[Paper Review] CATS: The Hubble Constant from Standardized TRGB and Type Ia Supernova Measurements

D. Scolnic, A. G. Riess+8 more|arXiv (Cornell University)|Apr 13, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This paper presents CATS, a standardized algorithm for measuring the Tip of the Red Giant Branch (TRGB) using unsupervised edge-detection to reduce measurement variance across multiple fields. Applying it to SN Ia hosts and NGC 4258, the method yields $H_0 = 73.22 \pm 2.06$ km/s/Mpc, with systematic differences of ~2.0 km/s/Mpc due to SN sample updates and ~1.4 km/s/Mpc from TRGB standardization, clarifying the Hubble tension in late-universe measurements.

ABSTRACT

The Tip of the Red Giant Branch (TRGB) provides a luminous standard candle for constructing distance ladders to measure the Hubble constant. In practice its measurements via edge-detection response (EDR) are complicated by the apparent fuzziness of the tip and the multi-peak landscape of the EDR. As a result, it can be difficult to replicate due to a case-by-case measurement process. Previously we optimized an unsupervised algorithm, Comparative Analysis of TRGBs (CATs), to minimize the variance among multiple halo fields per host without reliance on individualized choices, achieving state-of-the-art $\sim$ $<$ 0.05 mag distance measures for optimal data. Further, we found an empirical correlation at 5$σ$ confidence in the GHOSTS halo survey between our measurements of the tip and their contrast ratios (ratio of stars 0.5 mag just below and above the tip), useful for standardizing the apparent tips at different host locations. Here, we apply this algorithm to an expanded sample of SN Ia hosts to standardize these to multiple fields in the geometric anchor, NGC 4258. In concert with the Pantheon$+$ SN Ia sample, this analysis produces a (baseline) result of $H_0= 73.22 \pm 2.06$ km/s/Mpc. The largest difference in $H_0$ between this and similar studies employing the TRGB derives from corrections for SN survey differences and local flows used in most recent SN Ia compilations but which were absent in earlier studies. SN-related differences total $\sim$ 2.0 km/s/Mpc. A smaller share, $\sim$ 1.4 km/s/Mpc, results from the inhomogeneity of the TRGB calibration across the distance ladder. We employ a grid of 108 variants around the optimal TRGB algorithm and find the median of variants is $72.94\pm1.98$ km/s/Mpc with an additional uncertainty due to algorithm choices of 0.83 km/s/Mpc. None of these TRGB variants result in $H_0$ less than 71.6 km/s/Mpc.

Motivation & Objective

• To reduce measurement variance in TRGB distance estimates by eliminating subjective, case-by-case choices in edge-detection.
• To standardize TRGB apparent magnitudes using an empirical tip-contrast ratio correlation across multiple halo fields.
• To quantify systematic uncertainties in $H_0$ arising from SN Ia sample choices and TRGB calibration inhomogeneity.
• To clarify the Hubble tension by isolating sources of discrepancy between TRGB-based $H_0$ measurements and early-universe predictions.
• To provide a reproducible, algorithmic framework for TRGB-based distance ladder analysis using a grid of 108 variants.

Proposed method

• The CATs algorithm applies unsupervised edge-detection to TRGB luminosity functions, minimizing variance across multiple halo fields per host galaxy.
• It uses a tip-contrast ratio (ratio of stars 0.5 mag below and above the tip) as an empirical standardization parameter, validated at 5σ confidence in the GHOSTS survey.
• The method is applied to 108 algorithmic variants of the TRGB detection process to quantify uncertainty from algorithmic choice.
• SN Ia distances are calibrated using TRGB distances to NGC 4258 as a geometric anchor, with the Pantheon+ sample used for SN Ia data.
• Systematic shifts in $H_0$ are quantified by comparing results to CCHP and EDD studies, isolating contributions from SN sample differences and TRGB calibration.
• A grid of 108 TRGB algorithm variants is used to compute the median $H_0$ and estimate uncertainty from algorithmic choices.

Experimental results

Research questions

• RQ1 How can TRGB measurements be standardized across multiple fields to reduce variance and improve reproducibility?
• RQ2 To what extent do SN Ia sample differences and peculiar flow corrections affect the derived $H_0$ value in TRGB-based ladders?
• RQ3 What is the impact of TRGB calibration inhomogeneity across the distance ladder on $H_0$?
• RQ4 Can an empirical tip-contrast ratio correlation reliably standardize TRGB apparent magnitudes across different host galaxies?
• RQ5 How does algorithmic choice in edge-detection affect the final $H_0$ value, and what is the uncertainty from this source?

Key findings

• The baseline CATS analysis yields $H_0 = 73.22 \pm 2.06$ km/s/Mpc using the Pantheon+ SN Ia sample and standardized TRGB measurements.
• The SN-related component contributes $\sim 2.0$ km/s/Mpc to the $H_0$ difference compared to earlier studies, primarily due to updated SN samples and peculiar flow corrections.
• The TRGB calibration inhomogeneity contributes $\sim 1.4$ km/s/Mpc to the $H_0$ difference, highlighting the need for standardized calibration.
• The median $H_0$ across 108 algorithmic variants is $72.94 \pm 1.98$ km/s/Mpc, with an additional $0.83$ km/s/Mpc uncertainty from algorithmic choice.
• No TRGB variant in the grid produces $H_0 < 71.6$ km/s/Mpc, indicating robust consistency across algorithmic approaches.
• The study confirms that TRGB from edge-detection must be treated as a standardizable, not a standard, candle, due to empirical correlations in tip morphology.