Planck Constraint on Relic Primordial Black Holes
  We investigate constraints on the abundance of primordial black holes (PBHs)
in the mass range 10^{15}-10^{17} g using data from the Cosmic Microwave
Background (CMB) and MeV extragalactic gamma-ray background (EGB). Hawking
radiation from PBHs with lifetime greater than the age of the universe leaves
an imprint on the CMB through modification of the ionization history and the
damping of CMB anisotropies. Using a model for redshift dependent energy
injection efficiencies, we show that a combination of temperature and
polarization data from Planck provides the strongest constraint on the
abundance of PBHs for masses \sim 10^{15}-10^{16} g, while the EGB dominates
for masses \gtrsim 10^{16} g. Both the CMB and EGB now rule out PBHs as the
dominant component of dark matter for masses \sim 10^{16}-10^{17} g. Planned
MeV gamma-ray observatories are ideal for further improving constraints on PBHs
in this mass range.
