647 research outputs found
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
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