Cosmological Perturbations of Axion with a Dynamical Decay Constant

TL;DR

This paper explores how a time-dependent axion decay constant can enhance small-scale isocurvature perturbations, potentially leading to axionic domain wall formation that could overclose the universe.

Contribution

It reveals that a dynamical decay constant not only avoids topological defects during high-scale inflation but also amplifies small-scale perturbations, causing domain walls without cosmic string boundaries.

Findings

Enhanced small-scale axion isocurvature perturbations can exceed potential periodicity.

Such perturbations can produce axionic domain walls independent of cosmic strings.

These walls may overclose the universe, posing cosmological challenges.

Abstract

A QCD axion with a time-dependent decay constant has been known to be able to accommodate high-scale inflation without producing topological defects or too large isocurvature perturbations on CMB scales. We point out that a dynamical decay constant also has the effect of enhancing the small-scale axion isocurvature perturbations. The enhanced axion perturbations can even exceed the periodicity of the axion potential, and thus lead to the formation of axionic domain walls. Unlike the well-studied axionic walls, the walls produced from the enhanced perturbations are not bounded by cosmic strings, and thus would overclose the universe independently of the number of degenerate vacua along the axion potential.