Footprint of Two-Form Field: Statistical Anisotropy in Primordial Gravitational Waves

TL;DR

This paper predicts that a two-form field during inflation produces statistically anisotropic gravitational waves with unique spherical moment signatures, potentially detectable by future CMB experiments.

Contribution

It introduces a novel inflationary scenario where a two-form field causes distinctive anisotropic gravitational wave signatures, including specific spherical moments.

Findings

Gravitational waves are statistically anisotropic with non-zero hexadecapole and tetrahexacontapole moments.

Tensor-to-scalar ratio can reach approximately 10^{-3}.

Distinctive signatures are testable in upcoming CMB experiments.

Abstract

We study the observational signatures of two-form field in the inflationary cosmology. In our setup a two-form field is kinetically coupled to a spectator scalar field and generates sizable gravitational waves and smaller curvature perturbation. We find that the sourced gravitational waves have a distinct signature: they are always statistically anisotropic and their spherical moments are non-zero for hexadecapole and tetrahexacontapole, while the quadrupole moment vanishes. Since their amplitude can reach $\mathcal{O}(10^{-3})$ in the tensor-to-scalar ratio, we expect this novel prediction will be tested in the next generation of the CMB experiments.