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The notion of a Big Bang implies that the
Universe was once in a state of high density that it no longer occupies today - in short, that it has changed. The idea of a changing Universe
(as opposed to a static, eternal one) got a big boost from several sciences
during the nineteenth century.
Geologists began to realize that the Earth's crust had evolved and that
it had taken a very long time to create the surface of the Earth at present
geological rates. Paleontologists saw
countless species in the fossil record come and go and concluded that the
logbook of terrestrial species had profoundly changed. Charles Darwin capped it all by arguing
persuasively that terrestrial life and its environment had both evolved
massively over a very long period of time.
Although the idea of an evolving Earth was in
the air, by the turn of the century astronomers had not yet measured the age of
any celestial object, and few in fact had any inkling of a Universe out there
beyond our Galaxy (most thought the Universe was the Galaxy). Two events early in the century shattered
this landscape and ushered in the era of scientific cosmology.
The first was Einstein's discovery of general
relativity (GR) in 1915-1917. GR is central to cosmology because it is a
theory of gravity, and gravity is unique among the four forces of nature for
its long-range
character - stars and galaxies pull on one another by gravity clear across the
Universe. Moreover, unlike Newton's theory of gravity, GR links the very geometry of
space to the distribution of matter and energy within it. Matter and energy are each a source of
gravity in GR, and the local curvature of spacetime is determined by their
local densities. The large-scale topology
of the Universe is thus determined by its contents, and the two have to be
solved for simultaneously. This
imposes a degree of self-consistency not present in Newton, making for a much
tighter theory.
Gravity is inherently dynamic - it pulls on
things and makes them move. A little reflection shows that the only form of
motion that is consistent with large-scale uniformity of the Universe is either
global
expansion or contraction.
Strangely, it took twelve years after the promulgation of GR for this
idea to sink in. One reason was
Einstein himself. In developing the
basic equations of GR, he noticed a freedom to insert an ad hoc term that at that
time corresponded to no known gravitational force. This famous term, called the cosmological constant (Λ)
corresponds to a large-scale repulsive form of gravity (repulsive if
the right sign for Λ is chosen). I
will return to Λ later, but for now I want to stress that the classical
Λ-force of Einstein does not depend on the presence of matter or energy
for its source - it just simply is, at the same strength throughout all
space and time. Einstein was attracted
to Λ because of his belief at the time - the year was 1917 and no
contrary evidence was as yet in hand - that the Universe was static and unchanging. The repulsive “anti-gravity” of Λ could
then be chosen precisely to balance the attractive force of normal gravity and
“prop the Universe up”.
Twelve years later, in 1929, Edwin Hubble
announced that the Universe was in fact expanding. This was the second event that marked the beginning of scientific
cosmology. Actually, Hubble himself
spoke only of “the recession of the nebulae” and avoided the word “expansion”
as an unwarranted extrapolation beyond the data. We owe the concept of
expansion to the Belgian cleric Georges Lemaître. Lemaître had been playing
around with solutions to Einstein's GR equations during the 1920'sand had discovered (with Eddington) that Einstein's static Λ-model was in
fact unstable - it was balanced, but on a knife edge. The slightest mismatch in the value of
Λ either way would trigger either a dizzying collapse or a wild expansion. In such evolving models, Lemaître noticed
that the velocities of galaxies would vary in proportion to their distances,
the same law of proportionality discovered two years later for the real
universe by Hubble. Perhaps the famous
“Hubble law” should really be called “Lemaître's law.”
Primed by his earlier work, Lemaître seized
on Hubble's discovery as evidence for a real physical expansion, and in 1931 he
produced the first cosmological model based on actual data - Hubble's. His model had a repulsive Λ and was just entering its rapid expansion phase at the
present time. It also contained
substantial ordinary matter, whose gravity, Lemaître showed, would generate a
“singularity” at a finite time in the past. Lemaître termed
this initial high-density state the “primeval atom.” He even described the exit from this state as a “bang” although
he did not embellish it with the word “big.”
(That came later, from the British steady-state cosmologist Fred Hoyle,
who invented the term “Big Bang”as a derisive dismissal of it. To him, it was an aesthetically unappealing rival to his
steady state cosmology - more on this later.)
Label aside, the concept of a Big Bang had entered cosmology.
Contributed by: Dr. Sandra Faber
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