Reconciling $H_0$ tension in a six parameter space?

TL;DR

The paper investigates whether a six-parameter emergent dark energy scenario (PEDE) can alleviate the longstanding $H_0$ tension without expanding the parameter space beyond that of flat $\Lambda$CDM. By constraining PEDE with Planck 2015 CMB, Pantheon SNIa, BAO, Riess $H_0$, DES 3×2pt, and CMB lensing (via CosmoMC) and comparing to $\Lambda$CDM across multiple dataset combinations, the authors show that $H_0$ can be reconciled at 68% CL for some cases (notably CMB alone) with an improved $\chi^2$, while for combined datasets the tension is reduced but the fit quality sometimes worsens relative to $\Lambda$CDM. The results indicate a nuanced, dataset-dependent picture: PEDE can mimic late-time acceleration and push $H_0$ upward without new degrees of freedom, but global fits do not uniformly outperform $\Lambda$CDM, and Bayesian evidence largely favors $\Lambda$CDM except in the specific case of CMB+Riess $H_0$ where PEDE gains support. Overall, PEDE represents a compelling six-parameter alternative worth further exploration, including perturbation treatment and potential dark sector interactions, to robustly address $H_0$ and $S_8$ tensions.

Abstract

Consistent observations indicate that some of the important cosmological parameters measured through the local observations are in huge tension with their measurements from the global observations (within the minimal $Λ$CDM cosmology). The tensions in those cosmological parameters have been found to be either weakened or reconciled with the introduction of new degrees of freedom that effectively increases the underlying parameter space compared to the minimal $Λ$CDM cosmology. It might be interesting to investigate the above tensions within the context of an emergent dark energy scenario proposed recently by Li and Shafieloo~\cite{Li:2019yem}. We find that the tension on $H_0$ is clearly alleviated within 68\% confidence level with an improvement of the $χ^2$ for CMB, for the above emergent dark energy model having only six free parameters similar to the spatially flat $Λ$CDM model. The tension on $H_0$ is still alleviated for every combined datasets considered in the work, however, such alleviation occurs by worsening the $χ^2$ compared to the $χ^2$ for $Λ$CDM model obtained for the same combined dataset.

Figures (3)

• Figure 1: 1D marginalized posterior distributions of all the free and derived parameters of the PEDE model as well as 2D joint contours at 68% and 95% confidence level are shown for various observational datasets.
• Figure 2: We compare the observational constraints on PEDE and $\Lambda$CDM models obtained from the CMB data alone.
• Figure 3: We compare the CMB temperature power spectra (left graph) and the matter power spectra (right graph) computed for the PEDE and $\Lambda$CDM models taking the best-fit values of the model parameters summarized in Table \ref{['tab:PEDE']} and \ref{['tab:LCDM']}.