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### CAMB: fixing sigma8

Posted: 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

### Re: CAMB: fixing sigma8

Posted: 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.

### Re: CAMB: fixing sigma8

Posted: 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...

### CAMB: fixing sigma8

Posted: March 11 2006
Hi Antony and Anze,

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

cheers, Pablo

### CAMB: fixing sigma8

Posted: 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?