CAMB: fixing sigma8

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 Joined: September 25 2004
 Affiliation: Institut d'Estudis Espacials de Catalunya
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CAMB: fixing sigma8
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 largescales...
thanks, Pablo
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 largescales...
thanks, Pablo

 Posts: 1512
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: CAMB: fixing sigma8
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 rerun CAMB after multiplying your original scalar_amp(1) by the (ratio of your desired \sigma_8 to the output \sigma_8) squared.

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 Joined: September 24 2004
 Affiliation: Brookhaven National Laboratory
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Re: CAMB: fixing sigma8
Just a safety warning: amp \propto sigma_8, but the constant of proportionality of course varies with other cosmological parameters varyiing...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 rerun CAMB after multiplying your original scalar_amp(1) by the (ratio of your desired \sigma_8 to the output \sigma_8) squared.

 Posts: 29
 Joined: September 25 2004
 Affiliation: Institut d'Estudis Espacials de Catalunya
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CAMB: fixing sigma8
Hi Antony and Anze,
thanks for the tip. I tried it and It actually works ;)
cheers, Pablo
thanks for the tip. I tried it and It actually works ;)
cheers, Pablo

 Posts: 25
 Joined: April 17 2007
 Affiliation: APC, Paris 7 / OBSPM
CAMB: fixing sigma8
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?