I'm going to analyze large-scale characteristics of the CMB and currently trying to get familiar with data evaluation and its simulation by SYNFAST.

I'm using the WMAP−5 foreground reduced temperature maps per frequency band, available on LAMBDA (e.g. wmap_band_forered_iqumap_r9_5yr_V

_{v3}.fits). Thereby, I observed something strange when comparing the map to Gaussian simulations. To get started, I wanted to compare the rms temperature fluctuations on large scales of real maps with their simulations.

First of all, I multiply a map (e.g. the V map) by the appropriate mask for temperature analysis (wmap_ext_temperature_analysis_mask_r9_5yr_v3.fits, also on LAMBDA) and degrade it to Healpix resolution 8 (via the Healpix fortran routine UD_GRADE). Now, the map is pixelized on a scale of about 7 degrees.

Then, I calculated the map's rms temperature fluctuation on this scale, just by averaging the square of the temperature fluctuations delta_T in each pixel over the whole map and taking the square-root in the end.

This value is to be compared to Gaussian simulations, created by the Healpix facility SYNFAST based on the WMAP−5 best fit Cl spectrum (e.g. computed by CAMB with the parameter file given by LAMBDA, see below). I expected the following: Since the best-fit Cls originate from the true WMAP-maps, my real rms temperature fluctuation should be (nearly) the same as the mean value of many Gaussian simulations running through the same masking and degrading procedure.

But that is not what I observe!

The mean value of the rms temperature fluctuations of a bunch of Gaussian simulations is about a factor of 1.4 too large.

I cannot find any explanation for this discrepancy. The instrument noise can be neglected on this large scale and could only correct into the wrong direction, I guess.

Do I understand something wrong or can anybody see a mistake?

Any hints are welcome!

Cheers,

Maik

If it helps you, here is the parameter fie for CAMB as downloaded from LAMBDA:

#########################################################

output_root = bestfit

get_scalar_cls = T

get_vector_cls = F

get_tensor_cls = F

COBE_normalize = F

CMB_outputscale = 7.425625e12

get_transfer = T

do_nonlinear = 0

l

_{max_scalar}= 2000

k_eta_max_scalar = 4000

do_lensing = T

lensing_method = 1

w = −1

cs2_lam = 1

hubble = 0.724365E+02

use_physical = T

ombh2 = 0.226823E−01

omch2 = 0.108085E+00

omnuh2 = 0

omk = 0

temp_cmb = 2.725

helium_fraction = 0.24

massless_neutrinos = 3.04

massive_neutrinos = 0

nu_mass_eigenstates = 1

nu_mass_degeneracies = 0

nu_mass_fractions = 1

transfer_high_precision = T

transfer_kmax = 2

transfer_k

_{per_logint}= 0

transfer_num_redshifts = 1

transfer_interp_matterpower = T

transfer_power_var = 7

transfer_redshift(1) = 0

transfer_filename(1) = transfer_out.dat

transfer_matterpower(1) = matterpower.dat

reionization = T

re_use_optical_depth = T

re_optical_depth = 0.890432E−01

re_delta_redshift = 0.5

re_ionization_frac = 1

pivot_scalar = 0.002

pivot_tensor = 0.002

initial_power_num = 1

scalar_spectral_index(1) = 0.960927E+00

scalar_nrun(1) = 0

scalar_amp(1) = 2.41147e−09

RECFAST_fudge = 1.14

RECFAST_fudge_He = 0.86

RECFAST_Heswitch = 6

initial_condition = 1

scalar_output_file = scalCls.dat

lensed_output_file = lensedCls.dat

FITS_filename = scalCls.fits

accurate_polarization = T

accurate_reionization = T

accurate_BB = T

do_late_rad_truncation = F

do_tensor_neutrinos = T

feedback_level = 1

massive_nu_approx = 3

number_of_threads = 2

accuracy_boost = 2

l_accuracy_boost = 2

l_sample_boost = 2

####################################################

And here is one sample parameter file for SYNFAST:

#####################################################

simul_type = 1

nsmax = 512

nlmax = 1024

iseed = 70

beam_file = beam_V1.fits

infile = Cls.fits

outfile = ../temp/bestfit_512_10.fits

######################################################

(the beam shouldn't matter on the large scales I analyze, but nonethelesse, I've calculated the window functions by squaring the beam transfer function, available on LAMBDA)[tex][/tex]