Non-Gaussianity from Inflation: Theory and Observations
N. Bartolo, E. Komatsu, S. Matarrese, A. Riotto
TL;DR
This review analyzes how inflation generates primordial perturbations and how non-Gaussian signatures, parameterized by $f_{\rm NL}$, arise in various scenarios. It develops a comprehensive second-order perturbation framework, distinguishes adiabatic and entropy modes, and derives evolution through reheating for standard, curvaton, and inhomogeneous reheating models. The work then connects theory to observations by detailing how $f_{\rm NL}$ and related non-Gaussian features imprint on the CMB, including the bispectrum and trispectrum, and discusses future prospects with Planck and beyond. Overall, it shows that while single-field slow-roll inflation predicts small non-Gaussianities, alternative scenarios and post-inflationary dynamics can yield observable signatures, offering powerful tests of early-universe physics.
Abstract
This is a review of models of inflation and of their predictions for the primordial non-Gaussianity in the density perturbations which are thought to be at the origin of structures in the Universe. Non-Gaussianity emerges as a key observable to discriminate among competing scenarios for the generation of cosmological perturbations and is one of the primary targets of present and future Cosmic Microwave Background satellite missions. We give a detailed presentation of the state-of-the-art of the subject of non-Gaussianity, both from the theoretical and the observational point of view, and provide all the tools necessary to compute at second order in perturbation theory the level of non-Gaussianity in any model of cosmological perturbations. We discuss the new wave of models of inflation, which are firmly rooted in modern particle physics theory and predict a significant amount of non-Gaussianity. The review is addressed to both astrophysicists and particle physicists and contains useful tables which summarize the theoretical and observational results regarding non-Gaussianity.