Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity

TL;DR

This paper develops a general framework to analyze large-scale anomalies in the CMB as signatures of non-Gaussian primordial fluctuations, extending previous models to include multiple superhorizon modes and scale-dependent effects.

Contribution

It introduces a comprehensive analytic approach to quantify CMB anisotropies caused by non-Gaussianity without assuming single superhorizon modes, incorporating scale dependence and validating predictions with simulations.

Findings

Non-local and scale-dependent local non-Gaussianity models produce scale-dependent power modulations.

The analytic predictions match numerical simulations of CMB maps.

Posterior distributions for bispectrum parameters can be derived from observed monopole and dipole modulations.

Abstract

We derive a general expression for the probability of observing deviations from statistical isotropy in the cosmic microwave background (CMB) if the primordial fluctuations are non-Gaussian and extend to superhorizon scales. The primary motivation is to properly characterize the monopole and dipole modulations of the primordial power spectrum that are generated by the coupling between superhorizon and subhorizon perturbations. Unlike previous proposals for generating the hemispherical power asymmetry, we do not assume that the power asymmetry results from a single large superhorizon mode. Instead, we extrapolate the observed power spectrum to superhorizon scales and compute the power asymmetry that would result from a specific realization of non-Gaussian perturbations on scales larger than the observable universe. Our study encompasses many of the scenarios that have been put forward as possible explanations for the CMB hemispherical power asymmetry. We confirm our analytic predictions for the probability of a given power asymmetry by comparing them to numerical realizations of CMB maps. We find that non-local models of non-Gaussianity and scale-dependent local non-Gaussianity produce scale-dependent modulations of the power spectrum, thereby potentially producing both a monopolar and a dipolar power modulation on large scales. We then provide simple examples of finding the posterior distributions for the parameters of the bispectrum from the observed monopole and dipole modulations.