Primordial backgrounds of relic gravitons
  The diffuse backgrounds of relic gravitons with frequencies ranging between
the aHz band and the GHz region encode the ultimate information on the primeval
evolution of the plasma and on the underlying theory of gravity well before the
electroweak epoch. While the temperature and polarization anisotropies of the
microwave background radiation probe the low-frequency tail of the graviton
spectra, during the next score year the pulsar timing arrays and the wide-band
interferometers (both terrestrial and hopefully space-borne) will explore a
much larger frequency window encompassing the nHz domain and the audio band.
The salient theoretical aspects of the relic gravitons are reviewed in a
cross-disciplinary perspective touching upon various unsettled questions of
particle physics, cosmology and astrophysics.
