## CosmoMC: fast parameters

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

### CosmoMC: fast parameters

Hello everyone,

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

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9 parameters &#40; 6 slow &#40; 0 semi-slow&#41;,  3 fast &#40; 0 semi-fast&#41;&#41;
If I change the batch2/params_CMB_defaults.ini file from

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param&#91;ns&#93; = 0.96 0.8 1.2 0.004 0.002
to

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param&#91;ns&#93; = 1.0
I get

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8 parameters &#40; 5 slow &#40; 0 semi-slow&#41;,  3 fast &#40; 0 semi-fast&#41;&#41;
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: 1646
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: CosmoMC: fast parameters

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