[astro-ph/0605004] Model selection forecasts for the spectral index from the Planck satellite

 Authors: Cédric Pahud, Andrew R Liddle, Pia Mukherjee, David Parkinson Abstract: The recent WMAP3 results have placed measurements of the spectral index n_S in an interesting position. While parameter estimation techniques indicate that the Harrison-Zel'dovich spectrum n_S=1 is strongly excluded (in the absence of tensor perturbations), Bayesian model selection techniques reveal that the case against n_S=1 is not yet conclusive. In this paper, we forecast the ability of the Planck satellite mission to use Bayesian model selection to convincingly exclude (or favour) the Harrison-Zel'dovich model. [PDF]  [PS]  [BibTex]  [Bookmark]

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Antony Lewis
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[astro-ph/0605004] Model selection forecasts for the spectra

This paper looks at how well Planck can do at distinguishing n_s \ne 1 models from n_s=1.

Why are the authors using the Gaussian likelihood in Eq. 1 rather than the true likelihood function? It is well known that the Gaussian approximation is not very good, and the true function is just as easy to compute with an effective full sky. (it's given explicitly in e.g. Eq 5 of astro-ph/0502469, though I certainly wasn't the first to write it down). Do you get out the correct posterior constraints using this function?

Pia Mukherjee
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Affiliation: University of Sussex

[astro-ph/0605004] Model selection forecasts for the spectra

Hi Antony,

We used Gaussian likelihoods, for no reason other than that we thought that the non-Gaussianity of the likelihood functions for C_2 - C_10 will not make a significant difference. But you're right, it is as easy to use the true likelihood function. We'll re-run the analysis through and get back. Our guess is that the result wont change much (as the posteriors look alright), but we'll see.

Pia Mukherjee
Posts: 3
Joined: May 02 2006
Affiliation: University of Sussex

[astro-ph/0605004] Model selection forecasts for the spectra

We have recomputed everything using the improved likelihood function. The updated paper is on astro-ph today. The numbers changed a little, the paper/results are qualitatively unchanged.