The zero-point of the extragalactic distance scale, defined by about two dozens of nearby, late-type galaxies, has remained nearly unchanged for the last decade, in spite of the advent of new techniques and great efforts. The distances are essentially tied to trigonometric parallax stars and hence independent of the Hyades modulus; they are consistent with RR Lyr stars. The mean zero-point is therefore probably secure to better than 10%.
All known secondary distance indicators are still affected by zero-point errors, by problems in the definition of their relation between distance indicator and absolute magnitude (or linear size), and/or by selection bias. The effect of the very important selection bias (Malmquist effect), which causes a seemingly non-linear expansion field, is illustrated by two examples. To test for any true deviations from a linear expansion the Hubble diagram of nearly bias-free first-ranked cluster galaxies and supernovae Ia is shown; this imposes stringent limits on any non-linearity of the Hubble flow within v<5000 km s−1.
After freeing the available distances of field galaxies from selection bias and after reducing them to a common zero-point, one finds HO=55–65. Several distance indicators require a best Virgo cluster modulus of (m-M)=31.60, which implies for the Coma cluster (m-M)=35.38 and, with v(Coma)=7217 km s−1, HO=60. Supernovae Ia and first-ranked cluster galaxies out to large distances give HO (global)=53. Thus the evidence from clusters and field galaxies is best satisfied by HO=55; the assigned mean error of ±7 is to indicate a 3σ range of 35<HO<75.
Purely physical methods to determine extragalactic distances have modest weight yet; they will contribute eventually much to the determination of HO.
If HO were as large as 100, several paradoxa would arise. The Milky Way would have a very high supernova frequency, our Galaxy and M31 would be oversized, the baryon density would fall short to bind clusters, and Friedman universes were excluded.
Because all systematic errors have conspired and probably still conspire to measure HO too high, the true value could well be 40. Until new, decisive evidence becomes available, it is suggested for all practical purposes to use HO=50.