Merger rate of supermassive primordial black hole binaries

The probability that the primordial black hole (PBH) binaries formed in the early Universe can be affected by the Hubble expansion of background, which is non-negligible when the number density of PBHs is very low (it is actually this case for supermassive PBHs). In this paper, taking into account the effect of cosmic expansion on the comoving distance of PBH pairs, we worked out the merger rate of PBHs with any extended mass function. The torques by all PBHs and linear density perturbations are also considered. It is found that the merger rate of PBH, $M\gtrsim 10^6M_\odot$, binaries is significantly lower for $f_\text{pbh}\lesssim 0.01$ than expected.