Secondary graviton spectra and waterfall-like fields
  The secondary spectra of the gravitons induced by a waterfall-like field are
computed and the general bounds on the spectral energy density of the tensor
modes of the geometry are translated into explicit constraints on the amplitude
and slope of the waterfall spectrum. The obtained results are compared with the
primary gravitational wave spectra of the concordance model and of its
neighboring extensions as well as with the direct Ligo/Virgo bounds on
stochastic backgrounds of relic gravitons. Space-borne interferometers (such as
Lisa, Bbo, Decigo) seem to be less relevant but their potential implications
are briefly outlined.
