Light Particles with Spin in Inflation

TL;DR

This paper explores the theoretical possibility and phenomenological implications of light spin-2 particles during inflation, which can alter gravitational wave signals and generate distinctive non-Gaussianities in the cosmic microwave background.

Contribution

It demonstrates how light spin-2 particles can evade the Higuchi bound through coupling with the inflaton, and analyzes their effects on inflationary observables.

Findings

Mixing with gravitons modifies the tensor power spectrum.

Presence of spin-2 particles induces angle-dependent non-Gaussianities.

Potential for sizeable tensor non-Gaussianities and distinctive bispectrum and trispectrum signatures.

Abstract

The existence of light particles with spin during inflation is prohibited by the Higuchi bound. This conclusion can be evaded if one considers states with a sizeable coupling with the inflaton foliation, since this breaks the de Sitter isometries. The action for these states can be constructed within the Effective Field Theory of Inflation, or using a CCWZ procedure. Light particles with spin have prescribed couplings with soft inflaton perturbations, which are encoded in consistency relations. We study the phenomenology of light states with spin 2. These mix with the graviton changing the tensor power spectrum and can lead to sizeable tensor non-Gaussianities. They also give rise to a scalar bispectrum and trispectrum with a characteristic angle-dependent non-Gaussianity.