Circular Polarization of Gravitational Waves from Early-Universe Helical Turbulence

TL;DR

This paper uses numerical simulations to analyze the circular polarization of gravitational waves generated by helical turbulence in the early Universe, revealing dependence on initial conditions and potential for detecting early parity violations.

Contribution

It provides the first detailed numerical analysis of gravitational wave polarization from helical turbulence, considering various initial conditions and turbulence types.

Findings

Spectral polarization degree peaks at twice the source wavenumber.

Maximum polarization reaches nearly 100% in fully helical decaying turbulence.

Detection of polarization can reveal early Universe parity violation mechanisms.

Abstract

We perform direct numerical simulations to compute the net circular polarization of gravitational waves from helical (chiral) turbulent sources in the early Universe for a variety of initial conditions, including driven (stationary) and decaying turbulence. We investigate the resulting gravitational wave signal assuming different turbulent geneses such as magnetically or kinetically driven cases. Under realistic physical conditions in the early Universe we compute numerically the wave number-dependent polarization degree of the gravitational waves. We find that the spectral polarization degree strongly depends on the initial conditions. The peak of the spectral polarization degree occurs at twice the typical wavenumber of the source, as expected, and for fully helical decaying turbulence, it reaches its maximum of nearly 100\% {\it only} at the peak. We determine the temporal evolution of the turbulent sources as well as the resulting gravitational waves, showing that the dominant contribution to their spectral energy density happens shortly after the activation of the source. Only through an artificially prolonged decay of the turbulence can further increase of the gravitational wave amplitude be achieved. We estimate the detection prospects for the net polarization, arguing that its detection contains {\it clean} information (including the generation mechanisms, time, and strength) about the sources of possible parity violations in the early Universe.