# Influence of Unobservable Modes on Correlation Functions during Inflation

## TL;DR

This paper investigates how unobservable super-Hubble modes influence observed correlation functions during inflation, revealing potential biases in cosmological parameter inference and effects on higher-order correlations.

## Contribution

It provides a detailed analysis of the impact of unobservable modes on correlation functions in Quasi-Single Field Inflation, including the bispectrum and higher-order functions, highlighting biases and shape modifications.

## Key findings

- Unobservable modes can bias the inferred mass of hidden sector fields by about 10%.
- Exchange-like diagrams modify the bispectrum shape, unlike contact-like diagrams.
- Couplings between observable and unobservable modes affect higher-order correlation functions.

## Abstract

Coupling between sub- and super-Hubble modes can affect the locally observed statistics of our universe. In the context of Quasi-Single Field Inflation, we can compute correlation functions and derive the influence of those unobservable modes on observed correlation functions as well as on the inferred cosmological parameters. We study how different classes of diagrams affect the bispectrum in the squeezed limit; in particular, while contact-like diagrams leave the scaling between the long and short modes unchanged, exchange-like diagrams do modify the shape of the bispectrum. We show that the mass of the hidden sector field can hence be biased by an unavoidable cosmic variance that can reach a 1-$\sigma$ uncertainty of $\mathcal{O}(10\%)$ for a weakly non-Gaussian universe. Finally, we go beyond the bispectrum and show how couplings between unobservable and observable modes can affect generic correlation functions with arbitrary order non-derivative self-interactions.