Adam G. Riess, Lucas M. Macri, Samantha L. Hoffmann, Dan Scolnic, Stefano Casertano, Alexei V. Filippenko, Brad E. Tucker, Mark J. Reid, David O. Jones, Jeffrey M. Silverman, Ryan Chornock, Peter Challis, Wenlong Yuan, Peter J. Brown, Ryan J. Foley

Research output: Contribution to journalArticle


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 magnitude-redshift 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.

Original languageEnglish (US)
Article number56
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2016


  • cosmological parameters
  • cosmology: observations
  • distance scale
  • galaxies: distances and redshifts

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Riess, A. G., Macri, L. M., Hoffmann, S. L., Scolnic, D., Casertano, S., Filippenko, A. V., Tucker, B. E., Reid, M. J., Jones, D. O., Silverman, J. M., Chornock, R., Challis, P., Yuan, W., Brown, P. J., & Foley, R. J. (2016). A 2.4% DETERMINATION of the LOCAL VALUE of the HUBBLE CONSTANT. Astrophysical Journal, 826(1), [56]. https://doi.org/10.3847/0004-637X/826/1/56