Cosmic microwave background limits on accreting primordial black holes
  Interest in the idea that primordial black holes (PBHs) might comprise some
or all of the dark matter has recently been rekindled following LIGO's first
direct detection of a binary-black-hole merger. Here we revisit the effect of
accreting PBHs on the cosmic microwave background (CMB) frequency spectrum and
angular temperature/polarization power spectra. We compute the accretion rate
and luminosity of PBHs, accounting for their suppression by Compton drag and
Compton cooling by CMB photons. We estimate the gas temperature near the
Schwarzschild radius, and hence the free-free luminosity, accounting for the
cooling resulting from collisional ionization when the background gas is mostly
neutral. We account approximately for the velocities of PBHs with respect to
the background gas. We provide a simple analytic estimate of the efficiency of
energy deposition in the plasma. We find that the spectral distortions
generated by accreting PBHs are too small to be detected by FIRAS, as well as
by future experiments now being considered. We analyze Planck CMB temperature
and polarization data and find, under our most conservative hypotheses, and at
the order-of-magnitude level, that they rule out PBHs with masses >~ 10^2 M_sun
as the dominant component of dark matter.
