Axion Landscape and Natural Inflation

TL;DR

This paper explores how multiple axions create a landscape facilitating eternal and slow-roll inflation, with potential observable signatures like spectral index variations and non-Gaussianities, influenced by the axion decay constants and bubble nucleation.

Contribution

It analyzes the vacuum structure and inflationary dynamics in the axion landscape, highlighting the roles of natural and multi-natural inflation, and potential observational signatures.

Findings

Inflation can be driven by natural or multi-natural mechanisms.

Large decay constants approximate quadratic chaotic inflation.

Observable signatures include spectral index deviations and non-Gaussianities.

Abstract

Multiple axions form a landscape in the presence of various shift symmetry breaking terms. Eternal inflation populates the axion landscape, continuously creating new universes by bubble nucleation. Slow-roll inflation takes place after the tunneling event, if a very flat direction with a super-Planckian decay constant arises due to the alignment mechanism. We study the vacuum structure as well as possible inflationary dynamics in the axion landscape scenario, and find that the inflaton dynamics is given by either natural or multi-natural inflation. In the limit of large decay constant, it is approximated by the quadratic chaotic inflation, which however is disfavored if there is a pressure toward shorter duration of inflation. Therefore, if the spectral index and the tensor-to-scalar ratio turn out to be different from the quadratic chaotic inflation, there might be observable traces of the bubble nucleation. Also, the existence of small modulations to the inflaton potential is a common feature in the axion landscape, which generates a sizable and almost constant running of the scalar spectral index over CMB scales. Non-Gaussianity of equilateral type can also be generated if some of the axions are coupled to massless gauge fields.