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Originally Posted by Tim Thompson
That's the general idea. This thread is devoted to only one measurement, singled out for special treatment.
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Not true, this thread is devoted to the question of whether the Hubble Constant is actually a resolved matter or whether it is possible that H0 could be as high as 85. It makes sense to focus first on the HKP final report because it has been so influential.
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But how does one go about the task of figuring the probability that any one particular measurement is "right"? It can only be done by comparing the one measurement to a population of like measurements.
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But you have to look at the methods utilized by the different research groups, not just their final result. For example,
Ekholm et al 1999 find H0=53 from the TFR whereas
Tully&Pierce 2000 find H0=77 from the TFR. Their methods are different.
It is not good enough to compare the final H0 values, you must compare the distance estimates for clusters that are in common. You must look at the calibration procedures, sample sizes and other factors. Ekholm et al use large bias corrections whereas Tully&Pierce find bias corrections are small.
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In this case, the population of measurements given by Huchra serves that purpose. Because they are all derived from different methods, they don't all share the same systematics. So the spread of measurements is a fair representation not only of random uncertainties, but systematic uncertainties.
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As noted above and in previous posts, even utilizing the same distance indicator the studies will show a wide range of H0 values. I just noted such a case with the TFR. As another example, using Type II Sn
Hamuy 2003 find H0=81 but
Leonard et al 2003 find H0=57.
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It only needs to be shown that the given value (84) is significantly high compared to the population, to argue that it is unlikely. Note that I am only saying that it is improbable, not impossible. Just consider the inverse argument. If you are going to claim that this one measurement must be likely correct, then how do you explain all the others being unlikely? It seems unreasonable to me that they would be.
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Examples of the types of things that must be looked at are mentioned throughout my posts on this thread - starting with the bullets in the OP. The HKP used four Type II SN with 3 cepheid calibrators. They used 6 SBF distances to find H0. Their I-band TFR to A3574 is 62 Mpc whereas their Fundamental plane distance to the same cluster is 51 Mpc. They find a SBF distance to Coma of 102 Mpc whereas their I-TFR and FP distances are 86 Mpc to Coma.
You can dissect each study this way if you want. You'll find many studies or distance estimates are based upon small numbers of calibrators. Many studies have small sample sizes such as the HKP Type II SN and SBF estimates,
this paper , the type II SN papers I linked to above, and the estimates from lensing studies. Many studies are based upon assumptions that have not been resolved among the specialists (again the lensing studies). Many studies are basically a repeat of an earlier analysis with small changes by the same group of researchers. For example I linked to the TFR study of Ekholm et al who found H0=53 in 1999. The same group found H0=53-57 in
1997 using largely the same methods and data sets. So these are not two completely independent H0 estimates.
And finally it should not be forgotten that most of the H0 values are based upon the same Cepheid distance scale - so a systematic error in the zero point of the Cepheid distances is going to affect every one of those studies. As simple a change as adopting the maser distance to NGC 4258 would make Dr. Huchra's list look very different and H0=84 would no longer be on the high end but closer to the middle.