## A_planck nuisance parameter and the Planck lensing likelihood

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
Tristan L Smith
Posts: 21
Joined: November 14 2005
Affiliation: Swarthmore
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### A_planck nuisance parameter and the Planck lensing likelihood

Hello,

I have been combining the Planck lensing likelihood with other (non CMB) likelihoods and realized that the prior on A_planck can have a significant impact on the values of the scalar amplitude (A_s) that are probed. Reading through the Planck documentation, the statement that A_planck should have a Gaussian prior with mean 1 and standard deviation 0.0025 appears in many places.

Is this prior still justified when using the Planck lensing likelihood without other CMB power spectra data? If not, what would be an appropriate prior?

Best,
Tristan

Antony Lewis
Posts: 1787
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: A_planck nuisance parameter and the Planck lensing likelihood

The CMBmarged CMB only likelihood does not depend on that parameter, as the distribution of true power spectra given the observed CMB power spectra has already been marginalized out. Otherwise, usign the un-marged likelihood, it should be correct to include it, as the likelihood will depend on the theory CMB power spectra.