Isotropy theorem for cosmological vector fields

TL;DR

This paper proves that rapidly evolving homogeneous vector fields in an expanding universe have an average energy-momentum tensor that is isotropic and behaves like a perfect fluid, regardless of their intrinsic anisotropic evolution.

Contribution

It establishes a general isotropy theorem for the average energy-momentum tensor of homogeneous vector fields in cosmology, applicable to arbitrary potentials and polarization patterns.

Findings

Average energy-momentum tensor is always diagonal and isotropic for bounded, rapid evolution.

For power-law potentials, the equation of state is w=(n-1)/(n+1).

Vector fields can serve as dark matter or dark energy candidates.

Abstract

We consider homogeneous abelian vector fields in an expanding universe. We find a mechanical analogy in which the system behaves as a particle moving in three dimensions under the action of a central potential. In the case of bounded and rapid evolution compared to the rate of expansion, we show by making use of the virial theorem that for arbitrary potential and polarization pattern, the average energy-momentum tensor is always diagonal and isotropic despite the intrinsic anisotropic evolution of the vector field. For simple power-law potentials of the form V=\lambda (A^\mu A_\mu)^n, the average equation of state is found to be w=(n-1)/(n+1). This implies that vector coherent oscillations could act as natural dark matter or dark energy candidates. Finally, we show that under very general conditions, the average energy-momentum tensor of a rapidly evolving bounded vector field in any background geometry is always isotropic and has the perfect fluid form for any locally inertial observer.