CMB bounds on disk-accreting massive Primordial Black Holes
  Stellar-mass Primordial Black Holes (PBH) have been recently reconsidered as
a Dark Matter (DM) candidate, after the aLIGO discovery of several binary BH
mergers with masses of tens of $M_\odot$. Matter accretion on such massive
objects leads to the emission of high-energy photons, capable of altering the
ionization and thermal history of the universe. This in turn affects the
statistical properties of the cosmic microwave background (CMB) anisotropies.
Previous analyses have assumed spherical accretion. We argue that this
approximation likely breaks down and that an accretion disk should form in the
dark ages. Using the most up-to-date tools to compute the energy deposition in
the medium, we derive constraints on the fraction of DM in PBH. Provided that
disks form early on, even under conservative assumptions for accretion, these
constraints exclude a monochromatic distribution of PBH with masses above $\sim
2\, M_\odot$ as the dominant form of DM. The bound on the median PBH mass gets
more stringent if a broad, log-normal mass function is considered. A deepened
understanding of non-linear clustering properties and BH accretion disk physics
would permit an improved treatment and possibly lead to more stringent
constraints.
