[CAMB] parameters for neutrinos

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Minji Oh
Posts: 6
Joined: February 05 2015
Affiliation: UST/KASI

[CAMB] parameters for neutrinos

Post by Minji Oh » February 05 2015

Dear All,

I have a question about neutrino parameters in parameter file for camb.
(Recently, I downloaded the most recent version of camb.)

I thought that, if omnuh2=0.0, then no matter which numbers I use for the following parameter:
nu_mass_eigenstates
massive_neutrinos
nu_mass_fractions
nu_mass_degeneracies,
there is no change in P(k) matter power spectrum, because there is nothing to be distributed for each massive neutrino.

But, when I changed the values for these parameters like this:
nu_mass_eigenstates = 1 -> 0
massive_neutrinos = 1. -> 0.
nu_mass_fractions = 1. -> 0.
nu_mass_degeneracies = 1. -> 0.
with fixing omnuh2=0.0 and massless_neutrino = 0.0,
I found somehow big difference, where the first trial gives more suppression on small scales.
(During this comparison, share_delta_neff = T and F gave the same result.)

Just in case, here is my parameter file.
/cosmo_files.php?file=Minji_Oh/ ... shareT.ini



I checked several questions on neutrinos parameters in this forum and saw the comments. From the comments and within my understanding, I think massive_neutrino is not affected by massless_neutrino.

One thing among the comments which is not clear for me was "Massless neutrinos are always calculated". This may be the reason I cannot solve the above problem, but I'm not sure.

Can anybody give me some comments about this?
Which one is the right result among the above results?

Thanks in advance,
Minji Oh

Antony Lewis
Posts: 1941
Joined: September 23 2004
Affiliation: University of Sussex
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Re: [CAMB] parameters for neutrinos

Post by Antony Lewis » February 05 2015

You need to keep the total number of massive and massless neutrinos the same, otherwise you are changing N_eff. Setting the mass to zero just makes the massive neutrinos have zero mass, it doesn't remove them.

Minji Oh
Posts: 6
Joined: February 05 2015
Affiliation: UST/KASI

[CAMB] parameters for neutrinos

Post by Minji Oh » February 13 2015

Dear Antony Lewis,

Thank you for your reply~!

Sincerely,
Minji

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