Probing the initial state of inflation: analytical structure of cosmological correlators

TL;DR

This paper analyzes the analytic structure of inflationary correlation functions in de Sitter space to understand how initial quantum states influence observable signals, revealing new pole structures linked to initial particle excitations.

Contribution

It provides a detailed examination of how different initial states, including excited and coherent states, affect the pole structure of inflationary correlators, extending beyond the standard Bunch-Davies vacuum.

Findings

Initial states with excitations introduce new pole structures in correlators.

Coherent states can mimic Bunch-Davies vacuum in bispectrum absence.

Excited states lead to disconnected diagrams affecting observational signals.

Abstract

We study the analytic structure of in-in correlation functions in a deSitter background. The aim of this study is to probe the initial conditions for inflation through the features of correlation functions of the field fluctuations, and understand precisely how an in-in correlator responds to particles in the initial state. We emphasize that the choice of vacuua and the corresponding particle interpretation for these fluctuations is flexible, and we clarify the role of this choice at the level of calculations and their diagrammatic interpretation. We consider several possibilities aside from the standard Bunch Davies vacuum prescription for the initial state, and trace the change in pole structure as one begins adding excitations; starting from just a single particle, to highly excited states and special cases such as a coherent state. We illustrate - with the example of coherent states - the subtleties in concluding a Bunch Davies initial state from the absence of physical poles in the bispectrum, which is interesting in light of some recent literature. Initial states with a finite number of excitations are plagued with disconnected diagrams isolated in phase space, and we highlight their implications on the observation of these signals, and how the situation changes as one begins to excite more and more particles. We also comment about the implications of various initial conditions on the squeezed limit of the bispectrum. These new pole structures are a direct consequence of mixing of positive and negative frequency modes which is a characteristic of curved spacetimes; in particular, we see in detail how particles in an initial state replicate mode mixing structures. This study aims to clarify the missing details that link quantum and classical initial conditions, and sharpen our understanding of in-in correlators in inflation.