Gravitational Waves from Stochastic Scalar Fluctuations

TL;DR

This paper proposes a new mechanism where stochastic effects in spectator scalar fields during inflation generate a stochastic gravitational wave background, potentially observable by future detectors and relevant to recent NANOGrav signals.

Contribution

It introduces a novel scenario linking spectator scalar field fluctuations to gravitational wave production, compatible with current observations and future detection prospects.

Findings

Large curvature perturbations at small scales due to spectator fields.

Predicted gravitational wave background within detectable range of future experiments.

Potential explanation for NANOGrav's recent gravitational wave observations.

Abstract

We present a novel mechanism for gravitational wave generation in the early Universe. Light spectator scalar fields during inflation can acquire a blue-tilted power spectrum due to stochastic effects. We show that this effect can lead to large curvature perturbations at small scales (induced by the spectator field fluctuations) while maintaining the observed, slightly red-tilted curvature perturbations at large cosmological scales (induced by the inflaton fluctuations). Along with other observational signatures, such as enhanced dark matter substructure, large curvature perturbations can induce a stochastic gravitational wave background (SGWB). The predicted strength of SGWB in our scenario, $\Omega_{\rm GW}h^2 \simeq 10^{-20} - 10^{-15}$, can be observed with future detectors, operating between $10^{-5}$ Hz and 10 Hz. We note that, in order to accommodate the newly reported NANOGrav observation, one could consider the same class of spectator models. At the same time, one would need to go beyond the simple benchmark considered here and consider a regime in which a misalignment contribution is also important.