CAMB: fixing sigma8

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Post Reply
Pablo Fosalba
Posts: 29
Joined: September 25 2004
Affiliation: Institut d'Estudis Espacials de Catalunya
Contact:

CAMB: fixing sigma8

Post by Pablo Fosalba » March 10 2006

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
Posts: 1503
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

Re: CAMB: fixing sigma8

Post by Antony Lewis » March 10 2006

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
Posts: 183
Joined: September 24 2004
Affiliation: Brookhaven National Laboratory
Contact:

Re: CAMB: fixing sigma8

Post by Anze Slosar » March 10 2006

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
Posts: 29
Joined: September 25 2004
Affiliation: Institut d'Estudis Espacials de Catalunya
Contact:

CAMB: fixing sigma8

Post by Pablo Fosalba » March 11 2006

Hi Antony and Anze,

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

cheers, Pablo

Ivan Debono
Posts: 25
Joined: April 17 2007
Affiliation: APC, Paris 7 / OBSPM

CAMB: fixing sigma8

Post by Ivan Debono » July 15 2009

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?

Post Reply