Gravitational waves from primordial black holes collisions in binary   systems
  It was shown by (Nakamura et al. 1997), (Ioka et al. 1998), and (Sasaki et al. 2016) that primordial black holes (PBHs) binaries can form effectively at the cosmological stage of radiation dominance, and the merge of the PBHs in pairs can explain the gravitational wave burst GW150914. In this paper, the model is re-examined by considering the effect of inflationary dark matter density perturbations which produce additional tidal forces. As a result, the merge rate of PBHs binaries and the corresponding rate of the gravitational bursts are suppressed by the factor $\sim1.5-2$ in comparison with previous calculations. This rate matches the LIGO data if the PBHs constitute the $f\sim5\times10^{-4}-5\times10^{-3}$ fraction of dark matter. 
Yu.N. Eroshenko
astro-ph.CO astro-ph.HE