A 3% SOLUTION: DETERMINATION OF THE HUBBLE CONSTANT WITH THE<i>HUBBLE SPACE TELESCOPE</i>AND WIDE FIELD CAMERA 3

We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope to\ndetermine the Hubble constant (H0) from optical and infrared observations of\nover 600 Cepheid variables in the host galaxies of 8 recent Type Ia supernovae\n(SNe Ia), providing the calibration for a mag-z relation of 253 SNe Ia.\nIncreased precision over past measurements comes from: (1) more than doubling\nthe number of infrared observations of Cepheids in nearby SN hosts; (2)\nincreasing the sample of ideal SN Ia calibrators from six to eight; (3)\nincreasing by 20% the number of Cepheids with infrared observations in the\nmegamaser host NGC 4258; (4) reducing the difference in the mean metallicity of\nthe Cepheid comparison samples from \\Delta log [O/H] = 0.08 to 0.05; and (5)\ncalibrating all optical Cepheid colors with one camera, WFC3, to remove\ncross-instrument zero-point errors. Uncertainty in H0 from beyond the 1st rung\nof the distance ladder is reduced from 3.5% to 2.3%. The measurement of H0 via\nthe geometric distance to NGC 4258 is 74.8 \\pm 3.1 km s- 1 Mpc-1, a 4.1%\nmeasurement including systematics. Better precision independent of NGC 4258\ncomes from two alternative Cepheid absolute calibrations: (1) 13 Milky Way\nCepheids with parallaxes and (2) 92 Cepheids in the Large Magellanic Cloud with\nmultiple eclipsing binary distances, yielding 74.4 \\pm 2.5 km s- 1 Mpc-1, a\n3.4% uncertainty with systematics. Our best estimate uses all three\ncalibrations but a larger uncertainty afforded from any two: H0 = 73.8 \\pm 2.4\nkm s- 1 Mpc-1 including systematics, a 3.3% uncertainty. The improvement in H0,\ncombined with WMAP7yr data, results in a constraint on the EOS parameter of\ndark energy of w = -1.08 \\pm 0.10 and Neff = 4.2 \\pm 0.7 for the number of\nrelativistic species in the early universe. It also rules out the best-fitting\ngigaparsec-scale void models, posited as an alternative to dark energy.\n(abridged)\n