Gravitational wave background as a probe of the primordial black hole   abundance
  Formation of significant number of primordial black holes (PBHs) is realized if and only if primordial density fluctuations have a large amplitude, which means that tensor perturbations generated from these scalar perturbations as a second order effect are also large and comparable to the observational data. We show that pulsar timing observation could find/rule out PBHs with \sim 10^2 M_solar which are considered as a candidate of intermediate-mass black holes and that PBHs with mass range 10^{20-26} g, which serves as a candidate of dark matter, may be probed by future space-based laser interferometers and atomic interferometers. 
Ryo Saito, Jun'ichi Yokoyama