Inflation without an Inflaton III: non-Gaussian signatures

TL;DR

This paper explores non-Gaussian signatures in a novel inflationary model where scalar perturbations originate from primordial gravitational waves, revealing a suppressed non-Gaussianity at CMB scales due to the nonlinear generation mechanism.

Contribution

It introduces a new inflationary scenario where scalar modes are generated nonlinearly from tensor modes, and analyzes the resulting non-Gaussianity and bispectrum shape.

Findings

The bispectrum depends logarithmically on the ultraviolet cutoff.

Non-Gaussianity is enhanced in squeezed configurations.

Amplitude of non-Gaussianity is negligible at CMB scales.

Abstract

We investigate primordial non-Gaussianity in the Inflation without an Inflaton (IWI) framework, where scalar perturbations are generated at second order by primordial gravitational waves in Einstein gravity on an exact de Sitter (dS) background. Since scalar modes are produced nonlinearly from tensor modes, non-Gaussianity is an intrinsic prediction of the mechanism. We compute the corresponding scalar bispectrum, derive the relevant contribution to the three-point function of the scalar potential, and evaluate its shape numerically. We find that, unlike the scalar power spectrum, the bispectrum depends logarithmically on the ultraviolet cutoff set by the end of inflation, indicating a structural difference between the two- and three-point statistics in this scenario. Its shape is enhanced toward squeezed configurations, but its amplitude becomes strongly suppressed once the scalar power spectrum is normalized to the observed value. The resulting non-Gaussianity at CMB scales is therefore negligibly small, well below present observational sensitivity.