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How to calculate basic cosmological parameters

Page history last edited by Tom 13 years, 4 months ago

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How to calculate basic comological parameters


Once IDL and iCosmo are set up, you can calculate most cosmological parameters in 2 simple steps (an explanation is given below):


; (1) set fiducial parameters:



; (2) create cosmology parameters structure:




How to plot basic cosmological parameters


Once these cosmological parameters are calculated, you can easily plot them.  To plot the angular diameter distance and the linear power spectrum at redshift z=0.5 for example:         


; (3) To plot the angular diameter distance as a function of z, for a given cosmology

plt_cosmo, cosmo, 'z', 'da', linestyle=2


; (4) To plot the linear power spectrum at redshift z = 0.5, P(k, z=0.5):

plt_pk, cosmo, z=0.5, /linear


Explanation: calculating basic cosmological parameters


(1) Set Fiducial Parameters


Create a structure that contains all the fiducial values that are used in iCosmo.  At this stage users can input options here.  For this there are three input structures: cosmo_in, calc_in and expt_in:


cosmo_in: input cosmological parameter, the options are - 

H, Omega_B, Omega_m, Omega_L, w0, wa, n, Tau, Sigma8 and Curv 


calc_in: Input calculation parameter. Here you can decide which transfer function to use (using the fit_tk key word) and the nonlinear correction to use (the fit_nl key word). In the example fit_nl is set to 2 (Smith et al) and fit_tk is set to 0 (E&H).  The options are -

fit_NL, fit_TK, verbose, delta, n_l, l_ran, speed, linear, k_ran, n_k, n_lbin, nz_fn, nz_crs, ran_z and err


fit_nl: fitting formula for the non-linear powerspectrum

          (default 0:Peacock & Dodds, 1: Ma et al., 2: Smith et al.) 

fit_tk: fitting function for transfer function to use:    

          0: E&H without wiggles (default)   

          1: E&H with wiggles (UNSTABLE w/ NON-LINEAR CORRECTION)   

          2: BBKS as summarised by Peacock & Dodds (1997)


expt_in: Input experiment parametersproperties of a survey. The options are: 

sv1_N_ZBIN, sv1_ZERROR, sv1_Z_MED, sv1_NG,  sv1_A_SURVEY, sv1_EFF, sv1_SIG_INT, sv1_DNDZTYPE, sv1_DNDZP, sv1_DNDZZ, sv1_BIASTYPE, sv1_NS, sv1_SIGMAM, sv1_DELM, sv1_SNE_ZRAN, sv1_NAME, sv1_PROBES  (sv1 can also be changed to sv2 and sv3)


a_survey: survey area  [deg^2]

eff: masking efficiency

ng: galaxy surface density (without binning) [gals amin^-2]

z_m: galaxy median redshift (without binning)

 sig_int: intrinsic rms shear per component

n_zbin: number of redshift slices

     ;at the moment the BAO calculation does not take into account photometric errors

     gerror: Gaussian photo-z error (sigz=gerror*(1+z))

     f_cat: fraction of catastrophic failures

     z_delta: off set of a catastrophic failure 


To see the cosmological parameters:  help,fid.cosmo,/st

To see the calculation parameters:  help,fid.calc,/st

To see the parameters for survey:  help,fid.expt.sv1,/st



(2) Create a Cosmology Structure


Calculates the basic cosmology parameters using the setting in the fiducial structure.  This routine produces a structure which in turn contains three structure (help,cosmo,/st):


     cosmo.const - cosmological constants such as Omega_m and sigma8

     cosmo.evol   - cosmological parameters that evolve with redshift such as co-moving distance

     cosmo.pk     - 3D matter powerspectrum (both linear and non-linear) for a set of redshift.


To see the const parameters: help,cosmo.const,/st  

To see the evol parameters:   help,cosmo.evol,/st

To see the pk parameters:      help,cosmo.pk,/st


Explanation: plotting basic cosmological parameters


To plot the basic cosmological parameters you can either use the routines plt_cosmo or plt_pk.


3) Plotting cosmological parameters - apart from the power spectrum


To plot any cosmological parameter that evolves with redshift (expect the power spectrum), you can use the plt_cosmo routine in the following way: plt_cosmo, cosmo, 'x', 'y', where 'x' corresponds to the parameter you want on the x axis (here either 'z' or 'a'), 'y' corresponds to the parameter you want on the y axis (type icosmo_help, plt_cosmo for details).


All the usual IDL plotting commands are accepted. For example, to plot the angular diameter distance as a function of z: 

     plt_cosmo, cosmo, 'z', 'da', linestyle=2, thick=3


You can also overplot the angular diameter distance for a different cosmology:


     plt_cosmo, cosmo2, 'z', 'da', /over


(4) Plotting the power spectrum


When the 'cosmo' structure is created, the power spectrum is calculated and stored in 'cosmo.pk'.

To plot the power spectrum, simply type: 

     plt_pk, cosmo


To overplot the linear power spectrum at redshift z=0.5:


     plt_pk, cosmo, /linear, z=0.5, /over


Again, all the usual IDL plotting commands are accepted. 





  • red - Commands you can type into IDL
  • green - What IDL print
  • blue - other, e.g. keywords




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