The isotropic attractor solution of axion-SU(2) inflation: Universal isotropization in Bianchi type-I geometry

TL;DR

This paper demonstrates that axion-SU(2) inflation models naturally evolve towards isotropy in Bianchi type-I geometry, confirming the cosmic no-hair conjecture and resolving previous numerical issues with a new parametrization.

Contribution

It establishes the universal isotropization of axion-SU(2) inflation models and introduces a robust parametrization to accurately analyze anisotropic solutions.

Findings

Isotropic attractor solutions are generic in axion-SU(2) inflation.

Previous numerical breakdowns were due to parametrization singularities.

Anisotropies decay within a few e-folds, supporting the cosmic no-hair conjecture.

Abstract

SU(2) gauge fields coupled to an axion field can acquire an isotropic background solution during inflation. We study homogeneous but anisotropic inflationary solutions in the presence of such (massless) gauge fields. A gauge field in the cosmological background may pose a threat to spatial isotropy. We show, however, that such models $\textit{generally}$ isotropize in Bianchi type-I geometry, and the isotropic solution is the attractor. Restricting the setup by adding an axial symmetry, we revisited the numerical analysis presented in Wolfson et.al (2020). We find that the reported numerical breakdown in the previous analysis is an artifact of parametrization singularity. We use a new parametrization that is well-defined all over the phase space. We show that the system respects the cosmic no-hair conjecture and the anisotropies always dilute away within a few e-folds.