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 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.
Description
[1604.01424] A 2.4% Determination of the Local Value of the Hubble Constant
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