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Originally Posted by Tim Thompson
We can see from the data that a value of 84 km/sec/Mpc would lie significantly outside the indicated range of reported values (the lines drawn on the plot are clearly not 1-sigma uncertainties, and look more like 3-sigma uncertainties, but are probably neither). It certainly appears, based on these plotted data, that 84 km/sec/Mpc is an unreasonably high value, and significantly unlikely.
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Each research group will target a certain approach as a test of H0. The fact that multiple teams find H0 values ~ 70 doesn't rule out higher values. Tully&Pierce discussed this in their paper. They found H0=77 and asked why their lonely value on the higher side should be trusted and then went on to present their reasons why they believe their value is better. And they noted that if the Cepheid distance scale was calibrated to the geometric maser distance to NGC 4258 that they would get H0=86.
If you look at the approaches used it becomes a question of whose assumptions are better. The Sandage et al team assumes a significant impact from Malmquist, cluster population incompleteness and other biases throughout their analysis - and apply their correction methods they find H0 ~60.
Other research groups don't agree that the large bias corrections are needed and they find a "shorter" distance scale and thus larger H0 values.
Other methods such as time delays of gravitational lenses require assumptions about the distribution of dark matter in order to derive H0. For example,
Kochanek and Schechter find H0=48 for isothermal mass distributions, but the same lenses give H0=71 if they eliminate the DM halo and assume constant M/L ratios.
So it is very difficult to look at a list of H0 values and derive any conclusion other than the conclusion that the various methods utilized in the last 20 years give H0 values in the range of 40-90 km s-1 Mpc-1. Which methods are the best and lead to the most reliable values? That requires scrutiny of each study.
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Also keep in mind that all published values of H0 are here, not just those determined by the Cepheid P-L relationship. I think it is significant that the several methods used all agree in more or less ruling out a value of H0 that high.
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But - for example, if the geometric maser distance is adopted for NGC 4258, the distance scale shifts such that Tully&Pierce would get H0=86. In fact Tully&Pierce said there were arguments to be made for preferring the maser distance, but they kept their calibration based upon the LMC distance because that is what everyone else does and it would therefore be easier to compare their results with other studies.