# Crowded No More: The Accuracy of the Hubble Constant Tested with High Resolution Observations of Cepheids by JWST

**Authors:** Adam G. Riess, Gagandeep S. Anand, Wenlong Yuan, Stefano Casertano, Andrew Dolphin, Lucas M. Macri, Louise Breuval, Dan Scolnic, Marshall Perrin, Richard I. Anderson  
**arXiv:** [2307.15806](https://arxiv.org/abs/2307.15806)  
**Date:** 2023-07-28  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2307.15806v1/

## Key points

- The study uses high-resolution observations from the James Webb Space Telescope (JWST) to test the accuracy of previous observations made by the Hubble Space Telescope (HST) regarding extragalactic Cepheids and their impact on determining the Hubble constant.
- Two galaxies, NGC4258 and NGC5584, are focused on in the study as they have known distances based on maser-based geometric measurements and are representative of the sample used in previous HST studies.
- JWST's superior ability to separate sources from line-of-sight companions in the near-infrared region leads to a significant reduction in dispersion of Cepheid period-luminosity relations.
- This results in a higher precision for individual Cepheid measurements with an error rate reduced from 20% to 7%.
- Two-epoch photometry is performed to confirm identifications, assess JWST's photometric stability, and constrain Cepheid phases.
- The intercepts of the period-luminosity relations obtained from JWST observations are found to be in excellent agreement with those derived from HST data for both NGC4258 and NGC5584.
- Differences between them are measured as 0.00+/-0.03 magnitudes for NGC4258 and 0.02+/-0.03 magnitudes for NGC5584 respectively, indicating an insignificant discrepancy of only 0.02+/-0.04 magnitudes in determining the Hubble constant (H0).
- Various analysis variants consistently support these baseline results, suggesting that systematic errors in HST Cepheid photometry do not significantly contribute to current estimates of the Hubble Tension.

## Summary

In this study, the authors utilize high-resolution observations from the James Webb Space Telescope (JWST) to test the accuracy of previous observations made by the Hubble Space Telescope (HST) regarding extragalactic Cepheids and their impact on determining the Hubble constant. The researchers focus on two galaxies, NGC4258 and NGC5584, which have known distances based on maser-based geometric measurements and are representative of the sample used in previous HST studies. The JWST observations, conducted in two epochs and three filters, include more than 330 Cepheids in these galaxies. The advantage of using JWST over HST lies in its superior ability to separate sources from line-of-sight companions in the near-infrared (NIR) region due to low extinction. This results in a significant reduction in the dispersion of the Cepheid period-luminosity (P-L) relations by more than 2.5 times - decreasing from 0.45 to 0.17 magnitudes - leading to a higher precision for individual Cepheid measurements with an error rate reduced from 20% to 7%. The researchers also perform two-epoch photometry to confirm identifications, assess JWST's photometric stability, and constrain Cepheid phases. The intercepts of the P-L relations obtained from JWST observations are found to be in excellent agreement with those derived from HST data for both NGC4258 and NGC5584; differences between them being measured as 0.00+/-0.03 magnitudes for NGC4258 and 0.02+/-0.03 magnitudes for NGC5584 respectively - translating into an insignificant discrepancy of only 0.02+/-0.04 magnitudes in determining the Hubble constant (H0). Furthermore, various analysis variants such as different passband combinations, phase corrections, measured detector offsets and crowding levels consistently support these baseline results; indicating that systematic errors in HST Cepheid photometry do not significantly contribute to current estimates of the Hubble Tension.