|Slipher, Galaxies, and Cosmological Velocity Fields
|471, Origins of the Expanding Universe: 1912-1932
|Peacock, J. A.
|By 1917, V. M. Slipher had singlehandedly established a general
tendency for ‘spiral nebulae’ to be redshifted (21 out of 25
cases). From a modern perspective, it could seem surprising that the
discovery of the expansion of the universe was not announced at this
point. Examination of the data and arguments contained in 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 favor of
spiral nebulae as distant stellar systems. Nevertheless, Slipher's
data actually contain evidence at >8σ for a positive mean
velocity, even after subtracting the best-fitting dipole pattern owing
to motion of the observer. In 1929, Hubble provided distance estimates
for a sample of no greater depth, using redshifts due almost entirely
to Slipher. Hubble's distances turned out to be 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.