Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 \mu m with the Spitzer Space Telescope , we have obtained a new , high-accuracy calibration of the Hubble constant .
We have established the mid-IR zero point of the Leavitt Law ( the Cepheid Period-Luminosity relation ) using time-averaged 3.6 \mu m data for ten high-metallicity , Milky Way Cepheids having independently-measured trigonometric parallaxes .
We have adopted the slope of the PL relation using time-averaged 3.6 \mu m data for 80 long-period Large Magellanic Cloud ( LMC ) Cepheids falling in the period range 0.8 < log ( P ) < 1.8 .
We find a new reddening-corrected distance to the LMC of 18.477 \pm 0.033 ( systematic ) mag .
We re-examine the systematic uncertainties in H _ { 0 } , also taking into account new data over the past decade .
In combination with the new Spitzer calibration , the systematic uncertainty in H _ { 0 } over that obtained by the Hubble Space Telescope ( HST ) Key Project has decreased by over a factor of three .
Applying the Spitzer calibration to the Key Project sample , we find a value of H _ { 0 } = 74.3 with a systematic uncertainty of \pm 2.1 ( systematic ) km s ^ { -1 } Mpc ^ { -1 } , corresponding to a 2.8 % systematic uncertainty in the Hubble constant .
This result , in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe , yields a value of the equation of state for dark energy , w _ { 0 } = -1.09 \pm 0.10 .
Alternatively , relaxing the constraints on flatness and the numbers of relativistic species , and combining our results with those of WMAP7 , Type Ia supernovae and baryon acoustic oscillations yields w _ { 0 } = -1.08 \pm 0.10 and a value of N _ { eff } = 4.13 \pm 0.67 , mildly consistent with the existence of a fourth neutrino species .