On the Proper Use of the Reduced Speed of Light Approximation
  I show that the Reduced Speed of Light (RSL) approximation, when used
properly (i.e. as originally designed - only for the local sources but not for
the cosmic background), remains a highly accurate numerical method for modeling
cosmic reionization. Simulated ionization and star formation histories from the
"Cosmic Reionization On Computers" (CROC) project are insensitive to the
adopted value of the reduced speed of light for as long as that value does not
fall below about 10% of the true speed of light. A recent claim of the failure
of the RSL approximation in the Illustris reionization model appears to be due
to the effective speed of light being reduced in the equation for the cosmic
background too, and, hence, illustrates the importance of maintaining the
correct speed of light in modeling the cosmic background.
