Strong constraints on clustered primordial black holes as dark matter
  The idea of dark matter in the form of primordial black holes has seen a
recent revival triggered by the LIGO detection of gravitational waves from
binary black hole mergers. In this context, it has been argued that a large
initial clustering of primordial black holes can help alleviate the strong
constraints on this scenario. In this work, we show that on the contrary, with
large initial clustering the problem is exacerbated and constraints on
primordial black hole dark matter become overwhelmingly strong.
