Cold dark matter isocurvature perturbations: Constraints and model selection

Abstract

We use cosmic microwave background radiation (WMAP and ACBAR), large-scale structure (SDSS luminous red galaxies), and supernova (SNLS) data to constrain the possible contribution of cold dark matter isocurvature modes to the primordial perturbation spectrum. We consider three different admixtures with adiabatic modes in a flat {lambda}CDM cosmology with no tensor modes: fixed correlations with a single spectral index; general correlations with a single spectral index; and general correlations with independent spectral indices for each mode. For fixed correlations, we verify the WMAP analysis for fully uncorrelated and anticorrelated modes, while for general correlations with a single index we find a small tightening of the constraint on the fractional contribution of isocurvature modes to the observed power over earlier work. For generally correlated modes and independent spectral indices our results are quite different to previous work, needing a doubling of prior space for the isocurvature spectral index in order to explore adequately the region of high likelihood. Standard Markov-Chain Monte Carlo techniques proved to be inadequate for this particular application; instead, our results are obtained with nested sampling. We also use the Bayesian evidence, calculated simply in the nested-sampling algorithm, to compare models, finding the pure adiabatic model to bemore » favored over all our isocurvature models. This favoring is such that the logarithm of the Bayes factor, lnB<-2 for all models and lnB<-5 in the cases of fully anticorrelated modes with a single spectral index (the curvaton scenario) and generally correlated modes with a single spectral index.« less