Should we doubt the cosmological constant?

TL;DR

This paper evaluates the adequacy of the LambdaCDM model in cosmology using Bayesian doubt, finding strong evidence that it remains a sufficient description of current observations with little room for alternative models.

Contribution

It introduces Bayesian doubt to cosmological model comparison, providing an absolute measure of model performance against an unknown benchmark.

Findings

Bayesian doubt yields a Bayes factor B < 3 for unknown models versus LambdaCDM.

Posterior probability for doubt is less than 6%.

Probability for LambdaCDM increases to over 50% with data.

Abstract

While Bayesian model selection is a useful tool to discriminate between competing cosmological models, it only gives a relative rather than an absolute measure of how good a model is. Bayesian doubt introduces an unknown benchmark model against which the known models are compared, thereby obtaining an absolute measure of model performance in a Bayesian framework. We apply this new methodology to the problem of the dark energy equation of state, comparing an absolute upper bound on the Bayesian evidence for a presently unknown dark energy model against a collection of known models including a flat LambdaCDM scenario. We find a strong absolute upper bound to the Bayes factor B between the unknown model and LambdaCDM, giving B < 3. The posterior probability for doubt is found to be less than 6% (with a 1% prior doubt) while the probability for LambdaCDM rises from an initial 25% to just over 50% in light of the data. We conclude that LambdaCDM remains a sufficient phenomenological description of currently available observations and that there is little statistical room for model improvement.