universe (4 Mpc/h x 4 Mpc/h comoving) as a function of time (represented as a scale factor a). The evolution starts at z~20 (a~0.05) and stops at z=4 (a=0.2). The respective panels show:

1) upper left: hydrogen neutral fraction, stretch from 10-5 (black) to 1(pink); ionization fronts come out red.

2) lower left: gas density in units of average cosmic density, stretch from 10-1(black) to 102(pink);

3) lower right: gas temperature in K, stretch from 103(black) to 105(pink);

4) evolution of the ionizing intensity J21: x-axis is linear in z, y-axis is logarithmic in J21.

The value of the scale factor a is also shown in the middle bar.