Ultralight vector dark matter, anisotropies, and cosmological adiabatic modes

Abstract

In this work we study the background evolution and the early time behavior of scalar cosmologicalperturbations with an ultralight vector dark matter. We present a model for vector dark matter in ananisotropic Bianchi type I universe. Vector fields source anisotropies in the early Universe characterized bya shear tensor which rapidly decays once the fields start oscillating, making them viable dark mattercandidates. We present the set of equations needed to evolve scalar cosmological perturbations in the linearregime, both in synchronous gauge and Newtonian gauge. We show that the shear tensor has to be takeninto account in the calculation of adiabatic initial conditions.