CAMB: Setting massless neutrino density to zero

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Rui Xu
Posts: 3
Joined: March 19 2024
Affiliation: Peking University

CAMB: Setting massless neutrino density to zero

Post by Rui Xu » March 19 2024

Hi,

I'm trying to get familiar with using CAMB, so I would like to figure out its details with only a simple cosmological model where there are only matter, dark matter, and photons. I managed to remove massive neutrinos by setting mnu=0 in camb.CAMBparams.set_cosmology(). But I couldn't find a parameter that lets me to remove the massless neutrinos. Is it possible to remove the massless neutrino component in using CAMB? If yes, how can it be done?

Thanks!
Rui

Antony Lewis
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Affiliation: University of Sussex
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Re: CAMB: Setting massless neutrino density to zero

Post by Antony Lewis » March 19 2024

You can try setting nnu=0 (not sure it will work; not you also change BBN predictions if messing with number of neutrinos).
Antony

Rui Xu
Posts: 3
Joined: March 19 2024
Affiliation: Peking University

Re: CAMB: Setting massless neutrino density to zero

Post by Rui Xu » March 20 2024

Thank you for the reply. Yes, I tried to set nnu=0 or/and standard_neutrino_neff=0, but I still get nonzero delta_neutrino using get_time_evolution(). I guess the effect of neutrino is small and I probably can just ignore it at the moment.

Regards,
Rui

Antony Lewis
Posts: 1943
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

Re: CAMB: Setting massless neutrino density to zero

Post by Antony Lewis » March 20 2024

It will still evolve the fractional perturbations, but the contribution to the evolution should go to zero (because weighed by the background densities)

Rui Xu
Posts: 3
Joined: March 19 2024
Affiliation: Peking University

Re: CAMB: Setting massless neutrino density to zero

Post by Rui Xu » March 21 2024

Okay, that makes sense. Thanks for the explanation!

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