= Inverse Compton cooling
{title2=$P_e=4\sigma_TcU_\gamma k_B(T_e-T_\gamma)/(m_ec^2)$}
Thermal electrons can transfer energy to lower-energy radiation photons through <Compton scattering>. In the nonrelativistic Thomson regime, the net cooling power per electron is $4\sigma_TcU_\gamma k_B(T_e-T_\gamma)/(m_ec^2)$ for a blackbody photon bath. A sufficiently hot electron population therefore loses energy; a colder population is heated instead. The <Cosmic microwave background> radiation energy density grows as $(1+z)^4$, making this channel more important for high-redshift diffuse ionized gas.
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