5. THE HUBBLE CONSTANT

Since we have zero points in terms of both Mpc and km s^-1 we cannot help but infer a value for H0. The two estimators based on entire galaxy properties, TF and D_n - are consistent with one another and consistent with SBF in terms of relative distances. They give a zero point for SBF at the fiducial color of (V - I) = 1.15 of <_I⁰> = 5 log d (km/s) + 13.59 ± 0.07, where the error comes from the rms divided by (N - 1)^1/2.

Type Ia supernovae must be used with some care since the scatter is larger than one would expect from the errors. We do not use the recent calibration of SNIa using Cepheids, since that would again be circular. The average zero point derived from SNIa using the two assumptions about SNIa is 13.96 ± 0.17. We do find that SNIa distances match SBF distances much better when galaxies are compared individually rather than by group membership, and the galaxy by galaxy comparison gives a zero point which is smaller by about 0.20 mag.

Taken at face value, the D_n - , and TF zero point combines with the Cepheid zero point to give H₀ = 86 ± 4 km s^-1 Mpc^-1, whereas the SNIa give a Hubble constant of H₀ = 72 ± 6 km s^-1 Mpc^-1. These errors only reflect the statistical uncertainty, of course. If we regard these as statistically consistent with one another and average them, we get a combined zero point of 13.72 and H₀ = 81 ± 6 km s^-1 Mpc^-1. This final error estimate includes a contribution of 0.07 magnitude from the disagreement between the Cepheid and theory zero points (which we hope is indicative of the true accuracy of our calibrations), and an allowance of 0.13 magnitude for the uncertainty in the tie to the distant Hubble flow (judged from the scatter among the various methods).

The difficulty with the extragalactic distance game has not been primarily quality of data nor inadequate analysis, but rather an unwillingness to make falsifiable predictions. Therefore we offer Table 2, which lists the SBF distances to 12 nearby groups. The relative distances come from SBF only and are independent of any other distance estimator, and the zero point comes from the 10 Cepheid distances and 44 SBF distances.