The Homogeneous Scalar Field and the Wet Dark Sides of the Universe

Abstract

We study the possibility that a generalised real scalar field minimally coupled to gravity could explain both the galactic and the cosmological dark components of the universe. Within the framework of Einstein's Relativity we model static galactic halos by considering the most general action built from the scalar field and its first derivatives. Although the gravitational configuration is static, the scalar field may be either static, or homogeneous and linear in time. In the case of the static scalar field, the models we look at inevitably posses unphysical negative energies, and we are led to a sort of no-go result. In the case of the homogeneous scalar field, on the contrary, we find that compact objects with flat rotational curves and with the mass and the size of a typical galaxy can be successfully modeled and the Tully-Fisher relation recovered. We further show that the homogeneous scalar field deduced from the galactic halo spacetimes has an action compatible with the kinetic Unified Dark Matter models recently proposed by Scherrer. Therefore, such a homogeneous kinetic Unified Dark Matter not only may correctly mimic the galactic dynamics, but could also be used to model the present-day accelerated expansion in the universe.