Anharmonic Effects on the Squeezing of Axion Perturbations

TL;DR

This paper investigates how anharmonic effects influence the quantum squeezing and particle production of axion perturbations during inflation, revealing enhanced effects near the potential hilltop.

Contribution

It introduces a detailed analysis of anharmonic effects on axion perturbation squeezing, extending previous harmonic approximations in inflationary cosmology.

Findings

Anharmonic effects significantly enhance particle production.

Squeezing parameters increase near the potential hilltop.

The evolution of axion fluctuations is strongly affected by self-interactions.

Abstract

It is assumed in standard cosmology that the Universe underwent a period of inflation in its earliest phase, providing the seeds for structure formation through vacuum fluctuations of the inflaton scalar field. These fluctuations get stretched by the quasi-exponential expansion of the Universe and become squeezed. The aim of this paper is to deepen the understanding of the squeezing process, considering the effect of self-interactions. Axion-like particles can provide a useful setup to study this effect. Specifically we focus on the consequences that a non-trivial evolution of the background axion field has on the squeezing of the perturbations. We follow the evolution of the axion's fluctuation modes from the horizon exit during inflation to the radiation-dominated epoch. We compute Bogoliubov coefficients and squeezing parameters, which are linked to the axion particle number and isocurvature perturbation. We find that the quantum mechanical particle production and the squeezing of the perturbations are enhanced, if one accounts for anharmonic effects, i.e., the effect of higher order terms in the potential. This effect becomes particularly strong towards the hilltop of the potential.