Pseudoscalar N-flation and axial coupling revisited

TL;DR

This paper analyzes the axial coupling in N-flation models, focusing on its anisotropic signals and implications for large-scale magnetic fields, revealing challenges in reconciling theory with observations.

Contribution

It provides a detailed revisit of axial coupling dynamics in N-flation, highlighting the limitations for generating observable large-scale magnetic fields and anisotropy.

Findings

Anisotropic signals are confined to small scales and are generally undetectable.

Large-scale magnetic fields can be generated but have a blue spectrum incompatible with observations.

Enhanced anisotropy may conflict with the observed isotropic sky, constraining inflation scale.

Abstract

We revisit the dynamics of the axial coupling between many N-flatons and an Abelian gauge field, with special attention to its statistically anisotropic signal. The anisotropic power spectrum of curvature perturbations associated to the large wavelength modes of the gauge vector field is generally undetectable, since the anisotropy is confined to small scales. If the gauge field is the electromagnetic field, provided that the number of fields participating in the exponential expansion is large, it could be possible to generate sizable large scale magnetic fields. However, its spectrum is blue, and appreciable power on large scales implies an overly strong field on smaller scales, incompatibly with observations. Furthermore, the anisotropy is also markedly enhanced, and might be at odds with the isotropic observed sky. These aspects further demand that the scale of inflation is kept to a minimum.