Axion isocurvature fluctuations with extremely blue spectrum

TL;DR

This paper proposes a new axion model that produces isocurvature fluctuations with a blue spectrum at large scales and scale invariance at small scales, aligning with recent cosmological data.

Contribution

The model introduces a novel mechanism using Peccei-Quinn scalar field dynamics to generate a distinctive blue-tilted isocurvature spectrum.

Findings

Spectrum is blue at large scales, scale invariant at small scales

Consistent with recent cosmological analyses of mixed perturbations

Provides a theoretical framework for observed spectral index differences

Abstract

We construct an axion model for generating isocurvature fluctuations with blue spectrum, n_{iso}=2-4, which is suggested by recent analyses of admixture of adiabatic and isocurvature perturbations with independent spectral indices, n_{ad} \ne n_{iso}. The distinctive feature of the model is that the spectrum is blue at large scales while scale invariant at small scales. This is naturally realized by the dynamics of the Peccei-Quinn scalar field.