Particle Production of Vector Fields: Scale Invariance is Attractive

TL;DR

This paper shows that during inflation, a scale-invariant spectrum of vector field perturbations naturally emerges as an attractor, potentially leading to observable anisotropies in the universe's structure.

Contribution

It demonstrates that scale invariance of vector field perturbations during inflation is an attractor solution under general conditions, including for massive and massless fields.

Findings

Scale-invariant spectrum is an attractor during inflation.

Massless or light vector fields can induce anisotropic stress.

Potential for observable statistical anisotropy in curvature perturbations.

Abstract

In a model of an Abelian vector boson with a Maxwell kinetic term and non-negative mass-squared it is demonstrated that, under fairly general conditions during inflation, a scale-invariant spectrum of perturbations for the components of a vector field, massive or not, whose kinetic function (and mass) is modulated by the inflaton field is an attractor solution. If the field is massless, or if it remains light until the end of inflation, this attractor solution also generates anisotropic stress, which can render inflation weakly anisotropic. The above two characteristics of the attractor solution can source (independently or combined together) significant statistical anisotropy in the curvature perturbation, which may well be observable in the near future.