Can a spectator scalar field enhance inflationary tensor mode?

TL;DR

This paper investigates whether a spectator scalar field with a small sound speed can significantly enhance inflationary gravitational waves, concluding that observational constraints limit its impact compared to vacuum fluctuations.

Contribution

It analytically derives the power spectra of gravitational waves and curvature perturbations induced by a spectator scalar field with small sound speed during inflation.

Findings

Small sound speed amplifies curvature perturbation more than tensor mode

Induced gravitational waves are negligible compared to vacuum fluctuations

Current observations constrain the enhancement effect

Abstract

We consider the possibility of enhancing the inflationary tensor mode by introducing a spectator scalar field with a small sound speed which induces gravitational waves as a second order effect. We analytically obtain the power spectra of gravitational waves and curvature perturbation induced by the spectator scalar field. We found that the small sound speed amplifies the curvature perturbation much more than the tensor mode and the current observational constraint forces the induced gravitational waves to be negligible compared with those from the vacuum fluctuation during inflation.