Relic gravitational waves from light primordial black holes
  The energy density of relic gravitational waves (GWs) emitted by primordial
black holes (PBHs) is calculated. We estimate the intensity of GWs produced at
quantum and classical scattering of PBHs, the classical graviton emission from
the PBH binaries in the early Universe, and the graviton emission due to PBH
evaporation. If nonrelativistic PBHs dominated the cosmological energy density
prior to their evaporation, the probability of formation of dense clusters of
PBHs and their binaries in such clusters would be significant and the energy
density of the generated gravitational waves in the present day universe could
exceed that produced by other known mechanisms. The intensity of these
gravitational waves would be maximal in the GHz frequency band of the spectrum
or higher and makes their observation very difficult by present detectors but
also gives a rather good possibility to investigate it by present and future
high frequency gravitational waves electromagnetic detectors. However, the low
frequency part of the spectrum in the range $f\sim 0.1-10$ Hz may be detectable
by the planned space interferometers DECIGO/BBO. For sufficiently long duration
of the PBH matter dominated stage the cosmological energy fraction of GWs from
inflation would be noticeably diluted.
