CosmoMC: fast parameters

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
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Paolo Campeti
Posts: 5
Joined: January 09 2018
Affiliation: SISSA, Trieste

CosmoMC: fast parameters

Post by Paolo Campeti » January 09 2018

Hello everyone,

I'm using CosmoMC-Nov2016 exactly as downloaded from github. Running with action=4 I get

Code: Select allp>

9 parameters ( 6 slow ( 0 semi-slow),  3 fast ( 0 semi-fast))
If I change the batch2/params_CMB_defaults.ini file from

Code: Select allp>

param[ns] = 0.96 0.8 1.2 0.004 0.002
to

Code: Select allp>

param[ns] = 1.0
I get

Code: Select allp>

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

Antony Lewis
Posts: 1352
Joined: September 23 2004
Affiliation: University of Sussex
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Re: CosmoMC: fast parameters

Post by Antony Lewis » January 09 2018

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.

Paolo Campeti
Posts: 5
Joined: January 09 2018
Affiliation: SISSA, Trieste

CosmoMC: fast parameters

Post by Paolo Campeti » January 09 2018

Perfect, thank you very much for your quick answer!

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