Backreaction in Axion Monodromy, 4-forms and the Swampland

TL;DR

This paper explores backreaction effects in axion monodromy inflation models, showing that saxions influence the inflaton's field range and support the Swampland Conjecture, with implications for transplanckian field excursions in string theory.

Contribution

It demonstrates the universal logarithmic scaling of the inflaton field distance due to saxion backreaction in Type II string flux compactifications, refining the Swampland Conjecture.

Findings

Backreaction causes logarithmic scaling of field distance.

Hierarchy between inflaton and saxions is hard to achieve without invalidating the effective theory.

Transplanckian excursions are disfavored in consistent string compactifications.

Abstract

Axion monodromy models can always be described in terms of an axion coupled to 3-form gauge fields with non-canonical kinetic terms. The presence of the saxions parametrising the kinetic metrics of the 3-form fields leads to backreaction effects in the inflationary dynamics. We review the case in which saxions backreact on the K\"ahler metric of the inflaton leading to a logarithmic scaling of the proper field distance at large field. This behaviour is universal in Type II string flux compactifications and consistent with a refinement of the Swampland Conjecture. The critical point at which this behaviour appears depends on the mass hierarchy between the inflaton and the saxions. However, in tractable compactifications, such a hierarchy cannot be realised without leaving the regime of validity of the effective theory, disfavouring transplanckian excursions in string theory.