CAMB: computing logarithmic growth rate "f"

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Andreu Font-Ribera
Posts: 2
Joined: October 17 2016
Affiliation: UCL

CAMB: computing logarithmic growth rate "f"

Post by Andreu Font-Ribera » July 14 2020

Hi - I was looking for a way to compute the logarithmic growth rate "f(z)" , and Pablo Lemos pointed me to the following function in CAMB.

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 sigma2_vdelta_8 – array of v-delta8 correlation, so sigma2_vdelta_8/sigma_8 can define growth 
First, I think the documentation is misleading, I would consider growth to be

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 sigma2_vdelta_8/sigma_8**2 
Second, I couldn't figure out what "fluid" was being used here. Is this CMB+baryons (fluid=8), or does it include neutrinos (fluid=8)? I guess whenever we have massive neutrinos, "f" becomes ill-defined, and one should use f(z,k). Is there a standard / efficient way to compute f(z,k), specifying a particular fluid?

Thanks,
Andreu

Antony Lewis
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Joined: September 23 2004
Affiliation: University of Sussex
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Re: CAMB: computing logarithmic growth rate "f"

Post by Antony Lewis » July 15 2020

You can more directly use
https://camb.readthedocs.io/en/latest/results.html?highlight=get_fsigma8#camb.results.CAMBdata.get_fsigma8

(the other doc string should say "growth parameter f*sigma_8"). By default it is using config.transfer_power_var (defaults to baryons+CDM+neutrinos) and the CDM velocity. More generally you can use
https://camb.readthedocs.io/en/latest/results.html?highlight=get_sigmaR#camb.results.CAMBdata.get_sigmaR
with whatever variables you like.

However, in general it may be best to try to calculate the actual physical quantity being measured rather some rather ill-defined growth parameter.

For mode evolution you can use
https://camb.readthedocs.io/en/latest/results.html?highlight=get_redshift_evolution#camb.results.CAMBdata.get_redshift_evolution
or use the general matter power spectrum at specific requested redshifts
https://camb.readthedocs.io/en/latest/results.html?highlight=get_redshift_evolution#camb.results.CAMBdata.get_linear_matter_power_spectrum

Andreu Font-Ribera
Posts: 2
Joined: October 17 2016
Affiliation: UCL

Re: CAMB: computing logarithmic growth rate "f"

Post by Andreu Font-Ribera » July 16 2020

Thanks, I'll take a look at it and report back.

Cheers,
Andreu

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