http://arxiv.org/abs/1604.01424
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 observations of Cepheid variables in 10 new hosts of recent SNe~Ia, more than doubling the sample of SNe~Ia having a Cepheid-calibrated distance for a total of 18; these leverage the magnitude-redshift relation based on 300 SNe~Ia at z<0.15. All 18 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. The H_0 from each is 72.39+/-2.56, 71.93+/-2.70, 76.09+/-2.42, and 74.45+/-3.34 km/sec/Mpc, respectively. Our best estimate of 73.03+/-1.79 km/sec/Mpc combines the anchors NGC4258, MW, and LMC, and includes systematic errors for a final uncertainty of 2.4%. This value is 3.0 sigma higher than 67.3+/-0.7 km/sec/Mpc predicted by LambdaCDM with 3 neutrinos with a mass of 0.06 eV and the Planck data, but reduces to 1.9 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 the 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.
A. Riess, L. Macri, S. Hoffmann, et. al.
Thu, 7 Apr 16
25/51
Comments: submitted to ApJ, 63 pages, 16 figures, 8 tables
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