Probing the inflationary particle content: extra spin-2 field

TL;DR

This paper investigates how coupling an extra massive spin-2 field during inflation influences primordial tensor modes, leading to observable anisotropies and modifications in standard consistency relations, thus providing a way to probe additional particle content in the early universe.

Contribution

It introduces a ghost-free inflationary model with an extra massive spin-2 field and analyzes its effects on tensor correlators and observable anisotropies.

Findings

Extra spin-2 field modifies tensor bispectrum consistency relations.

Fossil fields induce anisotropies in the tensor power spectrum.

Massive spin-2 field leaves detectable imprints despite decay.

Abstract

We study how inflationary observables associated with primordial tensor modes are affected by coupling the minimal field content with an extra spin-2 particle during inflation. We work with a model that is ghost-free at the fully non-linear level and show how the new degrees of freedom modify standard consistency relations for the tensor bispectrum. The extra interacting spin-2 field is necessarily massive and unitarity dictates its mass be in the $m \gtrsim H$ range. Despite the fact that this bound selects a decaying solution for the corresponding tensor mode, cosmological correlators still carry the imprints of such "fossil" fields. Remarkably, fossil(s) of spin $\geq 1$ generate distinctive anisotropies in observables such as the tensor power spectrum. We show how this plays out in our set-up.