CAMB: Setting massless neutrino density to zero

[Rui Xu]

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]

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]

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]

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]

Okay, that makes sense. Thanks for the explanation!