Analytic results in aligned axion inflation

TL;DR

This paper provides analytic solutions for aligned axion inflation, demonstrating that it can produce phenomenologically viable inflationary trajectories within current observational constraints, with potential for multiple inflationary stages.

Contribution

It introduces compact analytic expressions for aligned axion inflation trajectories and explores their phenomenology, including post-inflationary evolution and multi-stage inflation scenarios.

Findings

Phenomenology within current observational constraints.

Sub-Planckian inflationary trajectory extension.

Potential for multiple inflationary stages.

Abstract

The original model of axion natural inflation produces a tensor-to-scalar ratio above the experimental limit. Aligned axion inflation admits inflationary trajectories that originate near a saddle point of the two-field potential, and terminate due to the instability of the orthogonal direction. The phenomenology of these solutions is within the current constraints, and a range of parameters will be probed by the next stage CMB experiments. We provide the analytic solution for these trajectories and very compact analytic expressions for the associated phenomenology. For parameters leading to the observed value for the scalar spectral tilt the extension of the inflationary trajectory is sub-Planckian. However, one eigenvalue of the axion kinetic matrix (in the basis that diagonalizes the potential) is trans-Planckian. Finally, we discuss the post-inflationary evolution after the instability. In some cases, the fields reach a second inflationary valley, connected to a minimum. Multiple stages of inflation might be a more general occurrence in multiple-field inflationary models with trajectories starting next to critical points.