Gravitational waves induced by scalar-tensor mixing

TL;DR

This paper investigates second-order gravitational waves generated by scalar-tensor interactions in the early Universe, highlighting their unique signatures, potential detectability, and implications for cosmology and gravitational wave observations.

Contribution

It provides a detailed analysis of scalar-tensor induced GWs, revealing their distinctive features and effects of primordial parity violation, which differ from scalar-scalar induced GWs.

Findings

Scalar-tensor induced GWs lack resonances and logarithmic running in low frequencies.

Primordial parity violation can influence the chirality of GWs in the UV region.

Potential divergences in GWs are identified and addressed.

Abstract

This paper explores the physics of second-order gravitational waves (GWs) induced by scalar-tensor perturbation interactions in the radiation-dominated Universe. We investigate the distinctive signatures of these GWs and their detectability compared to scalar-induced GWs. Unlike scalar-scalar induced GWs, scalar-tensor induced GWs do not present resonances or a logarithmic running in the low frequency tail in the case of peaked primordial spectra. But, interestingly, they partly inherit any primordial parity violation of tensor modes. We find that chirality in primordial GWs can lead to distinguishing effects in scalar-tensor induced GWs in the ultraviolet (UV) region. We also address a potential divergence in our GWs and explore possible solutions. This study contributes to our understanding of GWs in the early Universe and their implications for cosmology and GWs detection.