Non-Gaussian Stochastic Gravitational Waves from Phase Transitions

TL;DR

This paper explores how primordial isocurvature fluctuations can lead to non-Gaussian features in gravitational wave backgrounds from early universe phase transitions, potentially observable with future detectors.

Contribution

It demonstrates that primordial isocurvature can survive in gravitational wave backgrounds and produce detectable non-Gaussianity, differing from the CMB predictions.

Findings

Primordial isocurvature can induce non-Gaussianity in GWB.

Future detectors like DECIGO and BBO could observe these non-Gaussian signals.

First inflation-era non-Gaussianity detection might be possible with gravitational waves.

Abstract

Cosmological phase transitions in the primordial universe can produce anisotropic stochastic gravitational wave backgrounds (GWB), similar to the cosmic microwave background (CMB). For adiabatic perturbations, the fluctuations in GWB follow those in the CMB, but if primordial fluctuations carry an isocurvature component, this need no longer be true. It is shown that in non-minimal inflationary and reheating settings, primordial isocurvature can survive in GWB and exhibit significant non-Gaussianity (NG) in contrast to the CMB, while obeying current observational bounds. While probing such NG GWB is at best a marginal possibility at LISA, there is much greater scope at future proposed detectors such as DECIGO and BBO. It is even possible that the first observations of inflation-era NG could be made with gravitational wave detectors as opposed to the CMB or Large-Scale Structure surveys.