Anisotropic Gravitational Waves from Anisotropic Axion Rotation

TL;DR

This paper explores how a brief epoch of axion field dynamics in the early universe can generate a distinctive, anisotropic gravitational wave background that might be detectable in future observations.

Contribution

It introduces a novel scenario where transient axion rotation produces a large-scale anisotropic GW background, evading previous constraints and offering new observational signatures.

Findings

Generated a strong stochastic GW background with large-scale anisotropy.

Showed the scenario can evade bounds on large-scale perturbations.

Predicted GW signals within reach of future detectors.

Abstract

Gravitational waves (GWs) provide a powerful probe of the early universe due to their ability to free-stream across cosmic history. We study GW production in a compelling scenario where a rotating axion(-like) field becomes relevant for a brief period in the early universe before transitioning into a kination fluid and rapidly dissipating its energy through cosmic expansion. During this short epoch, the curvature perturbation can be predominantly sourced by the rotating axion and may significantly exceed the adiabatic component. Moreover, axion field perturbations grow on superhorizon scales during this phase. These effects can generate a strong stochastic background of induced GWs. This GW background also exhibits a pronounced large-scale anisotropy inherited from the axion fluctuations, serving as a distinctive signature of the scenario. Importantly, the transient nature of axion relevance enables this scenario to evade stringent bounds on large-scale perturbations. We analyze various observational constraints and find that both the amplitude and anisotropy of the resulting GW signal could be accessible to future detectors.