CAMB/CosmoMC: Matter Power Spectrum for w!=−1

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Tijmen de Haan
Posts: 7
Joined: May 30 2013
Affiliation: UC Berkeley

CAMB/CosmoMC: Matter Power Spectrum for w!=-1

Post by Tijmen de Haan » December 02 2015

I'm finding significant differences between the matter power spectrum obtained through a direct CAMB call, and that available through the PK%PowerAt function in CosmoMC.

Here's the minimal way I was able to reproduce the issue:
1) I downloaded the latest version of CAMB (as of the time of this post), changed "get_transfer = T" in params.ini and plotted the contents of the resulting "test_matterpower.dat"
2) I downloaded the latest version of CosmoMC and added a print statement that prints a k array and the output of PK%PowerAt at those values of k and z=0. I compiled and ran CosmoMC with a minimal .ini file that sets the cosmological parameters to those printed out by CAMB in step 1). I copied the output and overplotted. I find that the resulting matter power spectra P(k,z=0) agree to roughly 1%.
Image
3) I now changed only w=−1 to w=−1.5 in both the CAMB params.ini file and the CosmoMC .ini file and reran both. This results in a disagreement of nearly a factor of two around k~0.01, smoothly changing to percent level agreement at k~1.
Image

A colleague of mine independently verified this problem. Thanks in advance for any help or insights you can offer.

Antony Lewis
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Joined: September 23 2004
Affiliation: University of Sussex
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Re: CAMB/CosmoMC: Matter Power Spectrum for w!=-1

Post by Antony Lewis » December 02 2015

Are you sure the parameters being used by CAMB are the same?

in Case (1) for example, CosmoMC will be setting transfer_high_precision =T (and the kmax is probably also different)

In Case(2), If you just changed the w input parameter in Cosmomc, that will change H0 at fixed theta (since theta is also another input parameter, which I assume you left unchanged?). In the CAMB .ini file, H0 is specified directly instead of theta, so you will be changing w at fixed H0 (and hence varying theta).

Tijmen de Haan
Posts: 7
Joined: May 30 2013
Affiliation: UC Berkeley

CAMB/CosmoMC: Matter Power Spectrum for w!=-1

Post by Tijmen de Haan » December 03 2015

After updating theta along with w in CosmoMC so as to get a consistent H0, I find the two agree at the percent level. Thanks for the help!

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