A 2.4% DETERMINATION OF THE LOCAL VALUE OF THE HUBBLE CONSTANT^*

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 from 3.3% to 2.4%. The bulk of this improvement comes from new near-infrared (NIR) observations of Cepheid variables in 11 host galaxies of recent type Ia supernovae (SNeIa), more than doubling the sample of reliable SNeIa having a Cepheid-calibrated distance to a total of 19; these in turn leverage the magnituderedshift relation based on ∼300 SNeIa at z<0.15. All 19 hosts as well as the megamaser system NGC 4258 have been observed with WFC3 in the optical and NIR, thus nullifying cross-instrument zeropoint errors in the relative distance estimates from Cepheids. Other noteworthy improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC 4258, a larger sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to the LMC based on late-type detached eclipsing binaries (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 NGC 4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes measured with HST/FGS, HST/WFC3 spatial scanning and/or Hipparcos, and (iv) 2 DEBs in M31. The Hubble constant from each is 72.25±2.51, 72.04±2.67, 76.18±2.37, and 74.50±3.27 km s −1 Mpc −1 , respectively. Our best estimate of H 0 =73.24±1.74 km s −1 Mpc −1 combines the anchors NGC 4258, MW, and LMC, yielding a 2.4% determination (all quoted uncertainties include fully propagated statistical and systematic components). This value is 3.4σ higher than 66.93±0.62 km s −1 Mpc −1 predicted by ΛCDM with 3 neutrino flavors having a mass of 0.06eV and the new Planck data, but the discrepancy reduces to 2.1σ relative to the prediction of 69.3±0.7 km s −1 Mpc −1 based on the comparably precise combination of WMAP+ACT+SPT+BAO observations, suggesting that systematic uncertainties in CMB radiation measurements may play a role in the tension. If we take the conflict between Planck high-redshift measurements and our local determination of H 0 at face value, one plausible explanation could involve an additional source of dark radiation in the early universe in the range of ΔN eff ≈0.4-1. We anticipate further significant improvements in H 0 from upcoming parallax measurements of long-period MW Cepheids.

Cosmology in Mirror Twin Higgs and neutrino masses

“…This bound may be somewhat relaxed if the dark radiation scatters with a short mean free path, as opposed to free streaming like neutrinos [50]. It should also be noted that the recently observed tension between CMB observations and the measurement of the local Hubble expansion can be interpreted as a hint of a ∆N eff ∼ O(1) [51]. However, even after taking these…”

Observational calibration of the projection factor of Cepheids

“…At a later stage, accurate broadband epoch photometry will also be included in the Gaia data releases (see, e.g., Clementini et al 2016). Combining Gaia data with archival observations, the SPIPS technique will enable a very accurate calibration of the Pp relation of Cepheids, and therefore of their distance scale, which is still today an essential ingredient in determining the local value of H 0 (Riess et al 2016). For the nearest Cepheids of the Gaia and HST/WFC3 samples, the availability of interferometric angular diameters will significantly improve the quality of the determination of their parameters thanks to the resolution of the usual degeneracy between effective temperature and interstellar reddening.…”

Analysis of the Angular Dependence of Time Delay in Gravitational Lensing

“…The most important modern techniques arise from the study of the perturbations of the Cosmic Microwave Background (CMB) [7], the Supernovae 1A (SN 1A) [6] and the effect of Gravitational Lensing (GL) [8][9][10][11][12][13]. Nevertheless, the different measurements are not compatible one with each other, and still a slight tension about the correct value of H 0 does remain [14].…”

“…After these works, the measurements of H 0 refined more and more, to reach the most recent direct estimate of the expansion rate of the Universe: H 0 = (73.0 ± 1.8) km s −1 Mpc −1 [6]. The most important modern techniques arise from the study of the perturbations of the Cosmic Microwave Background (CMB) [7], the Supernovae 1A (SN 1A) [6] and the effect of Gravitational Lensing (GL) [8][9][10][11][12][13].…”