Quote:
Originally Posted by antoniseb
I think that the Cepheid distances are about as accurate as our knowledge about the distance to the LMC. We have observed and studies a very large number of Cepheids there, and so the statistics about their brightness is pretty solid. Our distance measurement to the LMC is not so far off as to allow the kind of error bars you are asking about.
In about ten years when the Gaia data is in, we'll have very accurate direct trigonometric measurements of the distances of quite a few local Cepheids. This will also help nail things down.
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The Cepheid distances provide zero point calibration for the other distance indicators. There are now ~30-40 galaxies with Cepheid distances ... but only a handful of those can be used to calibrate any given secondary distance indicator as I noted in the bullets in the OP. It doesn't take a very large systematic error in the calibration of the zero point of the Cepheid distances in combination with a systematic error intrinsic to the calibration of the secondary distance indicator and small numbers of Cepheid calibrators (3 for the Type II SN, 6 for the SBF and Type Ia SN) to push H0 to 85.
I derived H0=84 using the TFR and 2MASS Ks-band photometry for a sample of 318 spirals that met very strict selection criteria designed to eliminate galaxies that were likely to have large distance errors. The calibrator sample was 26 galaxies with distances based upon the same Cepheid distance scale the HKP used.