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.

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