Particle production during Inflation with a non-minimally coupled spectator scalar field

TL;DR

This paper explores how a non-minimally coupled spectator scalar field during inflation can amplify gravitational waves and alter curvature perturbations, with potential implications for future GW detection experiments.

Contribution

It introduces a model where a spectator scalar field coupled to gravity influences inflationary dynamics and gravitational wave production, highlighting observable signatures.

Findings

Amplification of gravitational waves at specific scales.

Oscillating features in the curvature perturbation spectrum.

Potential detectability of GW signals by future experiments like BBO and SKA.

Abstract

We study the inflationary model with a spectator scalar field $\chi$ coupled to both the inflaton and Ricci scalar. The interaction between the $\chi$ field and the gravity, denoted by $\xi R\chi^2$, can trigger the tachyonic instability of certain modes of the $\chi$ field. As a result, the $\chi$ field perturbations are amplified and serve as a gravitational wave (GW) source. When considering the backreaction of the $\chi$ field, an upper bound on the coupling parameter $\xi$ must be imposed to ensure that inflation does not end prematurely. In this case, we find that the inflaton's evolution experiences a sudden slowdown due to the production of $\chi$ particles, resulting in a unique oscillating structure in the power spectrum of curvature perturbations at specific scales. Moreover, the GW signal induced by the $\chi$ field is more significant than primordial GWs at around its peak scale, leading to a noticeable bump in the overall energy spectrum of GWs. It's worth noting that this bump predicted in the slow-roll inflationary scenario is unlikely to be detected by LISA and Taiji, but there is a slim chance it might approach the detection limits of GW experiments like BBO and SKA if we devise distinctive inflatonary potentials.