Information criteria for astrophysical model selection

Model selection is the problem of distinguishing competing models, perhaps featuring different numbers of parameters. The statistics literature contains two distinct sets of tools, those based on information theory such as the Akaike Information Criterion (AIC), and those on Bayesian inference such as the Bayesian evidence and Bayesian Information Criterion (BIC). The Deviance Information Criterion combines ideas from both heritages; it is readily computed from Monte Carlo posterior samples and, unlike the AIC and BIC, allows for parameter degeneracy. I describe the properties of the information criteria, and as an example compute them from WMAP3 data for several cosmological models. I find that at present the information theory and Bayesian approaches give significantly different conclusions from that data.Comment: 5 pages, no figures. Update to match version accepted by MNRAS Letters. Extra references, minor changes to discussion, no change to conclusion

Stability of multi-field cosmological solutions

We explore the stability properties of multi-field solutions of assisted inflation type, where several fields collectively evolve to the same configuration. In the case of noninteracting fields, we show that the condition for such solutions to be stable is less restrictive than that required for tracking in quintessence models. Our results, which do not rely on the slow-roll approximation, further indicate that to linear order in homogeneous perturbations the fields are in fact unaware of each other's existence. We end by generalizing our results to some cases of interacting fields and to other background solutions and dynamics, including the high-energy braneworld.Comment: 6 pages; v2: typos corrected, version accepted by PR

Nflation: multi-field inflationary dynamics and perturbations

We carry out numerical investigations of the dynamics and perturbations in the Nflation model of Dimopoulos et al. (2005). This model features large numbers of scalar fields with different masses, which can cooperate to drive inflation according to the assisted inflation mechanism. We extend previous work to include random initial conditions for the scalar fields, and explore the predictions for density perturbations and the tensor-to-scalar ratio. The tensor-to-scalar ratio depends only on the number of e-foldings and is independent of the number of fields, their masses, and their initial conditions. It therefore always has the same value as for a single massive field. By contrast, the scalar spectral index has significant dependence on model parameters. While normally multi-field inflation models make predictions for observable quantities which depend also on the unknown field initial conditions, we find evidence of a `thermodynamic' regime whereby the predicted spectral index becomes independent of initial conditions if there are enough fields. Only in parts of parameter space where the mass spectrum of the fields is extremely densely packed is the model capable of satisfying the tight observational constraints from WMAP3 observations.Comment: 6 pages RevTeX4, 4 figures included. Updated to match PRD accepted version. Analysis and conclusions unchanged. New references, especially astro-ph/0510441 which was first to give the general r=8/N resul