The first observations of wide-band interferometers and the spectra of
  relic gravitons
  Stochastic backgrounds of relic gravitons of cosmological origin extend from
frequencies of the order of the aHz up to the GHz range. Since the temperature
and polarization anisotropies constrain the low frequency normalization of the
spectra, in the concordance paradigm the strain amplitude corresponding to the
frequency window of wide-band interferometers turns out to be, approximately,
nine orders of magnitude smaller than the astounding signal recently reported
and attributed to a binary black hole merger. The backgrounds of relic
gravitons expected from the early Universe are compared with the stochastic
foregrounds stemming from the estimated multiplicity of the astrophysical
sources. It is suggested that while the astrophysical foregrounds are likely to
dominate between few Hz and 10 kHz, relic gravitons with frequencies exceeding
100 kHz represent a potentially uncontaminated signal for the next generation
of high-frequency detectors currently under scrutiny.
