Flat spectra of cosmic gravitons in the nHz and audio bands
  The spectra of the relic gravitons are customarily normalized in the
low-frequency domain where the signal of the concordance paradigm is expected
to peak and this is why their contribution to the temperature and polarization
anisotropies of the microwave background is only described by the tensor to
scalar ratio. If the consistency relations are broken, the same strategy is
accomplished by introducing the tensor spectral index as a further independent
parameter. When the dominant component of the spectral energy density is
distributed for frequencies much larger than the aHz, the logic behind this
conventional approach is much less compelling. The improved bounds in the audio
band and the current data from the pulsar timing arrays in the nHz region
motivate a new strategy for the absolute normalization of the cosmic background
of relic gravitons. After introducing a general four-dimensional action for the
analysis of the relic gravitons the new approach is illustrated in the case of
conventional and unconventional inflationary models.
