New Sources of Gravitational Waves during Inflation

TL;DR

This work shows that inflationary gravitational waves can arise not only from quantum fluctuations of spacetime but also from particle- and string-production events during inflation. By developing a general framework and examining multiple concrete mechanisms—production, decays, string dynamics, and Bremsstrahlung—the authors demonstrate that these sources can compete with or surpass the standard vacuum contribution, even when the inflationary energy scale is low. Consequently, a detected B-mode signal does not uniquely fix the inflationary Hubble scale, motivating new observational signatures such as non-Gaussianities and scale-structure in the tensor spectrum. The results open up a broader landscape of inflationary phenomenology where exotic microphysics in the X-sector leaves potentially detectable imprints on the gravitational-wave background.

Abstract

We point out that detectable inflationary tensor modes can be generated by particle or string sources produced during inflation, consistently with the requirements for inflation and constraints from scalar fluctuations. We show via examples that this effect can dominate over the contribution from quantum fluctuations of the metric, occurring even when the inflationary potential energy is too low to produce a comparable signal. Thus a detection of tensor modes from inflation does not automatically constitute a determination of the inflationary Hubble scale.

Figures (1)

• Figure 1: Light degrees of freedom can be particles (left figure) or higher dimensional defects such as strings (right figure).