Isotropization of the universe during inflation

TL;DR

This paper reviews how primordial inflation can erase anisotropies in the universe, explores the dynamics and observational implications of early anisotropic phases, and discusses models with vector fields that challenge the cosmic no-hair theorem.

Contribution

It provides a comprehensive analysis of anisotropic inflationary models, including their dynamics, perturbations, and potential observational signatures, along with models that evade the cosmic no-hair theorem.

Findings

Anisotropic expansion tends to decrease during inflation.

Primordial anisotropy affects perturbation quantization and observable properties.

Models with vector fields can sustain anisotropy despite the cosmic no-hair theorem.

Abstract

A primordial inflationary phase allows one to erase any possible anisotropic expansion thanks to the cosmic no-hair theorem. If there is no global anisotropic stress, then the anisotropic expansion rate tends to decrease. What are the observational consequences of a possible early anisotropic phase? We first review the dynamics of anisotropic universes and report analytic approximations. We then discuss the structure of dynamical equations for perturbations and the statistical properties of observables, as well as the implication of a primordial anisotropy on the quantization of these perturbations during inflation. Finally we briefly review models based on primordial vector field which evade the cosmic no-hair theorem.