Rapid Merger of Binary Primordial Black Holes: an Implication for
  GW150914
  We propose a new scenario for the evolution of a binary of primordial black
holes (PBHs). We consider a dynamical friction by ambient dark matter,
scattering of dark matter particles with a highly eccentric orbit besides the
standard two-body relaxation process to refill the loss cone, and interaction
between the binary and a circumbinary disk, assuming that PBHs do not
constitute the bulk of dark matter. Binary PBHs lose the energy and angular
momentum by these processes, which could be sufficiently efficient for a
typical configuration. Such a binary coalesces due to the gravitational wave
emission in a time scale much shorter than the age of the universe. We estimate
the density parameter of the resultant gravitational wave background.
Astrophysical implication concerning the formation of intermediate-mass to
supermassive black holes is also discussed.
