|Annu. Rev. Astron. Astrophys. 2010. 48:
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(1) Several nearby distance determination methods are now available that are of high precision, having independent systematics. These include Cepheid variables, the tip of the red giant branch (TRGB) stars, and the geometrically determined distances to maser galaxies.
(2) The Cepheid Period-Luminosity relation (Leavitt Law) now has an absolute calibration based on HST trigonometric parallaxes for Galactic Cepheids. This calibration and its application at near-infrared wavelengths significantly reduces two of the four leading systematic errors previously limiting the accuracy of the Cepheid-based distance scale: zero-point calibration and metallicity effects.
(3) The maser galaxy distances, TRGB distances and Cepheid distances agree to high precision at the one common point of contact where they can each be simultaneously intercompared, the maser galaxy NGC 4258, at a distance of 7.2 Mpc.
(4) Galactic Cepheid parallax and NGC 4258 maser calibrations of the distance to the LMC agree very well. Based on these measurements and other independent measurements, we adopt a true, metallicity-corrected distance modulus to the LMC of 18.39 ± 0.06 mag.
(5) HST optical and near-infrared observations of Cepheids in SNe Ia galaxies calibrated by the maser galaxy, NGC 4258, have decreased systematics due to calibration, metallicity and reddening in the SNe Ia distance scale, and increased the number of well-observed SN calibrators to six.
(6) The current calibration of the Cepheid and maser extragalactic distance scales agree to within their quoted errors, yielding a value of Ho = 73 ± 2 (random) ± 4 (systematic) km s-1 Mpc-1.
(7) Within a concordance cosmology (that is, matter = 0.27 and vacuum = 0.73) the current value of the Hubble constant gives an age for the Universe of 13.3 ± 0.8 Gyr. Several independent methods (globular cluster ages, white dwarf cooling ages, CMB anisotropies within a concordance model) all yield values in good agreement with the expansion age.
(8) Further reductions of the known systematics in the extragalactic distance scale are anticipated using HST, Spitzer, GAIA and JWST. A factor of two decrease in the currently identified systematic errors is within reach, and an uncertainty of 2% in the Hubble constant is a realistic goal for the next decade.
(9) A Hubble constant measurement to a few percent accuracy, in combination with measurements of anisotropies in the cosmic microwave background from Planck, will yield valuable constraints on many other cosmological parameters, including the equation of state for dark energy, the mass of neutrinos, and the curvature of the universe.