# Matter Temperature during Recombination

In several papers about Recombination, (for instance Peebles, Astrophysical Journal, vol. 153, eq. 39) it is claimed that the evolution of matter temperature follows the equation $$\frac{dT}{dt} = -2\frac{\dot{a}}{a}T + \frac{2}{3 K_B n} \epsilon$$ where the first term in the right hand side accounts for the expansion of the universe and $\epsilon$ is the energy injected into the gas per second per unit physical volume. How can I derive this equation? I suppose that this equation can be derived applying the First Principle of Thermodynamics to a comoving volume in the expanding universe or maybe taking the second moment of a Boltzmann equation. Any help or reference in which this equation is discussed would be very helpful.

• Are you talking about this paper? If so, the equation you reference in your question is not (as far as I can tell) the same equation as in the paper. What's more, that paper pretty much derives the equation throughout. Sep 21 '18 at 14:47
• The difference between the equation I wrote and the one in the paper is that in the latter what I called $\epsilon$ is explicitly calculated for Compton Scattering. Anyway I can't see where this equation is derived. Sep 21 '18 at 19:28
• I've voted for this question to be reopened - see meta discussion Sep 28 '18 at 22:17
• I found the answer anyway. The equation can be derived using the first principle of thermodynamics to a comoving volume and the equation of state for a perfect gas. Sep 29 '18 at 7:32