[Paper Review] A Lockdown Perspective on the Hubble Tension (with comments from the SH0ES team)

G. Efstathiou|arXiv (Cornell University)|Jul 21, 2020

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy11 references84 citations

TL;DR

This paper analyzes the Hubble tension by re-examining SH0ES Cepheid calibrations and distance anchors, identifies potential systematic biases in the Cepheid PL intercepts, and discusses the need for independent distance anchors (notably NGC 4258) to resolve the tension.

ABSTRACT

This is a transcript of a talk that I gave in Cambridge on 17th July 2020 on the `Hubble tension'. I review the SH0ES analyses by Riess and collaborators and point out some internal inconsistencies, including a discrepancy between the relative distances inferred from Cepheids of two of the primary geometric distance anchors, the Large Magellanic Cloud (LMC) and NGC 4258. I then ask `what would it take to make SH0ES compatible with early time measurements?'. The answer is a systematic bias of 0.1 - 0.15 mag in the intercept of the Cepheid period-luminosity relations of SH0ES galaxies. Such a bias resolves the Hubble tension, the tension between the distance anchors, and the difference between SH0ES and the tip of the red giant branch (TRGB) distance ladder, as measured and calibrated by Freedman and collaborators. I show that the difference between the TRGB and SH0ES values of H0 is caused mainly by a systematic calibration offset. In the short term, observational efforts should be focussed on improving the calibrations of the distance anchors and nearby galaxies, rather than trying to measure distance moduli to more supernovae host galaxies. I argue that an independent distance estimate to NGC 4258 is particularly critical. With such observations, it should be possible, on a relatively short timescale, to establish definitively whether the Hubble tension really exists.

Motivation & Objective

• Assess whether systematics could bias the SH0ES H0 measurement and how
• Evaluate consistency between SH0ES Cepheid data across revisions (R11 vs R16) and distance anchors
• Investigate tensions among distance anchors (LMC, NGC 4258, MW) and their impact on H0
• Examine the compatibility of SH0ES results with TRGB measurements and early-Universe inferences
• Propose observational priorities to resolve the Hubble tension in the near term

Proposed method

• Use the SH0ES Cepheid framework to translate magnitude offsets into H0 changes via δH0/H0 = 0.2 ln 10 δm (Eq. 2.1)
• Compare PL-relations slopes across galaxies; identify incompleteness bias at short periods (Eq. 2.3, 2.4)
• Perform object-by-object comparisons of R11 vs R16 Cepheids and photometry (Appendix A, Table 1)
• Jointly fit LMC and NGC 4258 Cepheids to constrain the intercepts and slopes (Eq. 3.3–3.4)
• Incorporate metallicity terms into PL-relations (Eq. 4.1) and examine impact on H0
• Explore the SH0ES degeneracy by simulating offsets in Cepheid intercepts (Fig. 4.2)

Experimental results

Research questions

• RQ1 Can systematic biases bias the SH0ES Cepheid calibration enough to reconcile SH0ES H0 with early-time measurements?
• RQ2 Are the LMC, NGC 4258, and MW distance anchors in tension within the SH0ES framework?
• RQ3 How do PL-slope incompleteness and color/photometry systematics affect the derived H0?
• RQ4 What observations are most critical to resolve the Hubble tension in the near term?

Key findings

• There is a reported tension between the SH0ES anchors LMC and NGC 4258 when calibrated together with SH0ES Cepheids (Δμ_N4258 ≈ 0.177 mag, ~3.5σ if metallicity is ignored)
• R16 photometry shows shallower PL slopes than expected, implying potential biases from incompleteness at short periods; enforcing a fixed slope b = -3.30 can shift H0 by about 1.7 km/s/Mpc
• The intercept offset between R11 and R16 (a_-3.3) suggests a systematic shift in H0 of about 2 km/s/Mpc, larger than quoted SH0ES errors
• Including metallicity corrections reduces the LMC–NGC 4258 tension to slightly over 2σ, indicating metallicity terms are important in the joint fit
• The joint fit with NGC 4258 and LMC anchors tends to favor higher H0 and shows statistical inconsistency with geometric anchor moduli, highlighting the “SH0ES degeneracy” where small photometric offsets can significantly affect results
• The broader SH0ES degeneracy cautions against simply averaging distance anchors when systematic differences exist between anchors.