In this talk, we will consider the massless Boltzmann equation on FLRW spacetimes with spatial topology $\mathbb{T}^3$ in the linear and decelerated expanding regimes, where the scale factor is $t^{q}$ with $q\in (0,1]$. In these backgrounds, the massless Boltzmann equation admits a non-stationary Maxwell-Jüttner equilibrium that maximises the entropy of the system. In this work, we show this equilibrium is non-linearly stable for the massless Boltzmann equation on these FLRW spacetimes in the case of hard ball interaction. We will further show a dissipation effect that occurs when the expansion rate is slow enough. This is joint work with Robert M. Strain (University of Pennsylvania) and Martin Taylor (Imperial College London).