Viscous absorption of ultra-high-frequency gravitons
  The high-frequency gravitons can be absorbed by the first and second
viscosities of the post-inflationary plasma as the corresponding wavelengths
reenter the Hubble radius prior to big-bang nucleosynthesis. When the total
sound speed of the medium is stiffer than radiation the rate of expansion still
exceeds the shear rate but the bulk viscosity is not negligible. Depending on
the value of the entropy density at the end of inflation the spectral energy
density of the relic gravitons gets modified in comparison with the inviscid
result when the frequency ranges between the kHz band and the GHz region. In
the nHz domain the spectrum inherits a known suppression due to neutrino
free-streaming but also a marginal spike potentially caused by a sudden
outbreak of the bulk viscosity around the quark-hadron phase transition, as
suggested by the hadron spectra produced in the collisions of heavy ions.
