## Camb AxionEffectiveFluid dark energy model

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Posts: 3
Joined: May 01 2020
Affiliation: CNRS LAL IJClab

### Camb AxionEffectiveFluid dark energy model

Hi !

I'm trying to use the AxionEffectiveFluid dark energy model from CAMB to generate spectra.

class camb.dark_energy.AxionEffectiveFluid[source]
Bases: camb.dark_energy.DarkEnergyModel
Variables:
w_n – (float64)
om – (float64)
a_c – (float64)
theta_i – (float64)

I noticed that varying the om parameter seems to change the value of . But I fixed this parameter while declaring the cosmology parameters. Do you know why the spectra are modified this way ?

difference_om.png (86.41 KiB) Viewed 379 times

Antony Lewis
Posts: 1659
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: Camb AxionEffectiveFluid dark energy model

Theta_MC is an approximation for standard cosmologies. Better to fix thetastar in this case if you want the sound horizon to actually stay constant (see https://camb.readthedocs.io/en/latest/model.html#camb.model.CAMBparams.set_cosmology).

Posts: 3
Joined: May 01 2020
Affiliation: CNRS LAL IJClab

### Re: Camb AxionEffectiveFluid dark energy model

Hi !

I tried to keep thetastar constant, but the two spectra are still moving the same way. I put the script I used in attached files (sorry for the .zip, i could not attach a .py file)
axion_camb.zip

Antony Lewis
Posts: 1659
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: Camb AxionEffectiveFluid dark energy model

If you change the dark energy model, you need to do so *before* calling set_cosmology if you are using cosmomc_theta or thetastar (because the solution for H0 to give required thetastar depends on the dark energy model). Alternatively use camb.set_params, e.g.

Code: Select all

pars = camb.set_params(dark_energy_model='AxionEffectiveFluid', thetastar=...)


Also your fluid om values are probably far too large to get a valid H0 solution, since this model decays rapidly at late times.