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CosmoMC: fast parameters
 
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Paolo Campeti



Joined: 09 Jan 2018
Posts: 5
Affiliation: SISSA, Trieste

PostPosted: January 09 2018  Reply with quote

Hello everyone,

I'm using CosmoMC-Nov2016 exactly as downloaded from github. Running with action=4 I get
Code:
9 parameters ( 6 slow ( 0 semi-slow),  3 fast ( 0 semi-fast))


If I change the batch2/params_CMB_defaults.ini file from
Code:
param[ns] = 0.96 0.8 1.2 0.004 0.002

to
Code:
param[ns] = 1.0


I get
Code:
8 parameters ( 5 slow ( 0 semi-slow),  3 fast ( 0 semi-fast))


So is the parameter ns a slow parameter? Shouldn't initial power spectrum parameters be fast parameters? How can I change it into a fast parameter?
thank you very much for your time and patience
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1351
Affiliation: University of Sussex

PostPosted: January 09 2018  Reply with quote

That depends on whether you are using non-linear lensing. If you are, the way it currently stores transfer functions as a function of k (not time), it has to recompute them if primordial parameters are changed and you want to be consistent in the non-linear corrections. In principle this could be changed so it stored lower-level quantities, then only the non-linear corrections would need to be recomputed, but I've not tried to do that.

You can turn off non-linear lensing and then make the primordial power spectrum parameters fast again if you are not concerted about the (fairly small, esp. if you are not using lensing reconstruction) non-linear corrections.
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Paolo Campeti



Joined: 09 Jan 2018
Posts: 5
Affiliation: SISSA, Trieste

PostPosted: January 09 2018  Reply with quote

Perfect, thank you very much for your quick answer!
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