Polarization of gravitational waves from helical MHD turbulent sources

Abstract

We use direct numerical simulations of decaying primordial hydromagnetic turbulence with helicity to compute the resulting gravitational wave (GW) production and its degree of circular polarization. We find a clear dependence of the polarization of the resulting GWs on the fractional helicity of the turbulent source and we show that driven magnetic fields produce GWs more efficiently than magnetic fields that are initially present, leading to larger spectral amplitudes. The helicity does not have a huge impact on the maximum spectral amplitude in any of the two types of turbulence considered. However, the GW spectrum at wave numbers away from the peak becomes smaller for larger values of the magnetic fractional helicity. The degree of circular polarization approaches zero at frequencies below the peak, and reaches its maximum at the peak. At higher frequencies, it stays finite if the magnetic field is initially present, and it approaches zero if it is driven. We predict that the spectral peak of the GW signal can be detected by LISA if the turbulent energy density is at least $\sim 3\%$ of the radiation energy density, and the characteristic scale is a hundredth of the horizon at the electroweak scale. We show that the resulting GW polarization is unlikely to be detectable by the anisotropies induced by our proper motion in the dipole response function of LISA. Such signals can, however, be detectable by cross-correlating data from the LISA-Taiji network for turbulent energy densities of $\sim 5\%$, and fractional helicity of 0.5 to 1. Second-generation space-based GW detectors, such as BBO and DECIGO, would allow for the detection of a larger range of the GW spectrum and smaller amplitudes of the magnetic field.

49 pages, 18 figures, 1 table, accepted in JCAP

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• The Scalar Singlet Extension of the Standard Model: Gravitational Waves versus Baryogenesis
• Generation of gravitational waves from freely decaying turbulence
• The Lunar Gravitational-wave Antenna: Mission Studies and Science Case
• Droplet collapse during strongly supercooled transitions
• Big Bang Nucleosynthesis Limits and Relic Gravitational Waves Detection Prospects
• Gravitational waves from first-order phase transitions: from weak to strong
• Low frequency tail of gravitational wave spectra from hydromagnetic turbulence
• Present and future of CosmoLattice
• Modified propagation of gravitational waves from the early radiation era
• Simulations of helical inflationary magnetogenesis and gravitational waves
• Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band
• LISA and $\gamma$-ray telescopes as multi-messenger probes of a first-order cosmological phase transition
• Alternative LISA-TAIJI networks: detectability of the isotropic stochastic gravitational wave background