Bootstrapping Multi-Field Inflation: non-Gaussianities from light scalars revisited

TL;DR

This paper develops analytic templates for primordial non-Gaussianities in multi-field inflation, incorporating boost symmetry breaking and late-time effects, to better understand the signatures of light scalar exchanges in cosmological observations.

Contribution

It introduces a systematic bootstrap-based method to classify and compute non-Gaussian shapes from massless scalar exchange, including boost-breaking scenarios and late-time cutoff effects.

Findings

Derived analytic templates for non-Gaussian shapes at any kinematic configuration.

Identified a new class of non-Gaussian shapes peaking at specific geometric configurations.

Compared bootstrap results with $ ext{delta}N$ formalism, highlighting missing contributions.

Abstract

Primordial non-Gaussianities from multi-field inflation are a leading target for cosmological observations, because of the possible large correlations generated between long and short distances. These signatures are captured by the local shape of the scalar bispectrum. In this paper, we revisit the nonlinearities of the conversion process from additional light scalars into curvature perturbations during inflation. We provide analytic templates for correlation functions valid at any kinematical configuration, using the cosmological bootstrap as a main computational tool. Our results include the possibility of large breaking of boost symmetry, in the form of small speeds of sound for both the inflaton and the mediators. We consider correlators coming from the tree-level exchange of a massless scalar field. By introducing a late-time cutoff, we identify that the symmetry constraints on the correlators are modified. This leads to anomalous conformal Ward identities, and consequently the bootstrap differential equations acquire a source term that depends on this cutoff. The solutions to the differential equations are scalar seed functions that incorporate these late-time growth effects. Applying weight-shifting operators to auxiliary "seed" functions, we obtain a systematic classification of shapes of non-Gaussianity coming from massless exchange. For theories with de Sitter symmetry, we compare the resulting shapes with the ones obtained via the $\delta N$ formalism, identifying missing contributions away from the squeezed limit. For boost-breaking scenarios, we derive a novel class of shape functions with phenomenologically distinct features. Specifically, the new shape provides a simple extension of equilateral non-Gaussianity: the signal peaks at a geometric configuration controlled by the ratio of the sound speeds of the mediator and the inflaton.