Spectrum of Perturbations in Anisotropic Inflationary Universe with Vector Hair

TL;DR

This paper investigates anisotropic inflation with coupled scalar and vector fields, analyzing background evolution, stability, and gravitational wave spectra, revealing anisotropy at large scales and potential links to CMB anomalies.

Contribution

It introduces a stable anisotropic inflation model with vector hair that preserves gauge symmetry and analyzes its perturbations and gravitational wave spectrum.

Findings

Anisotropic inflation model remains stable during evolution.

Gravitational wave spectrum exhibits statistical anisotropy at large scales.

Results may explain large-scale CMB anomalies.

Abstract

We study both the background evolution and cosmological perturbations of anisotropic inflationary models supported by coupled scalar and vector fields. The models we study preserve the U(1) gauge symmetry associated with the vector field, and therefore do not possess instabilities associated with longitudinal modes (which instead plague some recently proposed models of vector inflation and curvaton). We first intoduce a model in which the background anisotropy slowly decreases during inflation; we then confirm the stability of the background solution by studying the quadratic action for all the perturbations of the model. We then compute the spectrum of the $h_{\times}$ gravitational wave polarization. The spectrum we find breaks statistical isotropy at the largest scales and reduces to the standard nearly scale invariant form at small scales. We finally discuss the possible relevance of our results to the large scale CMB anomalies.