Statistically Anisotropic Tensor Modes from Inflation

TL;DR

This paper investigates how a spectator scalar field coupled to a U(1) gauge field during inflation can produce statistically anisotropic and linearly polarized gravitational waves with higher multipole moments, exceeding vacuum fluctuation levels.

Contribution

It introduces a model where sourced gravitational waves exhibit statistical anisotropy and polarization, with a specific higher multipole pattern independent of model parameters.

Findings

Sourced GWs can surpass vacuum fluctuation levels.

GWs exhibit statistical anisotropy and linear polarization.

Power spectrum has higher multipole moments proportional to a specific angular function.

Abstract

We consider the inflationary universe with a spectator scalar field coupled to a $U(1)$ gauge field and calculate curvature perturbation and gravitational waves (GWs). We find that the sourced GWs can be larger than the one from vacuum fluctuation and they are statistically anisotropic as well as linearly polarized. The GW power spectrum acquires higher multipole moments as $\mathcal{P}_h \propto (1-\cos^2\theta+\cos^4\theta-\cos^6\theta)$ irrespective of the model parameters.