Abstract
Hydrodynamic and magnetohydrodynamic (MHD) 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 MHD turbulence for 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 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.
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