= Temperature-density relation of photoionized intergalactic gas
{title2=$T=T_0(1+\delta)^{\gamma-1}$}
Diffuse gas in <photoionization equilibrium> is often approximately described by the displayed <temperature>-<density contrast> relation. Photoheating and expansion cooling produce its thermal history. It is an empirical model for low-density gas, not the adiabatic equation of state of every intergalactic phase. <Lyman-alpha forest> line widths constrain both the temperature at a reference density and the slope through their lower envelope.
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