Primordial Gravitational Waves Assisted by Cosmological Scalar Perturbations

TL;DR

This paper proposes a new method to detect primordial gravitational waves by searching for second-order tensor perturbations caused by nonlinear couplings, potentially measurable with future detector networks.

Contribution

It introduces a novel approach focusing on second-order tensor perturbations from scalar-tensor couplings, offering a new detection avenue for primordial gravitational waves.

Findings

Blue-tilted tensor spectral index predicted

Tensor-to-scalar ratio measurable with future detectors

Potential for high-precision measurement over a decade

Abstract

Primordial gravitational waves are a crucial prediction of inflation theory, and their detection through their imprints on the cosmic microwave background is actively being pursued. However, these attempts have not yet been successful. In this paper, we propose a novel approach to detect primordial gravitational waves by searching for a signal of second-order tensor perturbations. These perturbations were produced due to nonlinear couplings between the linear tensor and scalar perturbations in the early universe. We anticipate a blue-tilted tensor spectral index, and suggest that the tensor-to-scalar ratio can potentially be measured with high precision using a detector network composed of the ground-based Einstein Telescope and the space-borne LISA project on a decade timescale.