Universal Non-Gaussian Signatures from Transient Instabilities

TL;DR

This paper uncovers universal non-Gaussian signatures in the bispectrum caused by transient tachyonic instabilities during inflation, providing new computational methods and shape templates for observational tests.

Contribution

It presents the first exact numerical computation of bispectra from tachyonic instabilities in inflation, distinguishing behaviors for different mass regimes and introducing observationally relevant templates.

Findings

Universal magnification of folded bispectrum configurations.

Distinct correlation between folded and squeezed configurations in heavy mass cases.

Compatibility of the computed bispectrum with current observational limits.

Abstract

We identify universal signatures in the bispectrum arising from a transient tachyonic instability of entropic fluctuations during inflation, a phenomenon that naturally arises in hyperbolic field-space geometries. We perform exact numerical calculations directly at the level of fluctuations, without relying on a specific background model, and distinguish two cases. In the light case, with masses around the Hubble scale, our results provide the first-ever computation of the bispectrum due to such tachyonic instabilities. We find a universal magnification of the folded configuration, together with the known non-analytic scaling in the squeezed limit. As an illustrative example, we compute and analyse the bispectrum in angular inflation, demonstrating compatibility with current limits. In the heavy case, with masses well above the Hubble scale, the bispectrum exhibits a distinctive correlation between enhanced folded configurations and a `tachyonic resonance' in mildly squeezed limits, with the resonance scale set by the strength of the instability. While the main qualitative features are reproduced, we show that there exists no UV matching for which a single-field effective description, obtained by integrating out the entropic modes, accurately captures the bispectrum for all kinematic configurations. To facilitate observational applications, we introduce simple bispectrum shape templates suitable for current and forthcoming cosmological surveys. Our model-independent results allow for constraining non-standard inflationary attractors characterised by strongly non-geodesic motion.