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Growth factor from transfer_out.dat with CAMB
 
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Giacomo Baso



Joined: 26 May 2016
Posts: 3
Affiliation: University of Padua - Department of Physics and Astronomy

PostPosted: October 06 2016  Reply with quote

I'm a PhD student in computational cosmology but I'm new to CAMB, so forgive me if the question is trivial.
I have a need to compute the growth factor with CAMB, in preparation for a work with massive neutrinos, for which I don't have a analytical approximation in the transition region ~ knr.

From the README the output file <root>_transfer_out.dat contains the values of Delta_X (delta rho_X)/rho_X at the desired redshift, so it should be possible to run CAMB at a set of different redshifts and just take the ratio of Delta_tot(z=0) and Delta_tot(z) to obtain the growth factor at redshift z.
In a pure ΛCDM universe this ratio should not depend on the scale k. However, I find something like this:

The red horizontal line is the prediction from the linear theory, using the approximation from Lahav & Suto (2004): D(a) ~ a g(a)

High-k cut-off notwithstanding, there are two different values at different scales, neither of which is equal to the linear prediction. The difference gets bigger at higher redshifts, i.e. Δ D ~ 0.2 at z=20 and Δ D ~ 6 at z=99. Am I missing something? Or is there some physical effects at play here, or maybe a numerical instability?

I should also mention that CAMB gives me the following error every time I run it:
Code:
Note: The following floating-point exceptions are signalling: IEEE_UNDERFLOW_FLAG

The code was compiled with gfortran 5.4.0

I have the last version of CAMB, with the following parameter file:
Code:
DEFAULT(params.ini) # Version 0.1.2
#output_root is prefixed to output file names
output_root = planck/05.00
#What to do
get_scalar_cls = T
get_vector_cls = F
get_tensor_cls = F
get_transfer   = T
do_lensing     = F
#Main cosmological parameters, neutrino masses are assumed degenerate
use_physical   = F
omega_baryon   = 0.04825
omega_cdm      = 0.25886
omega_lambda   = 0.69289
omega_neutrino = 0
hubble         = 67.77
#Transfer function settings, transfer_kmax=0.5 is enough for sigma_8
#transfer_k_per_logint=5 samples fixed spacing in log-k
transfer_high_precision = T
transfer_kmax           = 100
transfer_k_per_logint   = 5
transfer_num_redshifts  = 1
transfer_redshift(1)    = 05.00
#which variable to use for defining the matter power spectrum and sigma8
#main choices are 2: CDM, 7: CDM+baryon+neutrino, 8: CDM+baryon, 9: CDM+baryon+neutrino+de perts
transfer_power_var = 8
#Increase accuracy_boost to decrease time steps, use more k values, etc.
#Decrease to speed up at cost of worse accuracy. Suggest 0.8 to 3.
accuracy_boost          = 2


Thanks!
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1183
Affiliation: University of Sussex

PostPosted: October 07 2016  Reply with quote

See

http://cosmocoffee.info/viewtopic.php?p=5385
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Giacomo Baso



Joined: 26 May 2016
Posts: 3
Affiliation: University of Padua - Department of Physics and Astronomy

PostPosted: October 12 2016  Reply with quote

Thanks for the reference, I'm not an expert on cosmological gauges but I see that's a physical effect that I should include (although it's probably outside the scale range we are interested with).

I'm still a little worried by the underflow exception, however. What's the cause of it?
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1183
Affiliation: University of Sussex

PostPosted: October 12 2016  Reply with quote

That's harmless and fairly normal (if something is exponentially small, it's fine to underflow it to zero).
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Giacomo Baso



Joined: 26 May 2016
Posts: 3
Affiliation: University of Padua - Department of Physics and Astronomy

PostPosted: October 12 2016  Reply with quote

Got it, thank you.
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