CAMB: calculating the evolution of delta_photon with respect to the redshift

[Rui Xu]

Hi,

I calculated the evolution of delta_photon with respect to the redshift for given values of k using CAMB 1.5.3. An example of the result is shown in the attached figure. At about z=400, the solution shown in the figure has a sudden change, from oscillation behaviour for z>400 changing to an almost flat evolution for z<400 with a quick drop at z=0. I don't understand how this flat evolution part for z<400 (as well as the quick drop at z=0) is obtained.

I've tried to write some code to solve the system of Boltzmann equations together with the Einstein field equations. I can recover the oscillation part of the evolution of delta_photon, but cannot get the sudden change to the flat evolution for z<400. Maybe for this z<400 region, the solution isn't actually obtained by solving the set of equations? I would really appreciate it if anyone can throw some light on it.

Thanks!
Rui

[Antony Lewis]

It's an approximation, described in the CAMB notes for CAMB implementation and https://arxiv.org/pdf/1104.2933

[Rui Xu]

I see. Thanks very much!