On the Proper Use of the Reduced Speed of Light Approximation

TL;DR

The paper clarifies that the Reduced Speed of Light approximation remains accurate for cosmic reionization modeling when used correctly, emphasizing the importance of applying it only to local sources and not the cosmic background.

Contribution

It demonstrates the proper application of the RSL approximation and clarifies misconceptions about its failure in certain reionization models.

Findings

RSL remains accurate if not below 10% of the true speed of light

Incorrect application of RSL to the cosmic background causes apparent failures

Proper use of RSL yields results consistent with full speed of light simulations

Abstract

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.