CMB hemispherical asymmetry from non-linear isocurvature perturbations

TL;DR

This paper explores how non-linear isocurvature perturbations from multi-field inflation could cause hemispherical asymmetry in the CMB temperature and polarization, focusing on models where matter isocurvature modes are uncorrelated with density perturbations.

Contribution

It introduces a generalized non-linear δN formalism to analyze non-Gaussianity from isocurvature perturbations and assesses their potential to produce observable CMB asymmetries.

Findings

Matter isocurvature perturbations can cause large-scale hemispherical asymmetry.

In curvaton models, significant asymmetry is difficult to achieve under current constraints.

Axion models may produce observable asymmetry due to uncorrelated isocurvature modes.

Abstract

We investigate whether non-adiabatic perturbations from inflation could produce an asymmetric distribution of temperature anisotropies on large angular scales in the cosmic microwave background (CMB). We use a generalised non-linear $\delta N$ formalism to calculate the non-Gaussianity of the primordial density and isocurvature perturbations due to the presence of non-adiabatic, but approximately scale-invariant field fluctuations during multi-field inflation. This local-type non-Gaussianity leads to a correlation between very long wavelength inhomogeneities, larger than our observable horizon, and smaller scale fluctuations in the radiation and matter density. Matter isocurvature perturbations contribute primarily to low CMB multipoles and hence can lead to a hemispherical asymmetry on large angular scales, with negligible asymmetry on smaller scales. In curvaton models, where the matter isocurvature perturbation is partly correlated with the primordial density perturbation, we are unable to obtain a significant asymmetry on large angular scales while respecting current observational constraints on the observed quadrupole. However in the axion model, where the matter isocurvature and primordial density perturbations are uncorrelated, we find it may be possible to obtain a significant asymmetry due to isocurvature modes on large angular scales. Such an isocurvature origin for the hemispherical asymmetry would naturally give rise to a distinctive asymmetry in the CMB polarisation on large scales.