The non-linear dynamics of axion inflation: a detailed lattice study

TL;DR

This paper investigates the complex non-linear dynamics of axion inflation using lattice simulations, revealing critical local effects and regimes that influence inflation's end and related phenomenology.

Contribution

It introduces gauge-invariant lattice techniques to accurately simulate local non-linear effects in axion inflation, surpassing previous homogeneous approximations.

Findings

Magnetic energy dominates over electric energy during strong backreaction.

The longitudinal mode becomes significantly excited in the non-linear regime.

A scale-dependent chiral imbalance is generated during inflation.

Abstract

We study in detail the fully inhomogeneous non-linear dynamics of axion inflation, identifying three regimes: weak-, mild-, and strong-backreaction, depending on the duration of inflation. We use lattice techniques that explicitly preserve gauge invariance and shift symmetry, and which we validate against other computational methods of the linear dynamics and of the homogeneous backreaction regime. Notably, we demonstrate that the latter fails to accurately describe the truly local dynamics of strong backreaction. We investigate the convergence of simulations of local backreaction, determining the requirements to achieve an accurate description of the dynamics, and providing useful parametrizations of the delay of the end of inflation. Additionally, we identify key features emerging from a proper local treatment of strong backreaction: the dominance of magnetic energy against the electric counterpart, the excitation of the longitudinal mode, and the generation of a scale-dependent chiral (im)balance. Our results underscore the necessity to accurately capture the local nature of the non-linear dynamics of the system, in order to correctly assess phenomenological predictions, such as e.g. the production of gravitational waves and primordial black holes.