Cosmological Perturbation Theory for streams of relativistic particles

Motion equations describing streams of relativistic particles and their properties are explored in detail in the framework of Cosmological Perturbation Theory. Those equations, 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 conditions - 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-Lema^{i}tre metric is then derived and analyzed. We show in particular that the momentum field $P_i(x)$ 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 invariance properties satisfied by such a system of equations, extending that known for streams of non-relativistic particles, namely the extended Galilean invariance.