CAMB: fixing sigma8

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Pablo Fosalba
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Joined: September 25 2004
Affiliation: Institut d'Estudis Espacials de Catalunya
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CAMB: fixing sigma8

Post by Pablo Fosalba »

Hi there,

is there any simple way of setting sigma8 to a ct. value for
Cls computations with CAMB ?
I am not sure how to control that from the parameter scalar_amp(1)
that is the regular input for fluctuation amplitude on large-scales...

thanks, Pablo
Antony Lewis
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Affiliation: University of Sussex
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Re: CAMB: fixing sigma8

Post by Antony Lewis »

Not directly. However if you set get_transfer = T, \sigma_8 will be computed for whatever your input scalar_amp(1). Since \sigma_8^2 scales like scalar_amp(1), you then just need to re-run CAMB after multiplying your original scalar_amp(1) by the (ratio of your desired \sigma_8 to the output \sigma_8) squared.
Anze Slosar
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Affiliation: Brookhaven National Laboratory
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Re: CAMB: fixing sigma8

Post by Anze Slosar »

Antony Lewis wrote:Not directly. However if you set get_transfer = T, \sigma_8 will be computed for whatever your input scalar_amp(1). Since \sigma_8^2 scales like scalar_amp(1), you then just need to re-run CAMB after multiplying your original scalar_amp(1) by the (ratio of your desired \sigma_8 to the output \sigma_8) squared.
Just a safety warning: amp \propto sigma_8, but the constant of proportionality of course varies with other cosmological parameters varyiing...
Pablo Fosalba
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Affiliation: Institut d'Estudis Espacials de Catalunya
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CAMB: fixing sigma8

Post by Pablo Fosalba »

Hi Antony and Anze,

thanks for the tip. I tried it and It actually works ;-)

cheers, Pablo
Ivan Debono
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Affiliation: APC, Paris 7 / OBSPM

CAMB: fixing sigma8

Post by Ivan Debono »

The constant of proportionality also varies with the redshift. So if I want to calculate P(k) at a certain (nonzero) z, do I use the normalisation for a P(k,z=0) spectrum i.e. the scalar amplitude for my desired sigma_8 at z=0?
Thaher Chowdhury
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Affiliation: Rensselaer Polytechnic Institute
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Re: CAMB: fixing sigma8

Post by Thaher Chowdhury »

Hi,

I am facing the same problem. I tried the above solution. However, it gives me errors like error halofit. How can I solve this issue?

Thanks,
Thaher
Antony Lewis
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Affiliation: University of Sussex
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Re: CAMB: fixing sigma8

Post by Antony Lewis »

Make sure your initial A_s or scalar_amp(1) is set to something sensible
Thaher Chowdhury
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Affiliation: Rensselaer Polytechnic Institute
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Re: CAMB: fixing sigma8

Post by Thaher Chowdhury »

Hello Antony Lewis,

The value I get from the run is sigma_8 = 1.03733754 * 10^(-2), which is relatively low. My desired sigma_8 is 0.807. I followed the above steps, and it gave me A_s = 1.27127556 * 10^(-5), which is very big for A_s. Now, it is giving me an error, as I mentioned. Is there any other way to fix sigma_8? Is there any specific parameter I am putting wrong for which I get this value for sigma_8?

Thank you for responding so quickly. Thank you for helping me out.

Regards,
Thaher
Antony Lewis
Posts: 1991
Joined: September 23 2004
Affiliation: University of Sussex
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Re: CAMB: fixing sigma8

Post by Antony Lewis »

Are need to set A_s sensibly as an input parameter.
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