?Jeremy Mould
MSSSO, Institute of Advanced Studies, ANU
in the presence of a velocity field which
exhibits large scale flows. The results of their numerical experiments can
be summed up in the following alarming illustration: ``Even if the local
expansion rate is known to be 80 
8 km/s/Mpc out to 30 h
Mpc in the
North Galactic Cap, the 95% confidence limits on the true global value of
H
is 50--128 km/sec/Mpc in a CDM model."
The Hubble Space Telescope Key Project to measure H
effects
the calibration of secondary distance indicators whose effective range
extends beyond the Coma cluster. At Coma the same rms difference has
fallen from 45% to 3% in CDM according to Turner's model.
Empirical evidence that real velocity fields present no larger problem than these calculations suggest is provided by:
(1) an all-sky survey of Tully-Fisher distances to clusters of galaxies
(2) an all-sky survey of brightest cluster galaxies (Lauer & Postman)
(3) the type II supernova expanding photospheres data set.
We can hypothesize a `bubble model' in which 
0.5,
that is, H
= 75
km/s/Mpc within the distance of the Coma cluster and H
= 50 outside that
radius.
The data appear to rule out a model with such a large difference between local and global H
.
Such a model is also heavily constrained by the isotropy of the
cosmic microwave background on 1
scales. One hundred Mpc, which
is approximately the radius of the volume enclosing Coma,
corresponds to 1
(12
) on the surface of last scattering for
= 1 (0.2). The requirement that
0.5 in a `bubble model' of this type implies a three
times larger density perturbation than is represented by the Great Attractor.
A Great Attractor would evolve from
= 1.7 
in an 
= 1 Universe. The measured 
1 
on 1.5
scales. A low density Universe would relax these constraints considerably.