Cosmic Reionization on Computers: Physical Origin of Long Dark Gaps in
  Quasar Absorption Spectra
  I explore the properties of "dark gaps" - regions in quasar absorption
spectra without significant transmission - with several simulations from the
Cosmic Reionization On Computers (CROC) project. CROC simulations in largest
available boxes (120 cMpc) come close to matching both the distribution of mean
opacities and the frequency of dark gaps, but alas not in the same model: the
run that matches the mean opacities fails to contain enough dark gaps and vice
versa.:( Never-the-less, the run that matches the dark gap distributions serves
as a counter-example to claims in the literature that the dark gap statistics
requires a late end to reionization - in that run reionization ends at z=6.7
(likely too early).
  While multiple factors contribute to the frequency of large dark gaps in the
simulations, the primary factor that controls the overall shape of the dark gap
distribution is the ionization level in voids - the lowest density regions
produce the highest transmission spikes that terminate long gaps. As the
result, the dark gap distribution correlates strongly with the fraction of the
spectrum above the gap detection threshold, the observed distribution is
matched by the simulation in which this fraction is 2%. Hence, the gap
distribution by itself does not constrain the timing of reionization.
