A 2.4% Determination of the Local Value of the Hubble Constant (1604.01424v3)

Abstract: We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant (H_0) from 3.3% to 2.4%. Improvements come from new, near-infrared observations of Cepheid variables in 11 new hosts of recent SNe~Ia, more than doubling the sample of SNe~Ia having a Cepheid-calibrated distance for a total of 19; these leverage the magnitude-z relation based on 300 SNe~Ia at z<0.15. All 19 hosts and the megamaser system NGC4258 were observed with WFC3, thus nullifying cross-instrument zeropoint errors. Other improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC4258, more Cepheids and a more robust distance to the LMC from late-type DEBs, HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes, and (iv) 2 DEBs in M31. H_0 from each is 72.25+/-2.51, 72.04+/-2.67, 76.18+/-2.37, and 74.50+/-3.27 km/sec/Mpc, respectively. Our best estimate of 73.24+/-1.74 km/sec/Mpc combines the anchors NGC4258, MW, and LMC, and includes systematic errors for a final uncertainty of 2.4%. This value is 3.4 sigma higher than 66.93+/-0.62 km/sec/Mpc predicted by LambdaCDM with 3 neutrinos with mass 0.06 eV and the Planck data, but reduces to 2.1 sigma relative to the prediction of 69.3+/-0.7 km/sec/Mpc with the combination of WMAP+ACT+SPT+BAO, suggesting systematic uncertainties in CMB measurements may play a role in the tension. If we take the conflict between Planck and H_0 at face value, one plausible explanation could involve an additional source of dark radiation in the early Universe in the range of Delta N_eff=0.4-1. We anticipate significant improvements in H_0 from upcoming parallax measurements.

• The paper presents a robust 2.4% local Hubble constant measurement by leveraging near-infrared Cepheid observations in 11 SNe Ia host galaxies.
• The study integrates four geometric distance calibrators, including megamasers and trigonometric parallaxes, to refine the cosmic distance ladder.
• Its findings highlight a 3.4σ tension with Planck’s ΛCDM predictions, suggesting potential new physics in cosmology.

A Critical Examination of a 2.4% Determination of the Local Value of the Hubble Constant

The paper authored by Riess et al. presents an extensive analysis to provide a precise measurement of the Hubble constant (H₀), reducing the uncertainty to 2.4%. This refinement predominantly stems from advancements in observing near-infrared Cepheid variables in 11 host galaxies of recent type Ia supernovae (SNe Ia), thus leveraging prior calibrated distances with an expanded sample size. The paper utilizes observations from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to establish a more robust local cosmic distance ladder.

The paper advances four geometric distance calibrations for Cepheids: megamasers in NGC 4258, detached eclipsing binaries (DEBs) in the Large Magellanic Cloud (LMC), trigonometric parallaxes for Milky Way Cepheids, and DEBs in M31. These calibrations yield varying results, with H₀ estimates ranging from 72.04 km/s/Mpc to 76.18 km/s/Mpc. The authors ultimately propose a combined estimate of H₀ derived from NGC 4258, the Milky Way, and the LMC data. This synthesized value stands at H₀ = {-1}, reflecting a deviation of 3.4σ from the ΛCDM-predicted value of 66.93 km/s/Mpc when considering Planck data.

The methodology emphasizes reducing cross-instrument zeropoint errors by using WFC3 observations across all 19 SNe Ia host galaxies and NGC 4258. Improvements over earlier work include a refined estimate of the maser distance to NGC 4258, a larger sample of Cepheids in the LMC, a robust distance to the LMC based on late-type DEBs, and new trigonometric parallaxes for Cepheids using HST observations. These contributions enable a detailed analysis of systematic uncertainties while leveraging an increased Cepheid sample to yield a more precise local H₀ value.

The analysis provides insights into several crucial astrophysical and cosmological parameters, including the Cepheid period-luminosity relationship and its potential metallicity dependency, though it does not find strong evidence for nonlinearity in the period-luminosity relation at P = 10 days for near-infrared Wesenheit magnitudes. Intriguingly, the paper highlights the possibility of systematic uncertainties in cosmic microwave background (CMB) radiation measurements contributing to the tension between local and cosmic determinations of H₀. If this local tension with Planck's CMB measurements is genuine, it may suggest new physical phenomena, such as additional dark radiation in the early Universe.

Theoretical implications underscore the importance of considering beyond standard model cosmology attributes, such as additional relativistic species or modified gravity theories, which could reconcile local measurements with those from the CMB. Practically, this research prescribes anticipated improvements in the measurement of H₀ from emerging methods, such as the upcoming parallax observations of long-period Cepheids via WFC3.

Given the paper’s meticulous treatment of systematic uncertainties, its synthesis of geometric and photometric anchor points, and its potential to inform discrepancies in cosmological models, it represents a significant contribution to our understanding of the Universe's expansion rate. Future endeavors may incorporate newer parallax measurements from missions like Gaia or refined cosmic distance ladder techniques to further narrow uncertainties and explore potential new physics elucidated by such high-precision results.