[CAMB] sigma8 calculation

[Stéphane Ilic]

Dear all,

I am having some issues concerning the computation of sigma8 in CAMB : using some cosmological model close to the current best-fit, I first ran CAMB and outputted the matter power spectrum, having the following parameters in my params.ini :

> transfer_high_precision = F
> transfer_kmax = 2
> transfer_k_per_logint = 0
> transfer_num_redshifts = 1
> transfer_interp_matterpower = T
> transfer_redshift(1) = 0
> transfer_filename(1) = transfer_out.dat
> #Matter power spectrum output against k/h in units of h^{-3} Mpc^3
> transfer_matterpower(1) = matterpower.dat

During the run, CAMB informs me that for this particular model :
> at z = 0.0000000E+00 sigma8 (all matter)= 0.8133983

However, I tried on the other hand to compute the value of sigma8 myself, using the following python code (N.B.: k_tab and Pk_tab are the tabulated value of the power spectrum as outputted by CAMB) :

> R = 8.
> int_sig = Pk_tab*(3.*(np.sin(k_tab*R)-k_tab*R*np.cos(k_tab*R))/(k_tab*R)**3.*k_tab)**2.
> integ = scipy.integrate.simps(int_sig,k_tab)
> print np.sqrt(integ/(2.*np.pi**2.))

Surprisingly, this gives me a different value for sigma8, namely 0.915113257117.

Am I missing something here ? Where is my mistake ?

Thanks in advance !

Cheers,

Stéphane

[Stéphane Ilic]

Hummm....

Somehow my problem solved itself...

Apologies for the waste of space !