HST/WFC3 obs. of Cepheids in SN Ia host gal.

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+/-3.27km/s/Mpc, respectively. Our best estimate of H_0_=73.24+/-1.74km/s/Mpc 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 3.4{sigma} higher than 66.93+/-0.62km/s/Mpc predicted by {Lambda}CDM with 3 neutrino flavors having a mass of 0.06eV and the new Planck data, but the discrepancy reduces to 2.1{sigma} relative to the prediction of 69.3+/-0.7km/s/Mpc 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 {Delta}N_eff_~0.4-1. We anticipate further significant improvements in H_0_ from upcoming parallax measurements of long-period MW Cepheids.