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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

Posted: October 06 2016

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!
Antony Lewis

Joined: 23 Sep 2004
Posts: 1352
Affiliation: University of Sussex

 Posted: October 07 2016
Giacomo Baso

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

 Posted: October 12 2016 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?
Antony Lewis

Joined: 23 Sep 2004
Posts: 1352
Affiliation: University of Sussex

 Posted: October 12 2016 That's harmless and fairly normal (if something is exponentially small, it's fine to underflow it to zero).
Giacomo Baso

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

 Posted: October 12 2016 Got it, thank you.
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