Computers and software • Clarification on Perturbation Variables in CAMB's `equations.f90`

Hello everyone,

I'm currently studying CAMB's perturbation equations (in equations.f90) and have a few clarifying questions:

1. The Fractional Density Perturbations:
What exactly are the fractional density perturbations (clxc, clxb, clxg, clxr, etc.) physically representing, and what is their significance? Similarly, what's the physical interpretation and role of the total perturbation variable dgrho?

2. The Heat Flux (dgq):
What is the "heat flux", labeled dgq in CAMB, and what physical role does it play?

3. Variable Assignments (e.g., clxb=ay(ix_clxb)):
What are the meaning and practical purpose of assignments like clxb=ay(ix_clxb) and vb=ay(ix_vb) within, for example, the `derivs` subroutine?

4. Definition of the Total Transfer function (line 2704):
In line 2704, there's the assignment: EV%OutputTransfer(Transfer_tot_de) = dgrho/grho_matter.
My question here is: Since dgrho includes dark energy perturbations, why is it normalized by grho_matter, which excludes dark energy?

Thank you in advance for your insights and help!

Best regards,
Dror

Statistics: Posted by Dror Berechya — March 25 2025

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