So in our GR lecture, we studied the $\Lambda$CMB model. And we neglected interactions between different contents of the universe and we found the following: The universe was first dominated by radiation, then matter, and later on cosmological constant.
Now the assumption of neglecting interactions seems reasonable now, but as we go back in time it is not and that's why we refer to statistical mechanics (as is mentioned in the text below, emphasis by me).
What I don't understand is that if the universe reached a thermal equilibrium in the radiation dominated epoch, why isn't it anymore?
With the knowledge we have so far, we can extrapolate back into the history of the universe until the temperature becomes high enough that interactions allow significant interchanges among the energy components of the universe.Probing back further would require assumptions about the particle interactions and the nature of physical laws themselves. Nevertheless, this apparent drawback, when taken to the extremity, turns into an advantage. As particles interact more strongly and rapidly, they are more likely to achieve thermal equilibrium. Thanks to that, we can use the powerful concept of thermal equilibrium, which allows us to describe a huge system, i.e. the universe, with only a few thermodynamical parameters such as temperature and chemical potentials. It is remarkable how much further we can go when armed with statistical mechanics.