# CMB B-mode auto-bispectrum produced by primordial gravitational waves

## TL;DR

This paper calculates the polarization bispectra in the CMB caused by primordial gravitational waves, identifying potential detectability of non-Gaussianities from specific scalar field couplings in inflation models.

## Contribution

It introduces a method to compute polarization bispectra from non-Gaussian gravitational waves in scalar-tensor theories using Laplace transformation.

## Key findings

- Future experiments can detect the new non-Gaussianity if the coupling parameter is very large.
- The new non-Gaussianity does not conflict with current observational data.

## Abstract

Gravitational waves from inflation induce polarization patterns in the cosmic microwave background (CMB). It is known that there are only two types of non-Gaussianities of the gravitaional waves in the most general scalar field theories having second-order field equations. One originates from the inherent non-Gaussianity in general relativity, and the other from a derivative coupling between the Einstein tensor and a kinetic term of the scalar field. We calculate polarization bispectra induced by these non-Gaussianities by transforming them into separable forms by virtue of the Laplace transformation. It is shown that future experiments can detect only the new one if the latter coupling parameter takes an extremely large value, which, however, does not cotradict the current observational data.