Slipher, galaxies, and cosmological velocity fields
  By 1917, V.M. Slipher had singlehandedly established a tendency for 'spiral
nebulae' to be redshifted (21 out of 25 cases). From a modern perspective, it
could seem surprising that the expansion of the universe was not announced at
this point. Examination of Slipher's papers shows that he reached a more subtle
conclusion: the identification of cosmological peculiar velocities, including
the bulk motion of the Milky Way, leading to a beautiful argument in favour of
nebulae as distant stellar systems. Nevertheless, Slipher's data actually
contain evidence at >8sigma for a positive mean velocity, even after
subtracting the dipole owing to the motion of the observer. In 1929, Hubble
estimated distances for a sample of no greater depth, using redshifts due
almost entirely to Slipher. Hubble's distances were flawed in two distinct
ways: in addition to an incorrect absolute calibration, the largest distances
were systematically under-estimated. Nevertheless, he claimed the detection of
a linear distance-redshift relation. Statistically, the evidence for such a
correlation is less strong than the simple evidence for a positive mean
velocity in Hubble's sample. Comparison with modern data shows that a sample of
more than twice Hubble's depth would generally be required in order to reveal
clearly the global linear expansion in the face of the 'noise' from peculiar
velocities. When the theoretical context of the time is examined, the role of
the de Sitter model and its prediction of a linear distance-redshift relation
looms large. A number of searches for this relation were performed prior to
Hubble over the period 1924-1928, with a similar degree of success. All were
based on the velocities measured by Slipher, whose work from a Century ago
stands out both for the precision of his measurements and for the subtle
clarity of the arguments he employed to draw correct conclusions from them.
