Constraints from microlensing experiments on clustered primordial black
  holes
  It has recently been proposed that massive primordial black holes (PBH) could
constitute all of the dark matter, providing a novel scenario of structure
formation, with early reionization and a rapid growth of the massive black
holes at the center of galaxies and dark matter halos. The scenario arises from
broad peaks in the primordial power spectrum that give both a spatially
clustered and an extended mass distribution of PBH. The constraints from the
observed microlensing events on the extended mass function have already been
addressed. Here we study the impact of spatial clustering on the microlensing
constraints. We find that the bounds can be relaxed significantly for
relatively broad mass distributions if the number of primordial black holes
within each cluster is typically above one hundred. On the other hand, even if
they arise from individual black holes within the cluster, the bounds from CMB
anisotropies are less stringent due to the enhanced black hole velocity in such
dense clusters. This way, the window between a few and ten solar masses has
opened up for PBH to comprise the totality of the dark matter.
