Local determination of the Hubble constant and the deceleration parameter (1906.11814v2)

Abstract: The determination of the Hubble constant $H_0$ from the Cosmic Microwave Background by the Planck Collaboration [Aghanim et al. 2018] is in tension at $4.2\sigma$ with respect to the local determination of $H_0$ by the SH0ES collaboration [Reid et al. 2019]. Here, we improve upon the local determination, which fixes the deceleration parameter to the standard $\Lambda$CDM model value of $q_0=-0.55$, that is, uses information from observations beyond the local universe. First, we derive the effective calibration prior on the absolute magnitude $M_B$ of Supernovae Ia, which can be used in cosmological analyses in order to avoid the double counting of low-redshift supernovae. We find $M_B = -19.2334 \pm 0.0404$ mag. Then, we use the above $M_B$ prior in order to obtain a determination of the local $H_0$ which only uses local observations and only assumes the cosmological principle, that is, large-scale homogeneity and isotropy. This is achieved by adopting an uninformative flat prior for $q_0$ in the cosmographic expansion of the luminosity distance. We use the latest Pantheon sample and find $H_0= 75.35 \pm 1.68 \text{ km s}^{-1} {\rm Mpc}^{-1}$, which features a 2.2% uncertainty, close to the 1.9% error obtained by the SH0ES Collaboration. Our determination is at the higher tension of $4.5\sigma$ with the latest results from the Planck Collaboration that assume the $\Lambda$CDM model. Furthermore, we also constrain the deceleration parameter to $q_0= -1.08 \pm 0.29$, which disagrees with Planck at the $1.9\sigma$ level. These estimations only use supernovae in the redshift range $0.023\le z\le 0.15$.

• The paper introduces a novel cosmographic method using a flat q₀ prior to recalibrate local H₀, highlighting a 4.5σ tension with ΛCDM.
• It refines the absolute magnitude calibration for Supernovae Ia from the Pantheon sample to prevent redundant use of low-redshift data.
• The findings imply that local universe metrics differ from CMB-inferred values, urging revisions to standard cosmological models and dark energy insights.

Analyzing Local Determinations of the Hubble Constant and Deceleration Parameter

The paper discussed herein provides an in-depth analysis of the local determination of the Hubble constant ($H_0$) and the deceleration parameter ($q_0$) through cosmography, focusing on disentangling these estimates from assumptions inherent in the standard cosmological model, $\Lambda$CDM. The core of this paper lies in the tension between the $H_0$ values deduced from the Cosmic Microwave Background (CMB) observations by the Planck Collaboration and those derived locally through the calibration of Supernovae Ia by the SH0ES Collaboration.

Methodological Approach

The authors embark on refining local $H_0$ estimations, traditionally relying on fixed $\Lambda$CDM values for $q_0$, by utilizing a cosmographic expansion parameterization. This method discards the rigid assumption of a single deceleration model and instead implements an uninformative flat prior for $q_0$. Such a framework facilitates a model-independent determination based predominantly on large-scale homogeneity and isotropy, thereby solely grounded in the cosmological principle.

The analysis leverages the previously defined effective calibration prior on the absolute magnitude ($M_B$) of Supernovae Ia to avoid potential double-counting of low-redshift supernovae in both the $H_0$ determinations and cosmological analyses.

Key Results

Significantly, the recalibrated local analysis using the Pantheon sample of supernovae yields a new $H_0$ estimation of $$75.35 \pm 1.68 \, \text{ km s}^{-1} \, \text{Mpc}^{-1}$$, showcasing a heightened tension with Planck's $\Lambda$CDM-derived $H_0$ value, reaching a discrepancy of $4.5\sigma$. This figure represents a stronger conflict compared to previous studies, indicating further depth in the ongoing $H_0$ discordance.

The paper proceeds to discuss the deceleration parameter $q_0$, finding $q_0 = -1.08 \pm 0.29$, which diverges from the Planck results at $1.9\sigma$. This deviation accentuates the hypothesis that $q_0$, much like $H_0$, may be under different influences at local levels, distinct from those assumed in the global $\Lambda$CDM paradigm.

Theoretical and Practical Implications

The results imply significant implications for the current understanding of dark energy and the overall expansion dynamics of the universe. A notable discord between local and CMB-inferred values suggests potential areas where existing models could be revisited or extended, possibly pointing towards new physics or the necessity to account for local inhomogeneities and anisotropies beyond what $\Lambda$CDM considers.

In terms of practical applications, the isolated calibration prior on $M_B$ stands to offer a robust tool for cosmological analyses, allowing for a less biased approach by preventing redundant usage of the same data points in multiple inferential steps.

Future Perspectives in Cosmology

Looking forward, the method presented may serve as a basis for more exhaustive assessment of non-standard cosmological models. There exists a need to bridge the observed discrepancies between local and cosmic-scale evaluations, potentially inspiring innovative approaches involving early dark energy theories or variations in the cosmic distance ladder's elements.

This paper consequently contributes to an enriched understanding of universe metrics and enkindles further discourse on addressing inconsistencies within standard cosmological interpretations. The community stands to gain from continued analyses extending similar methodologies to broader datasets or incorporating alternative cosmographic parameters.