Statistically-Anisotropic Tensor Bispectrum from Inflation

TL;DR

This paper proposes a model of anisotropic inflation involving a gauge field that generates distinctive, statistically anisotropic tensor non-Gaussianities in the cosmic microwave background, potentially observable with future experiments.

Contribution

It introduces a novel mechanism for generating anisotropic tensor bispectra during inflation through gauge field coupling, predicting unique polarization and multipole features.

Findings

Tensor bispectrum is strongly red-tilted.

Distinctive statistical anisotropies including higher multipole moments.

Dominant polarization mode combinations with amplitude dependence on multipole moments.

Abstract

We develop a possibility of generating tensor non-Gaussianity in a kind of anisotropic inflation, where a $U(1)$ gauge field is kinetically coupled to a spectator scalar field. Owing to this coupling, the coherent mode of the electric field appears and softly breaks the isotropy of the Universe. We compute the bispectrum of linearly-polarized tensor perturbations sourced by the gauge field and find that it is strongly red-tilted and has distinctive statistical anisotropies including higher-order multipole moments. Interestingly, the tensor bispectra with the specific combinations of linear polarization modes are dominant, and their amplitudes depend on the different sets of multipole moments. This new type of statistically-anisotropic tensor non-Gaussianity can be potentially testable with the upcoming cosmic microwave background B-mode polarization experiments.