## Power Spectra with CAMB

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Joseph Wraga
Posts: 6
Joined: February 22 2018
Affiliation: Drexel University

### Power Spectra with CAMB

I was going through the tutorial for CAMB (http://camb.readthedocs.io/en/latest/CAMBdemo.html ) and I notice the TT spectrum is shifted downward compared with the Planck 2015 results( https://arxiv.org/pdf/1502.02114.pdf ) . For example, the first peak in the demo is around 5300 $\mu K^2$, while for Planck the peak is around 5700 $\mu K^2$. I notice the same thing when trying to compare the BB spectrum with the best fit values (third entry in the data set column in http://irsa.ipac.caltech.edu/data/Planc ... lary-data/ ). In both cases it seems multiplying the CAMB results by a constant factor would make them match the Planck observation, but I do not see why that is the case.

Antony Lewis
Posts: 1644
Joined: September 23 2004
Affiliation: University of Sussex
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### Re: Power Spectra with CAMB

The demo is just using some random parameters. Specifically it probably doesn't set a realistic value of A_s. Always check with print(pars) that everything is correct after setting values.

Joseph Wraga
Posts: 6
Joined: February 22 2018
Affiliation: Drexel University

### Power Spectra with CAMB

Yes, that was the problem. Adjusting $A_s$ according to Planck (default value was slightly too small) makes the CAMB result match the best-fit model.
Thank you!