## CAMB: computing logarithmic growth rate "f"

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Andreu Font-Ribera
Posts: 2
Joined: October 17 2016
Affiliation: UCL

### CAMB: computing logarithmic growth rate "f"

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
Posts: 1618
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: CAMB: computing logarithmic growth rate "f"

You can more directly use

(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
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
or use the general matter power spectrum at specific requested redshifts