CosmoVerse Data Challenge: How Tip of the Red Giant Branch Analysis Choices Shape the Inferred Hubble Constant
Tip of the red giant branch (TRGB)-based determinations of the Hubble constant ($H_0$), the current expansion rate of the Universe, depend on choices made in selecting and analyzing stellar photometry and in assembling the sample of calibrator galaxies hosting Type Ia supernovae (SNe Ia). The CosmoVerse Data Challenge invites participants to analyze public photometric catalogs within a shared distance-ladder framework, opening each step of the measurement to the community and testing how these choices propagate into $H_0$. Participants independently selected photometric catalogs, galaxies, and TRGB measurement settings, while using a common SN Ia compilation and fitting procedure to convert distances into $H_0$. Supported by public notebooks and a training school, the challenge received 26 submissions from 22 participants. All submissions used the same analysis notebook, many participants were trainees at the school, and the true galaxy distances are unknown; the results therefore characterize the sensitivity of the submissions to analysis and sample-selection choices and are not a combined determination of $H_0$ or an estimate of the systematic uncertainty in published TRGB measurements. The inferred $H_0$ values have a median of $77.10\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and a standard deviation of $7.38\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ $(5.11\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ for the scaled median absolute deviation), $\sim 3.4$ $(\sim 2.4)$ times the typical quoted uncertainty of a submission. Differences in the measured tip of the anchor galaxy NGC 4258 add a coherent shift in the distance scale. The challenge shows how different choices within a common framework can produce substantially different values of $H_0$, and provides an open basis for tracing these differences and for making datasets and tools more transparent and accessible.