Primordial-black-hole mergers in dark-matter spikes
  It has been suggested that primordial black holes (PBHs) of roughly 30 solar
masses could make up the dark matter and if so, might account for the recent
detections by LIGO involving binary black holes in this mass range. It has also
been argued that the super-massive black holes (SMBHs) that reside at galactic
centers may be surrounded by extremely-dense dark-matter (DM) spikes. Here we
show that the rate for PBH mergers in these spikes may well exceed the merger
rate considered before in galactic dark-matter halos, depending on the
magnitudes of two competing effects on the DM spikes: depletion of PBHs due to
relaxation and replenishment due to PBHs in loss cone. This may provide a
plausible explanation for the current rate of detection of mergers of
30-solar-mass black holes, even if PBHs make up a subdominant contribution to
the dark matter. The gravitational-wave signals from such events will always
originate in galactic centers, as opposed to those from halos, which are
expected to have little correlation with luminous-galaxy positions.
