A Hint of a Blue Axion Isocurvature Spectrum?

TL;DR

This paper investigates a specific axion isocurvature spectrum with a blue tilt and a spectral break, fitting it to cosmological data, and finds a mild hint of its presence that could explain certain CMB anomalies.

Contribution

It introduces a model of axion isocurvature spectrum with a spectral break and tests its fit to Planck and BOSS data, suggesting a potential new component in cosmology.

Findings

Mild statistical hint for axionic blue-tilted isocurvature perturbations.

Best fit parameters compatible with QCD axions and inflation at ~10^8 GeV.

Possible explanation for low-ell vs. high-ell CMB anomalies.

Abstract

It is known that if the Peccei-Quinn symmetry breaking field is displaced from its minimum during inflation, the axion isocurvature spectrum is generically strongly blue tilted with a break transition to a flat spectrum. We fit this spectrum (incorporated into the "vanilla" $\Lambda$-CDM cosmological model) to the Planck and BOSS DR11 data and find a mild hint for the presence of axionic blue-tilted isocurvature perturbations. We find the best fit parameter region is consistent with all of the dark matter being composed of QCD axions in the context of inflationary cosmology with an expansion rate of order $10^{8}$ GeV, the axion decay constant of order $10^{13}$ GeV, and the initial misalignment angle of order unity. Intriguingly, isocurvature with a spectral break may at least partially explain the low-$\ell$ vs. high-$\ell$ anomalies seen in the CMB data.