The Vector Curvaton

TL;DR

This paper explores a vector curvaton model with a time-varying mass and kinetic function during inflation, showing it can produce scale-invariant, statistically anisotropic primordial perturbations consistent with observations.

Contribution

It introduces a vector curvaton with a non-canonical kinetic term that generates scale-invariant, statistically anisotropic curvature perturbations during inflation.

Findings

Spectra of vector perturbations are scale-invariant on superhorizon scales.

The model produces statistically anisotropic curvature perturbations.

An increasing vector field mass can lead to isotropic particle production.

Abstract

We analyze a massive vector field with a non-canonical kinetic term in the action, minimally coupled to gravity, where the mass and kinetic function of the vector field vary as functions of time during inflation. The vector field is introduced following the same idea of a scalar curvaton, which must not affect the inflationary dynamics since its energy density during inflation is negligible compared to the total energy density in the Universe. Using this hypothesis, the vector curvaton will be solely responsible for generating the primordial curvature perturbation \zeta. We have found that the spectra of the vector field perturbations are scale-invariant in superhorizon scales due to the suitable choice of the time dependence of the kinetic function and the effective mass during inflation. The preferred direction, generated by the vector field, makes the spectrum of \zeta depend on the wavevector, i.e. there exists statistical anisotropy in \zeta. This is discussed principally in the case where the mass of the vector field increases with time during inflation, where it is possible to find a heavy field (M >> H) at the end of inflation, making the particle production be practically isotropic; thus, the longitudinal and transverse spectra are nearly the same order which in turn causes that the statistical anisotropy generated by the vector field is within the observational bounds.