2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192977This article investigates the collisionless Boltzmann equation up to second order in the cosmological perturbations. It describes the gauge dependence of the distribution function and the construction of a gauge invariant distribution function and brightness, and then derives the gauge invariant fluid limit.36 pagesGeneral Relativity and Quantum CosmologyAstrophysicstext