Question: implementing scalar field (w≈0, tiny sound speed) instead of CDM

[joeyharper52]

Hi, I’m trying to run a test in CLASS where CDM is replaced by a scalar field behaving like pressureless matter.

Setup:

• Ω_cdm = 0
• scalar field with w ≈ 0
• sound speed c_s^2 ≈ 1e-10 at recombination

Goal:
Check how the CMB TT spectrum changes (especially the 3rd peak).

I don’t have experience modifying CLASS yet.

Would this be best implemented using:

1. fluid approximation?
2. k-essence module?
3. or requires modifying perturbation equations?

Any guidance would be really appreciated.