Effective Theory of Squeezed Correlation Functions

TL;DR

This paper introduces a systematic framework for analyzing the squeezed limit of inflationary correlation functions using an expansion in super-horizon degrees of freedom, applicable across various inflation models and relevant for large-scale structure observations.

Contribution

It develops a unified approach to study squeezed-limit correlation functions in inflation, applicable even with strongly coupled additional degrees of freedom, and provides insights into their observability.

Findings

Reproduces known results for single- and multi-field inflation.

Applicable to models with strongly coupled degrees of freedom.

Suggests higher detectability of squeezed non-Gaussianity in large-scale surveys.

Abstract

Various inflationary scenarios can often be distinguished from one another by looking at the squeezed limit behavior of correlation functions. Therefore, it is useful to have a framework designed to study this limit in a more systematic and efficient way. We propose using an expansion in terms of weakly coupled super-horizon degrees of freedom, which is argued to generically exist in a near de Sitter space-time. The modes have a simple factorized form which leads to factorization of the squeezed-limit correlation functions with power-law behavior in $k_{\rm long}/k_{\rm short}$. This approach reproduces the known results in single-, quasi-single-, and multi-field inflationary models. However, it is applicable even if, unlike the above examples, the additional degrees of freedom are not weakly coupled at sub-horizon scales. Stronger results are derived in two-field (or sufficiently symmetric multi-field) inflationary models. We discuss the observability of the non-Gaussian 3-point function in the large-scale structure surveys, and argue that the squeezed limit behavior has a higher detectability chance than equilateral behavior when it scales as $(k_{\rm long}/k_{\rm short})^\Delta$ with $\Delta<1$ -- where local non-Gaussianity corresponds to $\Delta=0$.