[Paper Review] The Concerning S$H_0$ES Hubble Constant

D. Majaess|arXiv (Cornell University)|Jan 23, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This paper challenges the robustness of the S$H_0$ES Hubble constant measurement ($H_0 \approx 73$ km/s/Mpc), arguing that persistent inconsistencies in Cepheid photometry, metallicity corrections, and distance estimates to NGC4258—despite 18 years of data—undermine claims of a genuine Hubble tension. It calls for an independent, blind reanalysis of HST Cepheid images to eliminate confirmation bias and unresolved systematic errors.

ABSTRACT

Concerns are raised regarding the S$H_0$ES results, and the present $H_0$ controversy. The S$H_0$ES $H_0 \simeq 73$ km/s/Mpc has remained relatively unaltered across $18$ years (2005-2023), despite marked shifts in maser and Cepheid distances to the keystone galaxy NGC4258 (M106), and changes in the slope, zeropoint, metallicity, and extinction terms tied to the Leavitt Law, and notwithstanding uncertain photometry for remote Cepheids spanning galaxies with highly inhomogeneous crowding and surface brightness profiles. Concerns raised regarding the S$H_0$ES findings by fellow researchers are likewise highlighted. An independent blind assessment of the entire suite of raw HST Cepheid images is warranted, while being mindful of extit{a priori} constraints and confirmation bias that unwittingly impact conclusions.

Motivation & Objective

• To question the reliability of the S$H_0$ES $H_0 \approx 73$ km/s/Mpc measurement amid persistent inconsistencies in Cepheid distance determinations.
• To highlight unresolved issues in the Leavitt Law, including variable zeropoint, slope, and metallicity dependence, which affect $H_0$ calibration.
• To emphasize the impact of photometric contamination and crowding in extragalactic Cepheid observations, particularly in high-surface-brightness regions.
• To call for an independent, blind re-evaluation of all raw HST Cepheid images to mitigate confirmation bias and a priori constraints.
• To question the validity of the Hubble tension claim given the lack of consensus on CMB and Cepheid-based $H_0$ estimates and ongoing systematic errors.

Proposed method

• Analyzes historical $H_0$ estimates from the Huchra database to identify long-standing uncertainties and instability in $H_0$ measurements.
• Reviews discrepancies in Cepheid distance measurements to NGC4258 using HST data, including conflicting results from Maoz et al. (1999), Newman et al. (2001), and Macri et al. (2006).
• Evaluates the Leavitt Law’s slope and zeropoint across different metallicities using data from the LMC, SMC, Milky Way, and NGC4258.
• Assesses the impact of photometric contamination from red clump giants and crowded fields on extragalactic Cepheid photometry.
• Compares S$H_0$ES results with alternative $H_0$ determinations from TRGB and Cepheid methods (e.g., Freedman & Madore, 2023a), highlighting convergence with CMB results.
• Proposes a blind, unbiased reanalysis of all raw HST Cepheid images to eliminate confirmation bias and systematic errors.

Experimental results

Research questions

• RQ1 Why has the S$H_0$ES $H_0$ value remained nearly unchanged at ~73 km/s/Mpc over 18 years despite evolving Cepheid and maser distance measurements to NGC4258?
• RQ2 To what extent do photometric contamination and crowding in HST images of remote Cepheids compromise the accuracy of $H_0$ determinations?
• RQ3 How do conflicting Leavitt Law parameters (slope, zeropoint, metallicity dependence) across different studies affect the consistency of $H_0$ calibration?
• RQ4 Can the observed Hubble tension be attributed to unresolved systematic errors rather than new physics?
• RQ5 What are the implications of degeneracies between metallicity, extinction, and blending for Cepheid distance measurements in galaxies like NGC4258 and Cen A?

Key findings

• The S$H_0$ES $H_0$ estimate of ~73 km/s/Mpc has remained largely unchanged over 18 years (2005–2023), despite significant shifts in maser and Cepheid distances to NGC4258.
• Cepheid distance measurements to NGC4258 show a 0.16 mag offset between two HST fields, indicating substantial field-to-field variation and potential contamination.
• The Leavitt Law slope ($\alpha$) for Cepheids is reported as $-3.3$ in metal-poor environments (e.g., Magellanic Clouds), contradicting S$H_0$ES’s earlier claim of $-2.98 \pm 0.07$ for near-solar metallicity.
• The S$H_0$ES team revised its earlier $\alpha = -2.60 \pm 0.24$ for NGC3021 Cepheids to $\alpha \simeq -3.3$, suggesting inconsistency in their own calibration approach.
• A metallicity-dependent zeropoint of $\gamma = -0.29 \pm 0.09$ mag/dex was proposed by Macri et al. (2006), but this conflicts with evidence from Majaess (2010) and others showing $W_{VI}$ functions are relatively insensitive to metallicity.
• Alternative $H_0$ determinations using the TRGB method (e.g., Freedman & Madore, 2023a) are consistent with CMB results, suggesting the S$H_0$ES $H_0$ may be biased high due to unresolved systematics.