Model selection forecasts for the spectral index from the Planck satellite

The recent WMAP3 results have placed measurements of the spectral index $n_{\mathrm{S}}$ in an interesting position. While parameter estimation techniques indicate that the Harrison–Zel’dovich spectrum $n_{\mathrm{S}}=1$ is strongly excluded (in the absence of tensor perturbations), Bayesian model selection techniques reveal that the case against $n_{\mathrm{S}}=1$ 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 favor) the Harrison–Zel’dovich model.