Electromagnetic probes of primordial black holes as dark matter
  The LIGO discoveries have rekindled suggestions that primordial black holes
(BHs) may constitute part to all of the dark matter (DM) in the Universe. Such
suggestions came from 1) the observed merger rate of the BHs, 2) their unusual
masses, 3) their low/zero spins, and 4) also from the independently uncovered
cosmic infrared background (CIB) fluctuations signal of high amplitude and
coherence with unresolved cosmic X-ray background (CXB). Here we summarize the
prospects to resolve this important issue with electromagnetic observations
using the instruments and tools expected in the 2020's. These prospects appear
promising to make significant, and potentially critical, advances. We
demonstrate that in the next decade, new space- and ground-borne
electromagnetic instruments, combined with concurrent theoretical efforts,
should shed critical light on the long-considered link between primordial BHs
and DM. Specifically the new data and methodologies under this program will
involve: I) Probing with high precision the spatial spectrum of
source-subtracted CIB with Euclid and WFIRST, and its coherence with unresolved
cosmic X-ray background using eROSITA and Athena, II) Advanced searches for
microlensing of Galactic stars by the intervening Galactic Halo BHs with OGLE,
Gaia, LSST and WFIRST, III) Supernovae (SNe) lensing in the upcoming surveys
with WFIRST, LSST and also potentially with Euclid and JWST, IV) Advanced
theoretical work to understand the details of PBH accretion and evolution and
their influence on cosmic microwave background (CMB) anisotropies in light of
the next generation CMB experiments, V) Better new samples and theoretical
understanding involving stability and properties of ultra faint dwarf galaxies,
pulsar timing, and cosmological quasar lensing.
