Anisotropy in solid inflation

TL;DR

This paper explores how solid inflation can sustain prolonged anisotropic expansion, challenging the cosmic no-hair conjecture, and analyzes the resulting curvature perturbations and their observational implications.

Contribution

It demonstrates that solid inflation can maintain anisotropy during inflation, providing a novel example that evades the cosmic no-hair conjecture with detailed perturbation analysis.

Findings

Solid inflation can sustain anisotropic solutions during inflation.

The model predicts enhanced bispectrum in the squeezed limit with angular dependence.

A phenomenological bound on anisotropy is derived.

Abstract

In the model of solid / elastic inflation, inflation is driven by a source that has the field theoretical description of a solid. To allow for prolonged slow roll inflation, the solid needs to be extremely insensitive to the spatial expansion. We point out that, because of this property, the solid is also rather inefficient in erasing anisotropic deformations of the geometry. This allows for a prolonged inflationary anisotropic solution, providing the first example with standard gravity and scalar fields only which evades the conditions of the so called cosmic no-hair conjecture. We compute the curvature perturbations on the anisotropic solution, and the corresponding phenomenological bound on the anisotropy. Finally, we discuss the analogy between this model and the f (phi) F^2 model, which also allows for anisotropic inflation thanks to a suitable coupling between the inflaton phi and a vector field. We remark that the bispectrum of the curvature perturbations in solid inflation is enhanced in the squeezed limit and presents a nontrivial angular dependence, as had previously been found for the f (phi) F^2 model.