18. The Expanding Universe. This thrilling epithet,
originally l'expansion de l'univers, was created in
1927 by Abbé Georges Édouard Lemaître, and cos-
mology has not been the same since. There had been
models of an expanding universe earlier, namely since
1922 (North, pp. 113ff.), but they were described in
words which had no appeal. However, an “expanding
universe” caught everybody's attention, and Arthur
Stanley Eddington soon began to write a book to fit
this title.
Most cosmologists of today aver that the universe
is expanding, meaning that the nebulae (that is,
galaxies) of the universe are receding from our galaxy,
that is, are moving away from our telescopes in their
lines of sight. By the Cosmological Principle (sec. 15
above), if applicable, it then follows that any nebula
is receding from the others.
The evidence adduced is the so-called red-shift in
the (visible) spectrum of a nebula, that is the displace-
ment of the total set of spectral “lines of absorption”
towards the “red” end of the spectrum and thus away
from its violet end. Also, the redistribution of the
spectral lines is such that it is possible to associate a
well-defined positive real number with each nebula.
On the assumption that the red-shift is caused by the
so-called “Doppler effect”—which asserts that a wave
emanating from a receding source gains in wavelength
in transit—this real number is proportional to the
velocity of the recession.
Working entirely within our galaxy, and using the
Doppler effect in this way, Sir William Huggins had
asserted already in 1868 that the star Sirius is moving
away from the Sun, and he calculated a velocity. The
assertion was later confirmed and the velocity found
tolerably good (Coleman, p. 48). But only the twentieth
century was equipped to apply this spectroscopic pro-
cedure to nebulae as a whole.
The red-shift appears to be the greater the fainter
the nebula, and in 1929 this led the American astrono-
mer Edwin P. Hubble to suggest that the velocity of
recession of a nebula is proportional to the nebula's
distance from our galaxy (North, p. 145). This is a
renowned law, called “Hubble's Law.” It also permits
a rough estimate of the limit of the observable universe,
and in 1963 the universe was thus estimated to be about
13 billion light years (Coleman, p. 65).
In 1935 the British astronomer E. A. Milne gave an
extremely simple derivation of Hubble's Law, within
a purely kinematic study of the motion of nebulae. He
treated the assemblage of nebulae of the universe
almost as if it were a large assemblage of particles
composing an expanding gas (North, p. 160).
It was this approach which suggested to Milne,
justifiably, to expressly introduce his Cosmological
Principle (sec. 15). Before Milne, the principle used
to be introduced, somewhat haphazardly, as the occa-
sion would arise; but in one form or another it had
appeared in virtually every cosmological theory since
the beginning of the century (North, p. 156).
Milne credited the Cosmological Principle to a re-
mark of Albert Einstein in 1931 that Alle Stellen des
Universums sind gleichwertig (ibid; “all the stars of the
universe are equivalent”), but philosophically the
principle had a long past. A certain version of it can
be identified in utterances of Nicholas of Cusa
(Jammer, p. 54), and a rather explicit passage is the
following:
The fabric of the world (machina mundi) will quasi have
its center everywhere and its circumference nowhere, be-
cause the circumference and the center are God who is
everywhere and nowhere
(Koyré, p. 17).
This statement refers both to the universe and to
God. But long before Cusa there was a statement about
God only, namely that he is “a sphere of which the
center is everywhere, and the circumference nowhere”
(sphaera infinita cuius centrum est ubique, circum-
ferencia nusquam). The saying occurs in the Book of
XXIV Philosophers, which appears to be a pseudo-
Hermetic compilation of the twelfth century; but the
Renaissance philosopher Marsilio Ficino attributes it
to Hermes Trismegistus, which would make the saying
even older, however shadowy the figure of this Hermes
may be (Yates, p. 247).
On the other hand, long after Cusa, Johannes Kepler
was opposed to a Cosmological Principle; or so it
would appear from the following utterance of his:
It will never be the case that the [starry heavens] would
appear to those whom we may imagine observing them from
these stars as they appear to us. From which it follows that
this place, in which we are, will always have a certain
peculiarity that cannot be attributed to any other place
in all this infinity
(Koyré, p. 67).
First impressions notwithstanding, it is possible that
this argument is only a special pleading for something
like the uniqueness of life on earth, which in itself is
not a contravention of the Cosmological Principle.
Even cosmologists today sometimes contemplate that
“our corner of the universe” may have some peculiari-
ties by which to explain certain occurrences that can-
not be explained otherwise. Altogether it appears that
the meaning, history, origin, and past interpretations
of the Cosmological Principle are still to be investi-
gated.
