Circular Polarization of Gravitational Waves from Early-Universe Helical
  Turbulence
  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.
