Non-Gaussianity and the Cosmic Microwave Background Anisotropies

TL;DR

This paper reviews how non-Gaussian features in the CMB anisotropies originate from primordial conditions and post-inflationary processes, analyzing their impact on cosmological observations and the challenges in isolating primordial signals.

Contribution

It provides a comprehensive pedagogical overview of second-order non-Gaussianity effects in the CMB, including analytical methods and contamination estimates from secondary sources.

Findings

Derived equations for second-order CMB anisotropies.

Quantified contamination of primordial NG by secondary effects.

Analyzed NG contributions at large and small angular scales.

Abstract

We review in a pedagogical way the present status of the impact of non-Gaussianity (NG) on the Cosmic Microwave Background (CMB) anisotropies. We first show how to set the initial conditions at second-order for the (gauge invariant) CMB anisotropies when some primordial NG is present. However, there are many sources of NG in CMB anisotropies, beyond the primordial one, which can contaminate the primordial signal. We mainly focus on the NG generated from the post-inflationary evolution of the CMB anisotropies at second-order in perturbation theory at large and small angular scales, such as the ones generated at the recombination epoch. We show how to derive the equations to study the second-order CMB anisotropies and provide analytical computations to evaluate their contamination to primordial NG (complemented with numerical examples). We also offer a brief summary of other secondary effects. This review requires basic knowledge of the theory of cosmological perturbations at the linear level.