%0 Journal Article %T Gauge invariant Boltzmann equation and the fluid limit %A Cyril Pitrou %J Physics %D 2007 %I arXiv %R 10.1088/0264-9381/24/24/001 %X This 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. %U http://arxiv.org/abs/0706.4383v3