CAMB: fixing sigma8
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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 large-scales...
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 large-scales...
thanks, Pablo
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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 re-run 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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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 re-run 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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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
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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?
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Re: CAMB: fixing sigma8
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
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
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- Posts: 1991
- Joined: September 23 2004
- Affiliation: University of Sussex
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Re: CAMB: fixing sigma8
Make sure your initial A_s or scalar_amp(1) is set to something sensible
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Re: CAMB: fixing sigma8
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
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
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Re: CAMB: fixing sigma8
Are need to set A_s sensibly as an input parameter.