Non-Gaussianity from isocurvature perturbations

TL;DR

This paper presents a formalism to analyze non-Gaussian features in both curvature and isocurvature perturbations, highlighting their distinct signatures in the CMB and exploring the QCD axion as a case study.

Contribution

It introduces a new formalism for studying non-Gaussianity in isocurvature perturbations and applies it to the QCD axion, showing potential observable signatures.

Findings

Isocurvature non-Gaussianity leaves distinct CMB signatures.

QCD axion can induce sizable non-Gaussianity at high inflationary scales.

Potential for detection in current or future CMB data.

Abstract

We develop a formalism to study non-Gaussianity in both curvature and isocurvature perturbations. It is shown that non-Gaussianity in the isocurvature perturbation between dark matter and photons leaves distinct signatures in the CMB temperature fluctuations, which may be confirmed in future experiments, or possibly, even in the currently available observational data. As an explicit example, we consider the QCD axion and show that it can actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} = O(10^9 - 10^{11})GeV.