Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM (1903.07603v2)

Abstract: We present an improved determination of the Hubble constant (H0) from Hubble Space Telescope (HST) observations of 70 long-period Cepheids in the Large Magellanic Cloud. These were obtained with the same WFC3 photometric system used to measure Cepheids in the hosts of Type Ia supernovae. Gyroscopic control of HST was employed to reduce overheads while collecting a large sample of widely-separated Cepheids. The Cepheid Period-Luminosity relation provides a zeropoint-free link with 0.4% precision between the new 1.2% geometric distance to the LMC from Detached Eclipsing Binaries (DEBs) measured by Pietrzynski et al (2019) and the luminosity of SNe Ia. Measurements and analysis of the LMC Cepheids were completed prior to knowledge of the new LMC distance. Combined with a refined calibration of the count-rate linearity of WFC3-IR with 0.1% precision (Riess et al 2019), these three improved elements together reduce the full uncertainty in the LMC geometric calibration of the Cepheid distance ladder from 2.5% to 1.3%. Using only the LMC DEBs to calibrate the ladder we find H0=74.22 +/- 1.82 km/s/Mpc including systematic uncertainties, 3% higher than before for this particular anchor. Combining the LMC DEBs, masers in NGC 4258 and Milky Way parallaxes yields our best estimate: H0 = 74.03 +/- 1.42 km/s/Mpc, including systematics, an uncertainty of 1.91%---15% lower than our best previous result. Removing any one of these anchors changes H0 by < 0.7%. The difference between H0 measured locally and the value inferred from Planck CMB+LCDM is 6.6+/-1.5 km/s/Mpc or 4.4 sigma (P=99.999% for Gaussian errors) in significance, raising the discrepancy beyond a plausible level of chance. We summarize independent tests which show this discrepancy is not readily attributable to an error in any one source or measurement, increasing the odds that it results from a cosmological feature beyond LambdaCDM.

• The paper refines the Hubble constant to 74.22 km/s/Mpc using 70 LMC Cepheids calibrated by DEBs for 1% precision.
• The study employs robust photometry and corrections for count-rate non-linearity to ensure consistent Cepheid distance measurements.
• The results reveal a significant 4.4σ discrepancy with the ΛCDM model, indicating potential new physics.

Large Magellanic Cloud Cepheid Standards and Implications for the Hubble Constant

The paper conducted by Riess et al. explores a refined determination of the Hubble constant ($H_0$), presenting a 1% precision calibration utilizing Cepheids in the Large Magellanic Cloud (LMC). Through this investigation, multiple enhancements were applied to tighten the constraints on $H_0$, providing compelling evidence that challenges the standard $\Lambda$CDM cosmological model.

Methodology

The determination of the Hubble constant is driven by refining the Cepheid Period-Luminosity (PL) relation, a cornerstone in the cosmic distance ladder. The paper implements a photometric survey using the Hubble Space Telescope (HST) across 70 long-period Cepheids in the LMC, spanning three bands: $F555W$, $F814W$, and $F160W$. The photometric observations were coordinated with DASH mode, ensuring efficient data capture even under suboptimal gyro conditions. Using redundancy measures such as these provided a systematic approach to maintaining data integrity across all observed samples.

The distances were anchored using the geometric distance of the LMC derived from 20 Detached Eclipsing Binaries (DEBs), offering a significant precision of 1.2%. By deploying a consistent photometric system on the HST observed samples and those observed in extragalactic Cepheids in the host galaxies of Type Ia supernovae (SNe Ia), the researchers were able to nullify variations introduced by different system zeropoints. A corrected understanding of Count-Rate Non-Linearity (CRNL) further improves this photometric consistency, offering a robust calibration for relative distances.

Results

This detailed methodological framework enabled the research team to identify a refined value of $H_0 = 74.22$ km/s/Mpc, accounting for both statistical and systematic uncertainties, which is considerably higher than the $67.4$ km/s/Mpc predicted from CMB observations calibrated by the $\Lambda$CDM model with Planck data. The discrepancy of $4.4\sigma$ underscores a significant inconsistency between early-universe projections and late-universe observational data—a phenomenon referred to as the "Hubble tension."

Implications

The implications of such a marked difference are substantial, challenging the foundational assumptions underpinning cosmological models. A notable feature is the strength of the calibration technique expanded through independent cross-verifications—a vital effort to spearhead a model-independent validation of Cepheid calibrations on the cosmic scale. This paper corroborates strong-lensing results and methodologies using alternate cosmic tracers, laying groundwork for potential explorations into non-standard physics, such as alterations of dark energy, additional relativistic particles, or novel interactions impacting cosmic expansion.

Future Perspectives

Resolving the existing tensions delineates a pathway toward an improved cosmic understanding. Future developments in Large Scale Structure (LSS) observations, enhanced GAIA data releases, and integrations with other standard candle methodologies, such as the Tip of the Red Giant Branch (TRGB) and Miras, would bolster the reliability and precision of extragalactic distance measurements. Additionally, engagements with gravitational wave bursts, acting as 'standard sirens', could introduce a new era of multi-modal cosmic observations, having potential to independently corroborate or redefine empirical $\Lambda$CDM estimations.

This paper evidences an era in cosmology fraught with challenges that require rigorous investigation into each element, quantifying uncertainties, and proposing reformative theories rooted in robust data. It is clear that this level of precision on measurements like the Hubble constant could illuminate new physical principles governing our universe. Through iterative engagements with cosmological data, refinements will continue to craft an empirical understanding to bridge the old with the emergent.