Status Report on the Chicago-Carnegie Hubble Program (CCHP): Measurement of the Hubble Constant Using the Hubble and James Webb Space Telescopes (2408.06153v3)

Abstract: We present the latest results from the Chicago-Carnegie Hubble Program (\cchp) to measure the Hubble constant, using data from the James Webb Space Telescope (JWST). The overall program aims to calibrate three independent methods: (1) Tip of the Red Giant Branch (TRGB) stars, (2) JAGB (J-Region Asymptotic Giant Branch) stars, and (3) Cepheids. To date, our program includes 10 nearby galaxies, hosting 11 Type Ia supernovae (SNe Ia) suitable for measuring the Hubble constant ($H_0$). It also includes the galaxy NGC 4258, whose geometric distance provides the zero-point calibration. In this paper we discuss our results from the TRGB and JAGB methods. Our current best (highest precision) estimate is $H_0$ = 70.39 $\pm$ 1.22 (stat) $\pm$ 1.33 (sys) $\pm$ 0.70 ($\sigma_{SN}$), based on the TRGB method alone, with a total of 24 SN Ia calibrators from both HST and JWST data. Based on our new JWST data only, and tying into SNe Ia, we find values of $H_0$ = 68.81 $\pm$ 1.79 (stat) $\pm$ 1.32 (sys) for the TRGB, and $H_0$ = 67.80 $\pm$ 2.17 (stat) $\pm$ 1.64 (sys) km/s/Mpc for the JAGB method. The distances measured using the TRGB and the JAGB method agree, on average, at a level better than 1%, and with the SH0ES Cepheid distances at just over the 1% level. Our results are consistent with the current standard LambdaCDM model, without the need for the inclusion of additional new physics. Future JWST data will be required to increase the precision and accuracy of the local distance scale.

• The paper presents three independent JWST-based methods—TRGB, JAGB, and Cepheids—to estimate the Hubble constant.
• It reports a combined H₀ of 69.96 ± 1.05 (stat) ± 1.12 (sys) km/s/Mpc with high consistency among TRGB and JAGB measures.
• The study underscores the need to minimize systematic uncertainties and improve calibration for resolving the local distance scale tension.

Overview of the Chicago-Carnegie Hubble Program's Measurement of the Hubble Constant Using JWST

The article by Freedman et al. provides detailed results from the Chicago-Carnegie Hubble Program (CCHP), which aims to estimate the Hubble constant (H₀) through the application of three independent astrophysical methodologies utilizing data obtained from the James Webb Space Telescope (JWST). The paper reports promising values of H₀ using (1) the Tip of the Red Giant Branch (TRGB), (2) J-region Asymptotic Giant Branch (JAGB) stars, and (3) classical Cepheid variables. Ten local galaxies serve as primary targets, housing type Ia supernovae (SNe Ia) that allow the calibration of the distance ladder up to the Hubble flow, anchored by the geometric distance to NGC 4258.

Key Findings

1. Distance Measurements:
   • The TRGB, JAGB, and Cepheid methods provide estimates of H₀ = 69.85 ± 1.75 (stat) ± 1.54 (sys), 67.96 ± 1.85 (stat) ± 1.90 (sys), and 72.05 ± 1.86 (stat) ± 3.10 (sys) km/s/Mpc respectively. These are combined for an estimated H₀ = 69.96 ± 1.05 (stat) ± 1.12 (sys) km/s/Mpc.
   • There is remarkable consistency at the 1% level between TRGB and JAGB distances, albeit with modest discrepancies (2.5-4%) when compared to Cepheid distance estimates.
2. Systematic and Statistical Uncertainties:
   • Systematic uncertainties primarily arise from the geometric calibration to NGC 4258, estimated at 1.5%, a parameter still needing further accuracy refinement.
   • The TRGB and JAGB methods show exceptional promise through their relative distance consistency and minimized red-leak/crowding effects when compared to Cepheids.
3. Implications for Cosmology:
   • This paper indicates a potential discord in local distance measurements (local distance scale tension) without requiring departures from standard ΛCDM cosmology given the close alignment with cosmologically inferred H₀ values from CMB measurements.
   • Importantly, results favor refinement in understanding astrophysical systematics before concluding the much-discussed "Hubble tension."

Critical Analysis

This work underlines several strengths in the application of JWST for high-precision cosmological studies, specifically in the determination of local astronomical distances. The CCHP employs a cohesive strategy leveraging homogeneous datasets from JWST, beneficial for minimizing cross-instrument calibration biases, thereby enhancing distance measurement reliability.

Methodological Robustness and Challenges

Significant thrust is placed on resolving internal and external inconsistencies within the cosmic distance ladder through parallel calibration endeavors via multiple independent methodologies. To know the root cause of the residual discrepancies among these methods (most notably with Cepheids), further empirical investigation is merited. Challenges remain in addressing systematic contributions and the external reproducibility of claimed precision, underlined by variance among independently measured distance indicators.

Future Prospects

Future work will undoubtedly benefit from extending these methodologies to a larger sample of galaxies within the JWST reach, paving the way for reducing uncertainty levels below the 1% threshold. Continued improvements in parallax measurements and external verification across alternative distance measures will be crucial steps for forthcoming experimental campaigns.

Ultimately, pursuits such as the CCHP play an instrumental role in deepening understanding, providing clarity on the local value of H₀ and simultaneously refining the cosmic expansion framework that underpins modern cosmology. Further work, particularly increasing sample sizes of SNe Ia calibrators and refining systematics, promises significant strides in cosmological precision measurements.