Cosmological collider non-Gaussianity from multiple scalars and $R^2$ gravity

TL;DR

This paper reviews how multiple scalar fields and $R^2$ gravity during inflation can produce primordial non-Gaussian signals detectable by cosmological colliders, highlighting the role of scalar mass and turn dynamics.

Contribution

It provides a critical review of the formalism linking inflationary theories with primordial non-Gaussianity, emphasizing the impact of multiple scalars and $R^2$ gravity on the bi-spectrum.

Findings

Scalar bi-spectrum can be at the gravitational floor level in many Planck-scale inflation models.

$R^2$ gravity with Hubble-scale scalar mass predicts mild non-Gaussian signals.

Multiple turns in inflation can produce signals significantly above the gravitational floor.

Abstract

Cosmological collider signals of primordial non-Gaussianity arise at tree level when an extra scalar has Hubble mass during inflation. We critically review the formalism finding that a large class of inflationary theories, based on Planck-scale physics, predict a scalar bi-spectrum around the gravitational floor level. This mild signal arises for example in $R^2$ gravity, in the regime where its gravitational scalar has Hubble-scale mass. Signals much above the gravitational floor arise in theories where scalars undergo multiple turns during inflation, thanks to sub-Planckian physics.