I'm currently reading about the Boltzmann equation, used for the early Universe.(adsbygoogle = window.adsbygoogle || []).push({});

The equation I end up with, after some simplifications is the following:

\begin{equation}

a^{-3}\frac{d}{dt}\left(n_1a^3\right) = n_1^{(0)}n_2^{(0)}\langle\sigma v\rangle\left[\frac{n_3 n_4}{n_3^{(0)}n_4^{(0)}} - \frac{n_1 n_2}{n_1^{(0)}n_2^{(0)}}\right]

\end{equation}

My problem is, I'm not actually sure what it tells me? Does it just tell me how species 1 evolves with time, when I'm taking interactions into account, or...?

I mean, it's used for non-equilibrium phenomenon, but does it tell me whether or not some particle is in, or out of equilibrium? Basically, I'm not quite sure what it tells me. So I was hoping someone could give a brief and clear explanation perhaps?

Thanks in advance.

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# Boltzmann equation for the early Universe

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