JWST Validates HST Distance Measurements: Selection of Supernova Subsample Explains Differences in JWST Estimates of Local H0 (2408.11770v2)

Abstract: JWST provides new opportunities to cross-check the HST Cepheid/SNeIa distance ladder, which yields the most precise local measure of H0. We analyze early JWST subsamples (~1/4 of the HST sample) from the SH0ES and CCHP groups, calibrated by a single anchor (N4258). We find HST Cepheid distances agree well (~1 sigma) with all 8 combinations of methods, samples, and telescopes: JWST Cepheids, TRGB, and JAGB by either group, plus HST TRGB and Miras. The comparisons explicitly include the measurement uncertainty of each method in N4258, an oft-neglected but dominant term. Mean differences are ~0.03 mag, far smaller than the 0.18 mag "Hubble tension." Combining all measures produces the strongest constraint yet on the linearity of HST Cepheid distances, 0.994+-0.010, ruling out distance-dependent bias or offset as the source of the tension at ~7 sigma. Yet, measurements of H0 from current JWST subsamples produce large sampling differences whose size and direction we can directly estimate from the full HST set. We show that Delta(H0)~2.5 km/s/Mpc between the CCHP JWST program and the full HST sample is entirely consistent with differences in sample selection. Combining all JWST samples produces a new, distance-limited set of 16 SNeIa at D<25 Mpc and more closely resembles the full sample thanks to "reversion to the mean" of larger samples. Using JWST Cepheids, JAGB, and TRGB, we find 73.4+-2.1, 72.2+-2.2, and 72.1+-2.2 km/s/Mpc, respectively. Explicitly accounting for SNe in common, the combined-sample three-method result from JWST is H0=72.6+-2.0, similar to H0=72.8 expected from HST Cepheids in the same galaxies. The small JWST sample trivially lowers the Hubble tension significance due to small-sample statistics and is not yet competitive with the HST set (42 SNeIa and 4 anchors), which yields 73.2+-0.9. Still, the joint JWST sample provides important crosschecks which the HST data passes.

• The paper demonstrates that HST Cepheid distances closely match JWST results, with mean differences of about 0.03 magnitudes in supernova subsamples.
• The study employs robust statistical methods, ruling out significant systematic biases with a linearity coefficient of 0.994 ± 0.010 at over 5σ confidence.
• By combining data from both telescopes, the researchers obtain local H0 estimates consistent with expectations, suggesting that the Hubble tension may stem from factors beyond distance measurement errors.

Overview and Analysis of {\it JWST} and {\it HST} Distance Measurement Comparisons

This paper provides a detailed comparison between supernova distance measurements obtained via the Hubble Space Telescope ({\it HST}) and the James Webb Space Telescope ({\it JWST}) as part of efforts to refine the local value of the Hubble constant (H$_0$). The researchers focus on the critical assessment of Cepheid and supernova (SN) observations, utilizing both telescopes to establish an intercomparison framework particularly around the "Hubble tension," which denotes the discrepancy between the locally measured H$_0$ and the value inferred from the cosmic microwave background (CMB) under the standard $\Lambda$CDM model.

The authors leverage a range of distance indicators, including Cepheids, Tip of the Red Giant Branch (TRGB), and C-rich AGB stars (JAGB), to cross-validate distance measurements independently obtained from both telescopes. Key to their approach is ensuring methodological consistency, whereby they calibrate {\it JWST} measurements with those from {\it HST} and examine the linearity and offsets of Cepheid distance measures to explore their potential role in the observed Hubble tension.

Key Findings

1. Measurement Consistency: The paper demonstrates that Hubble Space Telescope Cepheid distances align well with those obtained using {\it JWST} across multiple distance indicators, with mean differences approximating 0.03 magnitudes, significantly smaller than the 0.18 magnitude required to resolve the Hubble tension purely through distance discrepancies. This is a substantial result indicating minimal systematic bias between the telescopic results at the magnitudes necessary to explain the tension.
2. Statistical Robustness: The research presents strong statistical consistency between methods, ruling out a distance-dependent bias as a source of discord in the Hubble tension. The extracted linearity coefficient is $0.994 \pm 0.010$, effectively precluding systemic multiplicative or additive errors in the {\it HST} Cepheid distances capable of explaining the observed discrepancy to a confidence level exceeding 5$\sigma$.
3. Small-Sample Challenges: The investigators highlight the challenges associated with small sample sizes in the {\it JWST} observations, logically attributing observed fluctuations in $H_0$ estimates to sampling differences rather than inherent discrepancies between distance indicator methodologies. They argue that the relatively limited predictive power of current {\it JWST} samples restricts their capability to independently resolve the Hubble tension to a statistically significant extent.
4. Joint Data Insights: By combining all available {\it JWST} samples for SN hosts at distances $D\leq 25$ Mpc, the authors find a calculated $H_0$ of 72.6$\pm$2.0 km/s/Mpc, effectively matching expectations predicated on {\it HST} observations alone. This congruence reinforces the utility of {\it JWST} as a complementary tool for existing {\it HST}-based cosmic distance ladder models rather than a mechanism for standalone resolution of the tension.

Implications and Further Directions

This work importantly suggests that the resolution to the Hubble tension might not exclusively rest with methodological refinements in local astrophysical measurements but might instead point toward deeper cosmological conjectures necessitating further exploration. By affirming the consistency across independent methodologies and instruments, this paper compels the scientific community to consider alternative explanations, potentially involving new physics beyond $\Lambda$CDM or reaffirming the necessity to revisit CMB and model parameters critically.

Future endeavors could benefit from the expanded datasets that {\it JWST} promises, potentially alleviating the limitations of small sample size and enhancing the statistical precision of local measurements. Simultaneously, interdisciplinary collaboration with high-precision CMB measurements, as well as advancements in theoretical cosmology, remains imperative to converge on a cohesive understanding of cosmic expansion dynamics. The path forward will require integrating new observational strategies with continued theoretical innovation to address the complex web of evidence underpinning the Hubble constant conundrum.