A lower bound on the primordial black hole merger rate

We derive a lower bound on the merger rate of primordial black hole (PBH) binaries by estimating the maximal fraction of binaries that were perturbed between formation in the early Universe and merger, and computing a conservative merger rate of perturbed binaries. This implies robust constraints on the PBH abundance in the range 1–100M⊙. We further show that LIGO/Virgo design sensitivity has the potential to reach the PBH mass range of 10-2–103M⊙. The constraint from the merger rate of perturbed binaries is stronger if PBHs are initially spatially clustered.