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Who "discovered" the expansion of the Universe?



Recently, Leigh Palmer, in correctly identifying Milton Humason as having
worked his way "up the ladder" at Mt. Wilson in the message

Date: Thu, 6 Jan 2000 20:45:38 -0800
From: Leigh Palmer <palmer@SFU.CA>
Subject: Re: schooling and grades

It should also be pointed out that Milton Humason was a lowly
muleskinner who worked on hauling the 100" mirror for the Hooker
telescope up Mount Wilson. He reckoned that astronomy might be
an interesting occupation, and he was taken on by Edwin Hubble,
who showed him the ropes. Together they discovered the startling
expansion of the universe.

also inferred that Humason and Hubble were solely responsible for the
discovery of the expansion of the universe. This statement is found in many
contemporary introductory astronomy texts. Though I may be reading too
much into Lee's use of the word "discovered", a review of the discoveries
which led up to Hubble's announcement of evidence for the expansion of the
universe reveals at least two other individuals who deserve to share in the
credit for this announcement.

EXECUTIVE SUMMARY FOR THE HURRIED

Hubble and Humason are not the only astronomers due credit for this
discovery. The relatively unknown Vesto Slipher, working at Lowell
Observatory, started it all in 1914 by announcing the discovery of "nebular
redshifts". He contributed all of the radial velocity information which
Hubble used in his 1929 paper which proposed interpreting the data as
evidence for a general expansion of the universe. In that same paper,
Hubble credited Willem de Sitter for his 1917 discovery of a solution to
Einstein's General Relativity field equations which suggested the universe
itself was expanding. Therefore, de Sitter and Slipher deserve to share
credit with Hubble and Humason for the work they contributed to our
understanding of the dynamic universe in which we live.

THE DETAILS FOR THE UNHURRIED

The person who really discovered the "redshift" in the spectra of
_most_ bright galaxies and _first_ proposed they reflected real object
velocities was Vesto Slipher at Lowell Observatory. By 1914, he had
obtained spectra of 12 bright galaxies and had announced that, with the
exception of M31 in Andromeda whose spectrum was _blueshifted_ by 300 km
per second, all others exhibited spectra which were _redshifted_ with
respect to the Sun's velocity through space -- Slipher's local standard of
rest. Within a few years, astronomers began entertaining the idea that
these motions implied some kind of expansion away from at least the Sun's
position in space. Slipher continued taking spectra of the brighter
galaxies. By 1925 he had accumulated spectra for 40 galaxies, finding that
redshifted spectra were much more common than blueshifted ones. At that
time, Slipher concluded the wavelength shift was due to real movements of
the galaxies through space.

While Slipher was taking spectra at Lowell, Edwin Hubble was
establishing himself at Mt. Wilson. One of his first tasks was to observe
M31 for the presence of novae. As early as 1917, he had identified not a
nova but a potential Cepheid variable, a star whose _period_ of clocklike
brightness variations correlated with its _average_ total energy output per
second (called "luminosity"). Compared to the apparent brightnesses and
the estimated distances of then known Milky Way Cepheids, this star's
relative faintness implied a distance much larger than any estimated for
other "nebulous" Milky Way objects at that time.

Doubting his classification of this star as a Cepheid, Hubble sent for
comment a record of his observations and one photographic plate, with the
tentative Cepheid circled in ink, to Harlow Shapley, newly appointed
director at Harvard College Observatory (HCO) and the first astronomer to
estimate a distance to the Milky Way's center. Shapley returned the
observations and plate, apparently without comment but with the inked
circle around the tentative Cepheid erased!

Undaunted, Hubble continued observing M31 and finding probable Cepheid
variables. By 1925, he had sufficient data to convince all astronomers,
except perhaps Shapley, that he had found a population of Cepheid variables
in M31. Using the Cepheid variable "period-luminosity law", discovered
more than 25 years earlier by Herietta Levitt in her HCO study of the Large
and Small Magellanic Clouds, Hubble calculated a distance for M31 which was
very much further away than most astronomers had expected -- so very much
further away that a very large distance must separate it from the most
distant Milky Way objects known at that time, even further away than the
Magellanic Clouds. Even though further refinements in the early 1950's in
classifying Cepheid variables revised his M31 distance upward by a factor
of about 2, the distance he estimated for M31 in 1925 was the best evidence
that the Milky Way was not the entire universe but instead was only one of
many separate galaxies containing billions of stars each.

Hubble continued searching for Cepheids in other "spiral nebulae" --
still not universally recognized as galaxies on their own in 1925 -- but
found that only M31 and M33 were close enough to permit their
identification. Hubble then defined "secondary" distances indicators,
objects also found in M31 and M33 but which were brighter than Cepheids,
which could be used to estimate the distances to the more distant galaxies
for which Slipher had already determined radial velocities.

By 1929, Hubble had estimated the distances to 18 more distant
galaxies. Using Slipher's radial velocity data and his own distances, he
found a surprisingly simple, linear relation

v = Ho X d

between a galaxy's apparent recessional velocity v in kilometers per second
and its distance d in millions of parsecs, where Ho is called the Hubble
constant. These results were presented in 1929 at a meeting of the
National Academy of Sciences in a paper entitled, "A Relation between
Distance and Redial Velocity Among Extra-Galactic Nebulae". Hubble stated
this relation could be the observational counterpart of an expansion effect
found by Willem de Sitter as part of a solution to Einstein's General
Relativity field equations for an "empty" universe, a result de Sitter
published in 1917. Other theorists after de Sitter had obtained similar
expanding universes for different initial conditions, but this was unknown
to most astronomers until 1930. It was Hubble's reasoning from the data
assembled by 1929, along with these additional dynamic solutions, which
caused Einstein to abandon his preferred static universe in 1930.

Between 1929 and 1934, Hubble obtained additional galaxy data with
Milton Humason's assistance. In fact, the bulk of the photographic and
spectroscopic observational and data reduction work was completed by
Humason: Slipher no longer seems to be in the picture. By 1934, Humason
had obtained distances and velocities for 32 galaxies which added further
evidence for the validity of Hubble's relation.

To summarize, Vesto Slipher and Willem de Sitter deserve credit for
having performed the observational and theoretical work which permitted
Hubble to use his distance estimates for the brighter galaxies to propose
in 1929 that galaxy redshifts were evidence for our living in an expanding
universe. Humason's contributions to this conjecture were first published
in 1934 in a paper in which he added corroborating data to the Hubble
relation. If there are others who deserve similar credit for their work
during this period on the Hubble relation, I would like to hear about them.


Paul

******************************************
Paul M. Rybski, Ph.D., Associate Professor
Chair, Dept. of Physics, and
Director, Whitewater Observatory
University of WI-Whitewater
Whitewater, WI 53190-1790

Office Phone: (414) 472-5766
Office FAX: (414) 472-5633

Email address: rybskip@uwwvax.uww.edu
******************************************