Inflationary buildup of a vector field condensate and its cosmological consequences

TL;DR

This paper investigates the stochastic evolution of a vector field condensate during inflation, providing a physically motivated estimate of its magnitude and implications for cosmological signals like statistical anisotropy.

Contribution

It offers a new, physically grounded prediction for the vector condensate's magnitude during inflation, improving upon previous free parameter assumptions.

Findings

Estimate of vector condensate magnitude during inflation

Application to vector curvaton mechanism and observational constraints

Analysis of various scenarios including supergravity and parity violation

Abstract

Light vector fields during inflation obtain a superhorizon perturbation spectrum when their conformal invariance is appropriately broken. Such perturbations, by means of some suitable mechanism (e.g. the vector curvaton mechanism), can contribute to the curvature perturbation in the Universe and produce characteristic signals, such as statistical anisotropy, on the microwave sky, most recently surveyed by the Planck satellite mission. The magnitude of such characteristic features crucially depends on the magnitude of the vector condensate generated during inflation. However, the expectation value of this condensate has so-far been taken as a free parameter, lacking a definite prediction or a physically motivated estimate. In this paper, we study the stochastic evolution of the vector condensate and obtain an estimate for its magnitude. Our study is mainly focused in the supergravity inspired case when the kinetic function and mass of the vector boson is time-varying during inflation, but other cases are also explored such as a parity violating axial theory or a non-minimal coupling between the vector field and gravity. As an example, we apply our findings in the context of the vector curvaton mechanism and contrast our results with current observations.