Primordial black holes in interferometry
  If there is a substantial population of black holes distributed in space,
light rays passing in their vicinity will acquire random phases, leading to a
loss of coherence or "visibility" in interferometric observations. We give a
formula for this effect and consider its application, particularly for
observations on the CMB. We find that with earth-size baselines a non-zero
"visibility" on the CMB limits the mass of possible primordial black holes,
making up the dark matter, to approximately $M/M_\odot \leq 10{-1}$. Longer
baselines lead to more stringent limits and in principle with extreme lengths,
the method could possibly find positive evidence for primordial black holes. In
this case, however, all other kinds of phase averaging would have to be
constrained or eliminated.
