Measurements of the Hubble Constant: Tensions in Perspective (2106.15656v1)

Abstract: Measurement of the distances to nearby galaxies have improved rapidly in recent decades. The ever-present challenge is to reduce systematic effects, especially as greater distances are probed, and the uncertainties become larger. In this paper, we combine several recent calibrations of the Tip of the Red Giant Branch (TRGB) method. These calibrations are internally self-consistent at the 1% level. New Gaia Early Data Release 3 (EDR3) data provide an additional consistency check, at a (lower) 5% level of accuracy, a result of the well-documented Gaia angular covariance bias. The updated TRGB calibration applied to a distant sample of Type Ia supernovae from the Carnegie Supernova Project results in a value of the Hubble constant of Ho = 69.8 $\pm$ 0.6 (stat) $\pm$ 1.6 (sys) km/s/Mpc. No statistically significant difference is found between the value of Ho based on the TRGB and that determined from measurements of the cosmic microwave background. The TRGB results are also consistent to within 2$\sigma$ with the SHoES and Spitzer plus HST Key Project Cepheid calibrations. The TRGB results alone do not demand additional new physics beyond the standard Lambda-CDM cosmological model. They have the advantage of simplicity of the underlying physics (the core He flash) and small systematic uncertainties (from extinction, metallicity and crowding). Finally, the strengths and weaknesses of both the TRGB and Cepheids are reviewed, and prospects for addressing the current discrepancy with future Gaia, HST and JWST observations are discussed. Resolving this discrepancy is essential for ascertaining if the claimed tension in Ho between the locally-measured and the CMB-inferred value is physically motivated.

• The paper updates the absolute I-band TRGB magnitude and recalibrates the Hubble constant using independent geometric anchors.
• It integrates multiple methods including data from NGC 4258, Galactic globular clusters, and the Magellanic Clouds to ensure robust distance measurements.
• Results yield H0 = 69.8 ± 0.6 (stat) ± 1.6 (sys) km s⁻¹ Mpc⁻¹, aligning closer with CMB estimates and mitigating discrepancies with Cepheid-based measurements.

A Reassessment of the Hubble Constant: Examining Tip of the Red Giant Branch Calibrations

The determination of the Hubble constant ($H_0$), which describes the rate of expansion of the Universe, remains a central problem in cosmology. The paper focuses on an independent calibration method known as the Tip of the Red Giant Branch (TRGB) and its implications for resolving current discrepancies in measuring $H_0$. The key points addressed in the publication include updates to the absolute $I$-band magnitude calibration of the TRGB, and a reassessment of $H_0$ utilizing data from type Ia supernovae (SNe Ia).

TRGB Calibration and Methodology

The TRGB method leverages the fact that the tip of the red giant branch, where low-mass stars undergo a helium flash, acts as a standard candle. This feature provides a reliable distance measure due to its relative insensitivity to variables such as metallicity and extinction in certain bands. This research integrates several independent geometric calibrations, including:

• NGC 4258: Utilizing the galaxy's megamasers, a geometric distance serves as a robust anchor.
• Milky Way Globular Clusters: TRGB calibration via horizontal branch distance scale from Galactic globular clusters.
• Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC): Based on distances derived from detached eclipsing binaries (DEBs), including revised reddening corrections.

These calibrations collectively yield an absolute $I$-band TRGB magnitude, $M_I^{TRGB}$, with a high degree of consistency across various efforts.

Hubble Constant Calibration Using TRGB

This paper extends the TRGB calibration to 19 supernovae host galaxies, incorporating new data for the SNe Ia set from the Carnegie Supernova Project. The paper highlights the advantage of using the TRGB method—applicable to both elliptical and spiral galaxies—thereby avoiding biases related to galaxy type that affect other methods like Cepheids.

A detailed statistical analysis resulted in a recalibrated value of the Hubble constant from the TRGB method, determining $$H_0 = 69.8 \pm 0.6 \ (\text{stat}) \pm 1.6 \ (\text{sys}) \ \text{km s}^{-1}\text{Mpc}^{-1}$$. This value aligns more closely with CMB measurements, potentially resolving some tension between early and late Universe determinations of $H_0$.

Comparative Analysis with Cepheid-Based Measurements

Contrasts with Cepheid-based calibrations (e.g., those by SHoES) expose a $1.6\sigma$ discrepancy. Cepheid methods traditionally yield values of $H_0$ around 73-74 km s$^{-1}$ Mpc$^{-1$. Despite both TRGB and Cepheid methods sharing some base calibrations (such as with the LMC and NGC 4258), systematic uncertainties like crowding and metallicity affect them differently, particularly at the long distances probed through host galaxies of SNe Ia.

Implications and Future Directions

This work underscores the viability and precision of the TRGB method as a complementary or alternative distance scale measure. Notably, the paper suggests that the differences in $H_0$ estimates might originate from further distances along the SN Ia distance ladder, not from local calibration errors. The significance of reducing systematic uncertainties is emphasized, and the potential for high-precision measurements from future instruments, such as the James Webb Space Telescope (JWST) and upcoming Gaia data releases, could reconcile discrepancies among methodologies.

In conclusion, the paper positions the TRGB method as a promising tool for refining measurements of the Hubble constant, encouraging a broader application to diverse galaxy types to mitigate current $H_0$ tensions and ultimately enhancing our understanding of universal expansion.