Annu. Rev. Astron. Astrophys. 2010. 48: 673-710
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8. FUTURE IMPROVEMENTS

We summarize here the steps toward measuring the Hubble constant to a few percent accuracy. Most of these measurements should be feasible within the next decade.

1. Mid-infrared Galactic Cepheid parallax calibration with Spitzer and GAIA.

2. Mid-infrared calibrations of Galactic and nearby Cepheid galaxies and the infrared Tully-Fisher relation with Spitzer and JWST.

3. Increased numbers of maser distances.

4. Larger samples and improved systematics and modeling of strong gravitational lenses and Sunyaev-Zel-dovich clusters.

5. Higher-frequency, greater sensitivity, higher angular resolution measurements of the CMB angular power spectrum with Planck.

6. Measurements of baryon acoustic oscillations at a range of redshifts (e.g.,

BOSS [http://cosmology.lbl.gov/BOSS/], HETDEX [http://hetdex.org/], WiggleZ [http://wigglez.swin.edu.au/site/]), JDEM [http://jdem.gsfc.nasa.gov/], SKA [http://www.skatelescope.org/], DES [http://www.darkenergysurvey.org/], PanStarrs [http://pan-starrs.ifa.hawaii.edu/public/], LSST [http://www.lsst.org/lsst]).

7. Beyond 2020, detection of gravitational radiation from inspiraling massive black holes with LISA. Coupled with identification with an electromagnetic source and therefore a redshift, this method offers, in principle, a 1% measurement of H0.