How do gases of light behave in an expanding universe?

Photons and other massless particles follow the Boltzmann equation even in curved, expanding spacetime, and their equilibrium distribution shifts with time as the universe stretches. For a universe expanding as t^q (covering everything from static to decelerated expansion), this work proves that small perturbations away from equilibrium decay and never blow up — a rigorous mathematical guarantee of long-term stability. The result also confirms that near-vacuum states remain stable, shoring up a foundational assumption in relativistic kinetic theory relevant to early-universe cosmology.