Reconciling Planck with the local value of $H_0$ in extended parameter space (1606.00634v2)

Abstract: The recent determination of the local value of the Hubble constant by Riess et al, 2016 (hereafter R16) is now 3.3 sigma higher than the value derived from the most recent CMB anisotropy data provided by the Planck satellite in a LCDM model. Here we perform a combined analysis of the Planck and R16 results in an extended parameter space, varying simultaneously 12 cosmological parameters instead of the usual 6. We find that a phantom-like dark energy component, with effective equation of state $w=-1.29_{-0.12}^{+0.15}$ at 68 % c.l. can solve the current tension between the Planck dataset and the R16 prior in an extended $\Lambda$CDM scenario. On the other hand, the neutrino effective number is fully compatible with standard expectations. This result is confirmed when including cosmic shear data from the CFHTLenS survey and CMB lensing constraints from Planck. However, when BAO measurements are included we find that some of the tension with R16 remains, as also is the case when we include the supernova type Ia luminosity distances from the JLA catalog.

• The paper analyzes extended cosmological models beyond LambdaCDM to reconcile the discrepancy in the Hubble constant ($H_0$) values measured by Planck and local observations like Riess et al.
• By exploring a twelve-parameter space including variations in dark energy and neutrinos and combining diverse datasets (Planck, Riess et al., BAO, cosmic shear), the authors test alternative cosmological scenarios.
• A key finding suggests that a phantom-like dark energy equation of state ($w<-1$) could alleviate the $H_0$ tension, though this solution is constrained by Baryonic Acoustic Oscillation data.

Reconciling Planck with the Local Value of $H_0$ in Extended Parameter Space: An Overview

The paper "Reconciling Planck with the local value of $H_0$ in extended parameter space" by Eleonora Di Valentino, Alessandro Melchiorri, and Joseph Silk addresses a significant tension in modern cosmology between two disparate measurements of the Hubble constant, $H_0$. The respective values come from the Planck satellite's observations of the Cosmic Microwave Background (CMB) radiation and the direct local measurements by Riess et al. (2016). The Planck data, analyzed under the $\Lambda$CDM model, yields $H_0 = 67.27 \pm 0.66$ km/s/Mpc, while Riess et al.'s direct measurement suggests $H_0 = 73.24 \pm 1.74$ km/s/Mpc, a discrepancy exceeding three standard deviations.

To explore potential resolutions to this discrepancy, the authors conduct a meticulous analysis within an extended parameter space hypothesis, deviating from the standard six-parameter $\Lambda$CDM model and examining twelve parameters. The additional parameters include variations such as the dark energy equation of state, neutrino mass, and effective number of relativistic species. This modeling pivot allows investigation into frameworks beyond the classical dark energy characterization of a cosmological constant.

Key findings from the authors' expanded parameter space, incorporating datasets from Planck, Riess et al., as well as Baryonic Acoustic Oscillations (BAO) and cosmic shear data, underscore the plausibility of a phantom-like dark energy scenario. The analysis identifies an effective equation of state $w=-1.29_{-0.12}^{+0.15}$ that may mitigate the tension between Planck and the Riess et al. observations, although this solution is challenged by BAO data which reduces the statistical significance of a phantom-like scenario.

Theoretical implications posed by the paper include potential modifications to the dark energy framework, such as interactions or redshift dependencies akin to quintessence or modified gravity theories that exhibit an evolving equation of state. Additionally, the discussion touches on the possibility of systematic errors in current measurements needing reassessment. The implication is that any resolution of the $H_0$ tension should carefully consider both novel theoretical models and the robustness of observational methodologies.

This examination demonstrates the importance and complexity of reconciling diverse observational datasets in cosmology. Future directions could involve higher precision measurements from advanced satellite missions and terrestrial experiments, potentially enlightening the nature of dark energy or providing an impetus to recalibrate systematic methodologies employed in key cosmological observations. The paper's findings stimulate a continued dialogue about the fundamental composition and evolution of the universe, challenging and refining the contemporary cosmological model.