Model selection forecasts for the spectral index from the Planck satellite

The recent WMAP3 results have placed measurements of the spectral index nₒ in an interesting position. While parameter estimation techniques indicate that the Harrison--Zel'dovich spectrum nₒ=1 is strongly excluded (in the absence of tensor perturbations), Bayesian model selection techniques reveal that the case against nₒ=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 favor) the Harrison--Zel'dovich model.