http://arxiv.org/abs/1907.07569
The strong discrepancy between local distance ladder and Cosmic Microwave Background (CMB) estimates of the Hubble constant $H_0$ could be pointing towards new physics beyond the concordance $\Lambda$CDM model. Several attempts to address this tension through new physics rely on extended cosmological models, featuring extra free parameters beyond the 6 $\Lambda$CDM parameters. However, marginalizing over additional parameters has the effect of broadening the uncertainties on the inferred parameters, and it is often the case that within these models the tension is addressed due to larger uncertainties rather than a genuine shift in the central value of $H_0$. In this paper I consider an alternative viewpoint: what happens if one chooses to \textit{fix} the extra parameters to non-standard values instead of varying them? Focusing on the dark energy equation of state $w$ and the effective number of relativistic species $N_{\rm eff}$, I find that fixing $w \approx -1.3$ or $N_{\rm eff} \approx 3.95$ leads to a high-redshift estimate of $H_0$ in \textit{perfect} agreement with the local distance ladder estimate, without broadening the uncertainty on the former. These two figures can have interesting implications for model-building activity. While such non-standard models are strongly disfavoured with respect to the baseline $\Lambda$CDM model, Bayesian evidence considerations show that they nonetheless perform surprisingly better than the corresponding extended models where $w$ and/or $N_{\rm eff}$ are allowed to vary, when reducing the $H_0$ tension to the same level of statistical significance. Finally, I estimate dimensionless multipliers relating variations in $H_0$ to variations in $w$ and $N_{\rm eff}$, which can be used to swiftly repeat the analysis of this paper in light of future more precise local distance ladder estimate of $H_0$, should the tension persist.
S. Vagnozzi
Thu, 18 Jul 19
17/64
Comments: 23 pages, 10 figures. Comments on this non-standard hybrid Bayesian-frequentist approach are very welcome. The busy reader should skip to Fig. 1, 2, 4, 5, and 10
You must be logged in to post a comment.