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Edwin Hubble (1889-1953)
In high school and at the University of Chicago Hubble was
a star athlete as well as a scholar. Hubble was two years younger than most of
his classmates, but he was 6 feet 3 inches tall and very well coordinated.
Usually he placed in Big Ten dual track meets, in both the shot put and the
high jump.
Edwin Hubble
In 1910 Hubble went to Oxford
University as a Rhodes Scholar. It was
a high honor, awarded to a small
number of outstanding studentathlete-leaders. At Oxford Hubble
studied Roman law and Spanish,
competed in track and field events and
swam on the water polo team. He later
said he fought an exhibition boxing
match against the French national
champion, and did well enough that
promoters wanted him to train to fight
the world heavyweight champion.
Hubble also told of a duel with a
The University of Chicago
German naval officer whose wife had
flirted with the handsome Hubble — to 1909 intercollegiate
championship basketball
satisfy the officer's honor they
team. Hubble is on the
harmlessly discharged pistols in a
library. These stories may tell us less
left.
about actual events than about
Hubble's lifelong drive to promote a romantic image of himself.
After three years at Oxford, Hubble returned to his family home in
Hubble with his
sister Lucy in 1917.
Louisville, Kentucky. He taught physics and Spanish at a high school and also
became a member of the Kentucky bar, but never actually practiced law. In
1914 he returned to the University of Chicago and the Yerkes Observatory. He
hoped to finish his doctoral dissertation on a photographic investigation of
faint nebulae and take up a position at the Mount Wilson Observatory in the
summer of 1917.
In April, however, the United States
declared war on Germany. Hubble
rushed through his dissertation, took
his final oral exam, and reported to the
army for duty three days later. He
served in France and made the rank of
major before the war ended.
In 1919 Hubble finally joined the
Mount Wilson Observatory. There his
achievements made him the foremost
astronomer of the 20th century and
one of the most influential scientists of
all time in changing our understanding
of the universe. He demonstrated
conclusively, after centuries of fruitless
speculation by other astronomers, that
spiral nebulae are independent
galaxies at great distances beyond our
own galaxy. This great
accomplishment was only the starting
point for Hubble. Other scientists,
including Einstein, had assumed the
universe to be static. Hubble went on
to show that the universe is expanding.
Hubble (right) with two
other astronomers (James
Jeans, left, and Walter
Adams, middle) in front of
100-inch telescope on
Mount Wilson.
Hubble lived in San Marino, California near
Pasadena. He traveled to Europe several times
to give lectures and receive important honors.
Hubble partied in nearby Hollywood with
movie stars, including Charlie Chaplin and
Greta Garbo, and with famous writers,
including Aldous Huxley, Christopher
Isherwood, and Anita Loos. But his favorite
hobby was fishing.
World War II interrupted Hubble's
work on cosmology. He served as chief of
ballistics and director of the Supersonic Wind
Tunnels Laboratory at the Army Proving
Grounds in Maryland. He was awarded the
Medal of Merit for his war-time work. After
the war he supported the construction of the gigantic 200-inch telescope on
Palomar Mountain south of Mount Wilson, which helped put him on the cover
of Time magazine in 1948. In 1953, shortly after the 200-inch was completed,
Hubble died of a heart attack. It was too soon for conclusive answers from the
research program he had planned for the new telescope. Indeed different
measurements of the expansion of the universe did not reach broad agreement
until another great instrument was placed in orbit, named the Hubble Space
Telescope.
Hubble in the observer's cage located at
the top of the tube of the 200-inch
telescope on Palomar Mountain.
From Our Galaxy to Island Universes
At the beginning of the 20th century, astronomers were unsure
of the size of our galaxy. Generally, they believed it was not much greater than a
few tens of thousands of light years across, and perhaps considerably less. (A
light year, nearly six trillion miles, is the distance traveled in a year moving at
the speed of light in a vacuum.) Also, observations early in the 20th century
made it seem that our solar system was near the center of the galaxy. The way
astronomers were misled is explained here.
It is believed that the great mass of the stars … are arranged
in the form of a lens- or bun-shaped system … considerably
flattened towards one plane … the Sun occupies a fairly
central position.
—English astronomer Arthur Eddington, 1914 [Full
Quote]
Shapley's New Model of the Universe
Who Was
George Ellery Hale?
This vision of the universe was
soon replaced with a revolutionary
new conception, based largely on
the observations of the American
astronomer Harlow Shapley at the
Mount Wilson Observatory. The
astronomer and scientific
entrepreneur George Ellery Hale
had founded the observatory on a
mountain peak overlooking Los
Picture of Mt. Wilson
Angeles in 1904, and four years
later master instrument-builder
observatory on a postcard.
George Ritchey completed a 60inch reflecting telescope designed specifically for astronomical photography.
The first hint of a drastically revised understanding of
our galaxy came in 1916. Studying a "globular cluster,"—a group of hundreds
of thousands of stars—Shapley noticed faint blue stars. If they were similar to
bright blue stars near the Sun, they must be about 50,000 light years away to
explain why they looked so faint. He pushed ahead to establish distances
more conclusively using a new and ingenious method of measuring the
universe.
M22, a globular
cluster of many
thousands of stars.
By assuming that
certain types of
stars here were as
bright as similar
nearby stars whose
distances had been
measured, Shapley
could estimate the
distance to this far
object. (You can EXIT
this site to see NASA's
picture of M22.)
Who Was
Harlow Shapley?
Shapley built a new understanding of the universe by measuring distances to
stars based on properties of a type of variable stars called "Cepheids" (named
after the constellation Cepheus, in which a typical such star was first
noticed). They are giant stars, and thus visible to great distances. Each
Cepheid varies in brightness over time.
It is worthy of notice...that the brighter variables have
longer periods.
—Henrietta Swan Leavitt
In 1908 the American astronomer Henrietta Leavitt had pointed to a
remarkable rule that Cepheids obey. During routine comparisons of
photographs she discovered variable stars, brighter on some photographs
and fainter on other photographs taken at different times. Leavitt noticed
that the brighter the variable star, the longer its period.
The 16 variable stars Leavitt measured were all in the same group of
stars, the Small Magellanic Cloud. Thus they were all approximately the same
distance from the Earth. Therefore their apparent magnitudes (observed
brightness) were directly related to their absolute magnitudes (intrinsic
brightness, as it would be seen at some arbitrary standard distance). The
conclusion was a remarkable "period-luminosity relation"—the longer the
period, or time, from a Cepheid's maximum brightness to minimum and back
to maximum, the greater the intrinsic luminosity of the star.
The period-luminosity relation for Cepheid-type variable
stars—the curve showing how their brightness varies over
time - as established by Harlow Shapley in 1918. An
astronomer henceforth could observe the period, or time
from one maximum brightness to the next maximum, for any
other Cepheid variable star, and then read off the graph the
star's absolute magnitude. Comparison of this estimated
absolute ("real") magnitude with the observed apparent
magnitude yields the distance, since brightness diminishes
with the square of the distance.
Assuming that the system of globular clusters was sort of a galactic
skeleton, Shapley had the galactic outline, its size, and the place of the solar
system within it. The Sun was far toward one edge of the galactic plane, not
near the middle. He showed that the system of stars was ten or even a
hundred times larger than previous estimates, and that the Sun is many tens
of thousands of light years away from the center of the galaxy.
The system of globular clusters, which is coincident in
general, if not in detail, with the sidereal arrangement as a
whole, appears to be somewhat ellipsoidal.… The center of
the sidereal system is distant from the Earth …
—Shapley [Full quote]
Shapley used the period-luminosity relation to estimate distances. First, he
collected all the available data on Cepheid stars, from his own observations and
from other astronomers including Leavitt. The distance to some of the nearer
Cepheids had been measured, and thus Shapley could figure out their absolute
magnitudes. The only physics he needed was the simple rule that brightness
decreases with the square of the distance. Then Shapley graphed period versus
absolute magnitude.
[The period-luminosity curve is] based upon more than 230
stars, and, except for zero-point uncertainty [the uncertainty
in the distance measured by other methods to the nearer
Cepheids], is probably correct within one or two hundredths of
a magnitude.
—Harlow
Shapley
Shapley made the reasonable assumption that Cepheids in
distant globular clusters obey the same physics as nearby Cepheids. He observed
the periods of distant Cepheids, read off their presumed absolute magnitudes
from his graph of period versus luminosity, and compared that absolute
magnitude with the observed apparent magnitude. This produced distances to
many far-away Cepheids—and to the globular clusters in which they resided.
(Some globular clusters did not have Cepheids he could measure, and he used
other, cruder methods to estimate their distances.)
Shapley found that the globular clusters are arranged symmetrically around the
galaxy, about as many above the plane of the galaxy as below. The clusters
seemed to avoid the plane itself, the Milky Way. Shapley wrote that "this great
mid-galactic region, which is peculiarly rich in all types of stars, planetary
nebulae, and open clusters, is unquestionably a region unoccupied by globular
clusters." Shapley acknowledged that there was an alternative explanation.
Maybe globular clusters were not, as he believed, actually missing from the
region, but instead were hidden by clouds of absorbing matter along the spine of
the Milky Way.
our galaxy.
The distribution of globular
clusters as measured by
Shapley in 1918 (SIDE
VIEW). The shaded area is
the plane of our galaxy. The
position of the solar system
is marked by X, in the plane
of the galaxy near the left
side. Shapley marks the
globular clusters by black
circles above the plane and
white circles below. There
are about the same number
above and below the plane of
The galaxy as it was
understood in 1919 (TOP
VIEW), its shape and extent
outlined by globular clusters,
here shown projected on the
plane of the galaxy. The solar
system is in the small circle.
The dotted line is the major
axis of the galaxy, with its
center marked by the red X .
The large circles have radii
increasing by intervals of
10,000 parsecs, about 32,600
light years.
The dwindling significance of humans and their particular planet
had dwindled further still. Shapley noted a historical progression from belief
in a small universe, with humankind at its center, to a larger universe with the
Earth further from the center. The geometry had been transformed from
geocentric to heliocentric to a-centric. The psychological change was no less,
he insisted, from homocentric to a-centric. Some astronomers had long
doubted that the solar system was near the center of the galaxy and that
people enjoyed a privileged place in the universe. They felt that the odds,
given a random distribution, were small. Now Shapley gave this philosophical
position scientific substance.
The physical universe was anthropocentric to primitive
man.... the significance of man and the Earth in the sidereal
scheme has dwindled with advancing knowledge of the
physical world...
—Shapley [Full quote]
The Great Debate
Shapley's galaxy was far larger than any previous estimate (aside from
earlier guesses of an infinite stratum of stars). It might indeed be the entire
universe. For Shapley had showed that globular clusters were clearly part of the
galaxy, not independent island universes. Other nebulae (concentrations of
stars and dust), especially spiral-shaped ones, might still lie outside our galaxy.
But if they were similar in size to our now enormous galaxy, they seemed
implausibly large. Separate island universes were not impossible, but they
seemed less likely now that Shapley had multiplied the size of our galaxy many
fold.
From the new point of view our galactic universe appears as a
single, enormous, all-comprehending unit... The adoption of
such an arrangement leaves us with no evidence of a plurality
of stellar 'universes'.
—Shapley (Full
quote)
Shapley defended his conclusions in the so-called "Great Debate"
before the National Academy of Sciences on 26 April 1920. His major concern
was the size of the galaxy. His model of a drastically larger galaxy, with the
solar system far from its center, was largely correct. But he was on less solid
ground when he argued that the spiral nebulae, which seemed to be much
smaller, were part of our galaxy. His opponent, Heber Curtis, argued that the
galaxy could be as large as Shapley said, yet still be only one of many island
universes, if it happened by chance to be several times larger than the average.
Ultimately observations would prove Curtis correct, but in 1920 Shapley had
the stronger position. You can read more about the Great Debate here.
The centuries-old debate was resolved only by new scientific evidence,
produced using larger telescopes and new observational techniques, including
photography and spectroscopy. The key proponent of island universes was
Edwin Hubble, who like Shapley did his revolutionary work at the Mount
Wilson Observatory.
Who Was
Edwin
Hubble?
Writing in his doctoral thesis in 1917, Hubble noted that catalogs
already included some 17,000 small, faint nebulous objects that could ultimately be
resolved into groupings of stars. Perhaps 150,000 were within the reach of existing
telescopes. Yet, he wrote, "Extremely little is known of the nature of nebulae, and no
significant classification has yet been suggested; not even a precise definition has
been formulated." The way Hubble discovered to classify nebulae is described here.
After serving in World War I, Hubble joined the Mount Wilson Observatory staff.
There he took photographs of nebulae with the new 100-inch reflector, the most
powerful telescope in the world. Hubble discovered variable stars in an irregular
nebula (cataloged as NGC 6822). By now Shapley had left Mount Wilson for the
Harvard College Observatory. Hubble wrote to Shapley in 1923 to tell him of the
discovery. Hubble also said he was going to hunt for more variable stars and to
investigate their periods. Shapley wrote back, "What a powerful instrument the
100-inch is in bringing out those desperately faint nebulae."
The great spirals … apparently lie outside our stellar system.
—Edwin Hubble,
1917
Early in 1924 Hubble wrote to Shapley again. This time Hubble reported, "You
will be interested to hear that I have found a Cepheid variable [star] in the
Andromeda Nebula [M31]. I have followed the nebula this season as closely as the
weather permitted and in the last five months have netted nine novae and two
variables."
The irregular
nebula NGC 6822,
what would now be
called a nearby
dwarf galaxy.
Thanks to the 100inch telescope,
Hubble was able to
detect variable
stars here, although
it is (by modern
measures) 1.5
million light years
distant.
Pages 156-157 from
Edwin Hubble's
observation notebook.
It documents the
discovery of the first
Cepheid variable star
in the spiral nebulae
M31. Initially Hubble
When he found a Cepheid variable, Hubble realized he held the key to distance.
As Shapley had used the period-luminosity relation for Cepheids to find
distances to globular clusters in our galaxy, so Hubble could find the distance
to the spiral nebula M31.
The central region
of the spiral
nebula M31. This
plate was taken
with a 9-hour
exposure over two
nights in
September, 1920,
with the Mount
Wilson 100-inch
reflecting
telescope. The
Cepheid is in the
upper right
corner, marked
"VAR!" for
Variable.
M31: The
Andromeda
Galaxy.
The curve of luminosity
of the first Cepheid
variable star discovered
by Edwin Hubble in the
Andromeda Nebula, M31.
Using this he could
determine the nebula's
distance. Hubble
included this graph in his
19 February 1924 letter
to Harlow Shapley.
Hubble found that "the distance [to M31] comes out something over
300,000 parsecs." This was roughly a million light years, and several times
more distant than Shapley's estimate of the outer limits of our own galaxy.
Hubble continued: "I have a feeling that more [Cepheid] variables will be found
by careful examination of long exposures."
Here is the letter that has destroyed my universe.
—Shapley,
1924
On reading Hubble's letter, Shapley remarked to a colleague who
happened to be in his office, "Here is the letter that has destroyed my
universe." Shapley admitted that the large number of photographic plates that
Hubble had obtained were enough to prove that the stars were genuine
variables. By August, Hubble had still more variables to report. Shapley was
glad to see this definite solution to the nebula problem, even if it refuted earlier
evidence against spiral nebulae as island universes. Some of the evidence
against spiral nebulae as island universes was based on a mistake, as explained
here.
Hubble's discovery of Cepheid variable stars in spiral nebulae, and the distance
determination confirming that spiral nebulae are independent galaxies, were
officially announced on New Year's Day, 1925, at a meeting of the American
Astronomical Society. He followed this preliminary paper by further work over
the next four years, with convincingly voluminous detail. A good part of
Hubble's genius, and much of the acceptance that his revolutionary conclusions
commanded, were due to lots of hard work.
Before the 1920s ended, astronomers understood that the spiral
nebulae lie outside our own galaxy. In the previous decade Shapley had
multiplied the size of the universe by about ten times. Hubble multiplied it by
another ten - if not more. Hubble's universe was no longer the one allcomprehending galaxy envisioned by Shapley. Henceforth the universe was
understood to be composed of innumerable galaxies spread out in space,
farther than the largest telescope could see. Hubble next would show that the
universe is not static, as nearly everyone then believed, but is expanding. What
he had made infinite in space, he would make finite in time.
The Expanding Universe
Theories of a Static Universe
Never in all the history of science has there been a period when
new theories and hypotheses arose, flourished, and were
abandoned in so quick succession as in the last fifteen or twenty
years.
—Willem de Sitter,
1931
Looking back on the years of discovery and upheaval between
1915 and 1930, the Dutch astronomer Willem de Sitter rightly identified the period
as perhaps the most extraordinary ever known to physical scientists. In the theories
of relativity and the quantum, physicists starting with Albert Einstein had given
entirely new and astounding explanations of energy, matter, gravity, even space and
time. As astronomers tried to apply these new tools to cosmology, they were struck
with their own revelations. As described on the previous page, Harlow Shapley's
concept of the Big Galaxy was quickly followed by Edwin Hubble's proof of the
existence of island universes. Stranger still, at the end of the 1920s came the
realization that the universe is expanding.
The Dutch
astronomer Willem
de Sitter around
1898.
This realization came only after an uphill battle. In the early 20th century the
common worldview held that the universe is static — more or less the same
throughout eternity. Einstein expressed the general opinion in 1917 after de Sitter
produced equations that could describe a universe that was expanding, a universe
with a beginning. Einstein wrote him that "This circumstance irritates me." In
another letter, Einstein added: "To admit such possibilities seems senseless."
In his gravitational field equations, Einstein was just then providing a compact
mathematical tool that could describe the general configuration of matter and space
taking the universe as a whole. The peculiar curvature of space predicted in the
equations was quickly endorsed in famous experiments, and by the early 1920s
most leading scietists agreed that Einstein's field equations could make a
foundation for cosmology. The only problem was that finding a solution to these
simple equations — that is, producing a model of the universe — was a
mathematical nightmare.
I have erected but a lofty castle in the air... So let us be satisfied
and not expect an answer, and rather see each other again as
soon as possible!
– Einstein to de Sitter, 1916 [full
quote]
During 1916, in the
middle of World War I,
Einstein met with de Sitter in
neutral Holland. Stimulating and
criticizing each other, they
produced two cosmological
models, two different solutions to
the field equations. But both
models seemed to need special
adjustments. De Sitter's model
could be stable only if it contained
no matter. De Sitter hoped the
model might somehow be
adjusted to describe the real
universe, provided the density of
matter was close enough to zero.
The most striking thing about his
empty universe was an odd effect
on light — the farther one went
from the mathematical center
(the origin of coordinates), the
slower the frequency of light
vibrations. That meant that the
farther away an object was in this
odd universe, the more the light
coming from it would seem to
have a slowed-down frequency.
Albert Einstein (left) and
Willem de Sitter (right)
discussing at the blackboard
de Sitter's theory of the
expanding universe. This
picture was taken in 1932 at
the California Institute of
Technology.
Einstein's first try at a model likewise could not contain matter and be
stable. For the equations showed that if the universe was static at the outset,
the gravitational attraction of the matter would make it all collapse in upon
itself. That seemed ridiculous, for there was no reason to suppose that space
was so unstable.
Einstein found he could stabilize his model by adding a simple
constant term to the equations. If this constant was not zero, the model
would not have to collapse under its own gravity. This "cosmological
constant," Einstein admitted, was only "a hypothetical term." It was "not
required by the theory as such nor did it seem natural from a theoretical
point of view." In fact, "The term is necessary only for the purpose of making
possible a quasi-static distribution of matter."
The introduction of such a constant implies a considerable
renunciation of the logical simplicity of the theory... Since I
introduced this term, I had always a bad conscience... I am
unable to believe that such an ugly thing should be realized
in nature.
—Einstein to Lemaître, 1947
The Light from Distant Nebulae
The powerful belief in a static universe could only be
overturned by the weight of accumulating observations. The first of these
observations had already been reported in 1915. Probably the observation
was unknown to Einstein when he was developing his theory and
corresponding with de Sitter. World War I had disrupted communications
between the English-speaking nations and Germany, where Einstein
worked, while de Sitter had only a second-hand, incomplete report of the
crucial observation.
Vesto Slipher
How Slipher could
measure velocities
in the heavens
Andromeda
The observation had been made at the Lowell Observatory in Arizona. Its
founder, Percival Lowell, suspected that spectral lines seen in the light from
one species of nebula, the "planetary" nebulae, might also be found in the
spectra of spiral nebulae. In 1909 Lowell asked his assistant Vesto Slipher
to get spectra of spiral nebulae. Slipher initially doubted that it could be
done. Then he realized that for nebulae with their extended surfaces, in
contrast to the point images of stars, the critical instrumental factor was not
telescope size (rival Lick Observatory in California had a much larger
telescope) but camera "speed" — the exposure time needed to photograph
spectra of nebulae.
With a new camera, its speed increased by a factor of 30, on the
night of 17 September 1912 Slipher obtained a spectrogram for the
Andromeda Nebula. The spectrogram indicated that the nebula was
approaching the solar system at an amazingly high velocity. Slipher made
more observations, exposing the same photographic plate over multiple
nights (for example, 29, 30, and 31 December 1912). These yielded
velocities averaging 300 km/sec. That was so large that some astronomers
did not believe it possible.
Over the next two years, Slipher measured velocities for other spiral
nebulae. The first few measurements revealed approaching nebulae on the
south side of our galaxy and receding nebulae on the opposite side. Slipher
formed a "drift" hypothesis. He thought that it was our galaxy that was
moving relative to the nebulae, toward the south and away from the north.
However, observations of more spirals contradicted this. Receding spirals
were found on the south side of our galaxy as well as on the north side.
Slipher nevertheless clung to his drift hypothesis. Perhaps more
observations, he argued, would find at least a preponderance of
approaching nebulae on the south side, toward which he thought our galaxy
was moving.
A different interpretation of the velocities seen in spiral nebulae
Spiral Nebula in
Ursa Major (M. 101)
soon turned up. De Sitter's model of a static universe had a diminishing
frequency of light vibrations with increasing distance. Slipher had
calculated velocities by using the rule that the frequency of light observed
will change if the source of the light is moving rapidly away — but perhaps
this was an illusion. Perhaps distant objects were not really receding at
great speeds, but were only emitting a different frequency of light. Nothing
like that happened in Einstein's model of a static universe, so Slipher's
"Planetary"
like the one
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appear as ri
something l
orbit of a pl
They are no
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Spiral nebul
the one belo
side-on, are
known to be
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Milky Way, f
more distan
measurements might give a way to choose between the two models.
World War I had slowed communications, but by 1921 de Sitter knew of
Slipher's velocity measurements for 25 spiral nebulae. Only 3 were
approaching. They could be explained away as the result of large velocities
in random directions, superimposed on a much smaller systematic
recession. Still, de Sitter hesitated to draw any conclusions. Velocities were
known, but the other half of the predicted relation — the distances to the
nebulae — were unknown. This crucial information would be developed by
Edwin Hubble as he extended his measurements of "Island Universes"
(described on the previous page).
Hubble's Surprising Relationship
In 1928 Edwin Hubble attended a meeting of the International
Milton Humason on
Mount Wilson
How Hubble found
the distances
MORE about:
Hubble's distances
to the nebulae
were too small
Astronomical Union, held that year in Holland. There Hubble discussed
cosmological theories with de Sitter. Hubble returned to the Mount Wilson
Observatory determined to test de Sitter's theory. Hubble directed his assistant
Milton Humason, a gifted and meticulous observer, to study faint nebulae, which
would presumably be especially distant. Did the frequency of their light differ from
the light coming from closer nebulae? Now, a slower frequency corresponds to a
longer wavelength of light, that is, light closer to the red end of the spectrum. Thus
what Hubble and Humason were seeking was a displacement of lines in the
spectrum toward the red, what later came to be called a "red shift". Such a shift,
Humason later explained, was what "might be expected on de Sitter's theory of
curved space-time."
Humason obtained velocities and Hubble obtained distances. They found a linear
relationship — roughly speaking, the greater the receding velocity of a nebula, the
farther the distance to it. Their data were skimpy, and the interpretation was shaky
in detail. (Indeed it was later discovered that Hubble's distances to the nebulae were
only half the actual distances.) Indeed his figures disagreed with what scientists
already knew about the age of the universe. Nonetheless, the velocity-distance
relation was a bold and brilliant extrapolation.
The results establish a roughly linear relation between velocities
and distance among nebulae.
—Edwin Hubble,
1929
Not until the final paragraph of his 1929 paper did Hubble mention de
Sitter, or indeed theory at all. And then Hubble simply noted that the velocitydistance relation might represent the de Sitter effect and might be of interest for
cosmological discussion. Hubble emphasized the empirical, observational aspect of
his work. His chief goal was to convince skeptical readers that the velocity-distance
relation really existed.
Hubble's 1929 velocity-distance relation for 46
How o
the
Unive
Who Was
Edwin Hubble?
Hubble's campaign
to establish the
velocity-distance
relation
nebulae. The black dots and the solid line represent
the solution obtained from the 24 nebulae for which
individual distances were determined, using them
separately. Empty circles and the dashed line
represent the solution obtained by combining the
nebulae into groups. The cross represents the mean
velocity for a set of 22 nebulae whose distances
could not be estimated individually.
There is more to the advance of science than new observations and new theories.
Ultimately, people must be persuaded. In science, as in a court of law, advocates for
each side of an issue present the best case possible in an attempt to reach the truth.
A heroic image of pristine science may exclude the use of rhetorical skills. Advocacy,
however, is an integral part of real science. There can be no judgment until the
arguments are laid out clearly and energetically. Meanwhile Hubble carried on a
scientific campaign to pin down the velocity-distance relation beyond question with
improved observations .
New observations by Hubble and Humason … concerning the
redshift of light in distant nebulae make the presumptions near
that the general structure of the Universe is not static.
—Einstein
Hubble's undeniable observations that the light from nebulae showed
a red shift increasing with distance ruled out the possibility that Einstein's static
model represented the real universe. De Sitter's alternative static model, without
matter, was also ruled out by new observations. De Sitter had supposed that the
density of matter in the universe might be close enough to zero so that his model
could work. A new estimate, in 1927, of the mass of our galaxy caused de Sitter to
reexamine this assumption and reject it. There could be no more pretense that de
Sitter's model might correspond to reality. Einstein too soon acknowledged that the
red shift overthrew the old assumption of a static universe.
Rea
about
exam
of
Hubb
rhetor
ski
An Expanding Universe?
At a meeting in London of the Royal Astronomical Society early in 1930,
de Sitter admitted that neither his nor Einstein's solution to the field equations
could represent the observed universe. The English astronomer Arthur Eddington
next raised "one puzzling question." Why should there be only these two solutions?
Answering his own question, Eddington supposed that the trouble was that people
had only looked for static solutions.
Both the solutions [to the field equations, de Sitter's and
Einstein's] must be rejected, and as these are the only statical
solutions of the equations… the true solution represented in
nature must be a dynamical solution.
—Willem de Sitter,
1931
In fact a few astronomers had
Friedmann in
1922 or 1923.
been looking for other solutions to Einstein's
equations. Back in 1922, the Russian
meteorologist andmathematician Alexander
Friedmann had published a set of possible
mathematical solutions that gave a nonstatic universe. Einstein noted that this
model was indeed a mathematically possible
solution to the field equations. Later,
Friedmann would be hailed as an example of
great Soviet science. But through the 1920s,
neither Einstein nor anyone else took any
interest in Friedmann's work, which seemed
merely an abstract theoretical curiosity.
Most astronomers continued to take it for
granted that the real universe was static.
When Friedmann published again in 1924,
the paper, seen as a matter of pure relativity
theory with no astronomical interest, was
omitted from the annual survey of scientific
papers on astronomical topics. He could not
stand up for his ideas, for a year later he
died of typhoid fever, only 37 years old.
Georges Lemaître
The Belgian astrophysicist Georges Lemaître had also published a model of an
expanding universe, in 1927. Lemaître was a Catholic priest (from 1960 until his
death in 1966 he was president of the Pontifical Academy of Sciences). His
contribution to science is now celebrated, but at the time it made no impression.
Published in the little-read Annals of the Brussels Scientific Society, it was easily
overlooked. Those (including Eddington) who did read Lemaître's 1927 paper had
promptly forgotten it.
have found the true solution, or at least a possible solution,
I
which must be somewhere near the truth, in a paper... by
Lemaître... which had escaped my notice at the time.
—de Sitter to Shapley,
1930
Lemaître saw a report of the 1930 Royal Astronomical Society meeting and wrote to
Eddington, his former teacher, to remind him of the 1927 paper. Eddington now
recognized the value of Lemaître's study. Eddington shared Lemaître's paper with
de Sitter, who soon after wrote to Harlow Shapley at Harvard, "I have found the
true solution, or at least a possible solution, which must be somewhere near the
truth, in a paper… by Lemaître… which had escaped my notice at the time." Einstein
soon confirmed that Lemaître's work "fits well into the general theory of relativity."
In 1931 de Sitter publicly praised Lemaître's "brilliant discovery, the ‘expanding
universe'." In that same year, Lemaître went on to propose that the present universe
is the "ashes and smoke of bright but very rapid fireworks." We can now see this
"fireworks theory" (as it came to be called) as a first version of the "Big Bang" theory
of the origin of the universe.
What did it mean, that strange new phrase, expanding universe? It meant
that the light from distant nebulae was red-shifted not from some peculiar de Sitter
effect, but because the nebulae were actually moving away from us. This was not
because there is anything special about us — by now astronomers understood that
the nebulae are galaxies more or less like our own. Each of these galaxies was
moving away from all the other galaxies. Space itself was expanding between them.
There was no special point somewhere among the stars where the great expansion
had started—we and all other galaxies are inside that place. Thus the farther any
two galaxies were apart, the faster they continued to separate—which was precisely
Hubble's velocity-distance relation.
Cosmologists recognized at once that an expanding universe means that in the far
future the galaxies will be spread much farther apart. Looking back, long ago the
universe must have been far denser. Did time itself have a beginning? Hubble's few
measurements were enough to persuade the world's best scientists to take up a
radically new view of the nature, the origin, and the fate of the universe. Perhaps
scientists could take up this view so quickly because quantum and relativity theory
had prepared them for remarkable revelations. The recognition that the universe is
expanding was no less revolutionary — the culmination of a truly exceptional period
in the history of science.
The expansion of the universe is now seen as one of the great scientific
discoveries, and Hubble generally gets the credit. More precisely, however, Hubble
established an empirical formula that led the great majority of scentists to believe in
the expansion. It is an open historical and philosophical question in what sense
Hubble's correlation of data was a "discovery," and exactly how the claim that the
universe is expanding grew in scientists' minds.
Many observations have confirmed the model of an expanding universe that
Hubble's relationship validated. But Hubble should not be judged simply by which
of his conclusions are now believed to be correct. More important was the direction
he pointed out: using galaxies as a key to cosmic history. Hubble's work should be
appreciated for the assumptions it overthrew, and for the vistas it opened, as a
landmark accomplishment of human intellect.
Copyright ©2010.
Brought to you by the
Center for History of Physics,
a Division of the
American Institute of Physics