Non-Gaussianities of primordial perturbations and tensor sound speed

TL;DR

This paper explores how the sound speed of primordial gravitational waves influences the cross-bispectrum involving scalar and tensor perturbations, revealing a unique relationship unlike that of scalar auto-bispectra.

Contribution

It establishes a direct link between tensor sound speed and the scalar-tensor-tensor bispectrum, offering a new method to probe gravitational wave properties.

Findings

Tensor sound speed relates to the scalar-tensor-tensor bispectrum.

Contrasts with scalar perturbations where sound speed affects auto-bispectrum.

Scalar-tensor-tensor bispectrum can probe tensor sound speed.

Abstract

We investigate the relation between the non-Gaussianities of the primordial perturbations and the sound speed of the tensor perturbations, that is, the propagation speed of the gravitational waves. We find that the sound speed of the tensor perturbations is directly related not to the auto-bispectrum of the tensor perturbations but to the cross-bispectrum of the primordial perturbations, especially, the scalar-tensor-tensor bispectrum. This result is in sharp contrast with the case of the scalar (curvature) perturbations, where their reduced sound speed enhances their auto-bispectrum. Our findings indicate that the scalar-tensor-tensor bispectrum can be a powerful tool to probe the sound speed of the tensor perturbations.