Prepared for submission to JCAP Cosmological Perturbation Theory for streams of relativistic particles

Abstract. Motion equations describing streams of relativistic particles and their properties are explored in detail in the framework of Cosmological Perturbation Theory. Those equa-tions, derived in any metric both in the linear and nonlinear regimes, express the matter and momentum conservation. In this context we extend the setup of adiabatic initial condi-tions- that was initially performed in the Conformal Newtonian gauge- to the Synchronous gauge. The subhorizon limit of the nonlinear motion equations written in a generic perturbed Friedmann-Lemaître metric is then derived and analyzed. We show in particular that the momentum field Pi(x) is always potential in the linear regime and remains so at subhorizon scales in the nonlinear regime. Finally the equivalence principle is exploited to highlight in-variance properties satisfied by such a system of equations, extending that known for streams of non-relativistic particles, namely the extended Galilean invariance. ar X i