[Paper Review] The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration

Edvard Mörtsell, A. Goobar+2 more|arXiv (Cornell University)|May 24, 2021

Cosmology and Gravitation Theories3 references20 citations

TL;DR

This paper re-evaluates the Cepheid color-luminosity calibration used to determine the Hubble constant, proposing a data-driven, galaxy-specific approach instead of a universal relation. It finds $H_0 = 66.9 \pm 2.5$ km/s/Mpc using Wesenheit magnitudes—consistent with Planck—while another method yields $H_0 = 71.8 \pm 1.6$ km/s/Mpc, indicating that calibration choices introduce significant systematic uncertainty that obscures the true Hubble tension.

ABSTRACT

Motivated by the large observed diversity in the properties of extra-galactic extinction by dust, we re-analyse the Cepheid calibration used to infer the local value of the Hubble constant, $H_0$, from Type Ia supernovae. Unlike the SH0ES team, we do not enforce a universal color-luminosity relation to correct the near-IR Cepheid magnitudes. Instead, we focus on a data driven method, where the measured colors of the Cepheids are used to derive a color-luminosity relation for each galaxy individually. We present two different analyses, one based on Wesenheit magnitudes, a common practice in the field that attempts to combine corrections from both extinction and variations in intrinsic colors, resulting in $H_0=66.9\pm 2.5$ km/s/Mpc, in agreement with the Planck value. In the second approach, we calibrate using color excesses with respect to derived average intrinsic colors, yielding $H_0=71.8\pm 1.6$ km/s/Mpc, a $2.7\,\sigma$ tension with the value inferred from the cosmic microwave background. Hence, we argue that systematic uncertainties related to the choice of Cepheid color-luminosity calibration method currently inhibits us from measuring $H_0$ to the precision required to claim a substantial tension with Planck data.

Motivation & Objective

• To re-examine the Cepheid color-luminosity calibration method used in local $H_0$ determinations, particularly in the context of the Hubble tension.
• To address the assumption of a universal color-luminosity relation for Cepheids, which may introduce systematic biases in $H_0$ measurements.
• To test whether galaxy-specific, data-driven color-luminosity relations reduce uncertainty in $H_0$ estimates compared to standard universal calibrations.
• To assess whether the observed tension between local $H_0$ measurements and Planck's CMB-derived value is driven by calibration systematics rather than new physics.

Proposed method

• Instead of applying a universal color-luminosity relation, the method derives a unique color-luminosity relation for each galaxy based on observed Cepheid colors.
• Uses Wesenheit magnitudes—defined as $M_W = V - R imes (V - I)$—to combine extinction and intrinsic color corrections into a single, reddening-insensitive magnitude.
• Applies a second approach using color excesses relative to derived average intrinsic colors, allowing for galaxy-specific extinction corrections.
• Calibrates Cepheid distances using these galaxy-specific relations, then applies them to Type Ia supernovae to infer $H_0$ via the distance ladder.
• Employs a statistical framework to propagate uncertainties from Cepheid colors and magnitudes through to the final $H_0$ estimate.
• Compares results from both calibration methods to assess sensitivity to systematic choices in color correction.

Experimental results

Research questions

• RQ1 How does using galaxy-specific color-luminosity relations for Cepheids affect the inferred value of the Hubble constant?
• RQ2 To what extent do systematic uncertainties in Cepheid color calibration contribute to the observed Hubble tension?
• RQ3 Does the choice between universal versus data-driven color-luminosity relations significantly alter the $H_0$ determination?
• RQ4 Can a data-driven calibration method reconcile local $H_0$ measurements with the Planck CMB value?
• RQ5 What is the impact of intrinsic color variations and dust extinction diversity on $H_0$ precision in the local distance ladder?

Key findings

• Using a data-driven, galaxy-specific color-luminosity relation for Cepheids yields $H_0 = 66.9 \pm 2.5$ km/s/Mpc when employing Wesenheit magnitudes, which is consistent with the Planck value.
• When calibrating via color excesses relative to derived intrinsic colors, the method yields $H_0 = 71.8 \pm 1.6$ km/s/Mpc, indicating a $2.7\sigma$ tension with the Planck result.
• The large discrepancy between the two calibration methods highlights that systematic uncertainties in Cepheid color calibration are a dominant source of uncertainty in $H_0$ determination.
• The study concludes that current systematic errors in color-luminosity calibration prevent a definitive claim of tension between local and CMB-based $H_0$ values.
• The results suggest that resolving the Hubble tension may require more robust, data-driven calibration techniques rather than relying on universal relations.
• The sensitivity of $H_0$ to calibration method implies that future high-precision $H_0$ measurements must carefully account for galaxy-specific dust and color properties.