Primordial non-Gaussianities after Planck 2015: an introductory review

TL;DR

This review discusses how primordial non-Gaussianities serve as a probe of early universe physics, reviews theoretical predictions from inflationary models, and summarizes observational constraints from Planck 2015 data.

Contribution

It provides an accessible overview of computing non-Gaussianities in inflationary models and summarizes recent observational constraints and future prospects.

Findings

Planck 2015 data constrains primordial non-Gaussianities.

Different inflationary models predict distinct non-Gaussian signatures.

Future experiments aim to improve detection sensitivity.

Abstract

Deviations from Gaussian statistics of the cosmological density fluctuations, so-called primordial non-Gaussianities (NG), are one of the most informative fingerprints of the origin of structures in the universe. Indeed, they can probe physics at energy scales inaccessible to laboratory experiments, and are sensitive to the interactions of the field(s) that generated the primordial fluctuations, contrary to the Gaussian linear theory. As a result, they can discriminate between inflationary models that are otherwise almost indistinguishable. In this short review, we explain how to compute the non-Gaussian properties in any inflationary scenario. We review the theoretical predictions of several important classes of models. We then describe the ways NG can be probed observationally, and we highlight the recent constraints from the Planck mission, as well as their implications. We finally identify well motivated theoretical targets for future experiments and discuss observational prospects.