Low frequency tail of gravitational wave spectra from hydromagnetic turbulence

TL;DR

This paper models the low-frequency tail of gravitational wave spectra generated by hydromagnetic turbulence in the early Universe, highlighting differences between helical and nonhelical magnetic fields that could help distinguish their origins.

Contribution

It introduces a simple model for GW spectra from decaying MHD turbulence that aligns with numerical simulations and reveals spectral shape differences based on magnetic helicity.

Findings

The model reproduces numerical simulation results for GW spectra.

Spectral shapes differ between helical and nonhelical magnetic fields.

Differences in GW spectra can help identify magnetic field properties in the early Universe.

Abstract

Hydrodynamic and magnetohydrodynamic turbulence in the early Universe can drive gravitational waves (GWs) and imprint their spectrum onto that of GWs, which might still be observable today. We study the production of the GW background from freely decaying magnetohydrodynamic turbulence from helical and nonhelical initial magnetic fields. To understand the produced GW spectra, we develop a simple model on the basis of the evolution of the magnetic stress tensor. We find that the GW spectra obtained in this model reproduce those obtained in numerical simulations if we consider the detailed time evolution of the low-frequency tail of the stress spectrum from numerical simulations. We also show that the shapes of the produced GW frequency spectra are different for helical and nonhelical cases for the same initial magnetic energy spectra. Such differences can help distinguish helical and nonhelical initial magnetic fields from a polarized background of GWs -- especially when the expected circular polarization cannot be detected directly.