SUMMARY
At this point in our narrative, the reader may well
feel that modern
cosmology is a welter of conflicting
theories, all of which contain some
elements of truth,
but none of which gives a complete picture of the
actual universe. This, however, would be a wrong
conclusion to draw from
the present state of affairs.
It is true that a few years ago this would
have been
a fair assessment, since the observational evidence then
was
far too meager to permit us to choose from among
the various cosmologies
that stem from the basic field
equations. But even then, the common
heritage of all
of these theories (the general theory of relativity) indi-
cated that the basic differences among
them are more
apparent than real.
The situation in the early 1970's was quite different,
for a threshold had
been reached for a cosmological
breakthrough; as we have seen, enough
observational
evidence was available to show us that our universe
originated explosively, about ten to twenty billion years
ago, from a
highly condensed state. Even though we
still could not decide unequivocally
between an ex-
panding and an oscillating
universe on the basis of the
observational evidence, the major problem of
the origin
of the universe had been solved and we had a self-
consistent picture. It accounted not
only for the reces-
sion and distribution of
the distant galaxies but also
for many diverse phenomena, ranging from the
back-
ground radiation all around us in
space (the 3° K. iso-
tropic
radiation which we have already discussed) to the
formation of the stars
and the heavy elements.
The most remarkable thing about the state of matter,
whether in the form of
stars or interstellar dust and
gas all around us, is that it points to some
momentous
event that must have occurred some billions of years
ago and
which led to the pronounced differentiation
that we see now. Starting from
the “big bang,” to
which all these observations
point, we can now arrange
the succession of events that led to the present
state
of the universe into a well-ordered, meaningful, and
understandable sequence. After the original explosion,
when the temperature
was still very high, about 30%
of the primordial neutrons and protons were
fused into
He4, but the expanding gas cooled off much too rapidly
for
elements above He4 to be built up in any appreci-
able quantities, and these had to wait for the stellar
ovens
that were to be formed when the rapidly ex-
panding gas of hydrogen and helium was fragmented
into stars by
turbulence and the gravitational forces.
The fragmentation of the original hydrogen-helium
gaseous mixture into
galaxies and stars occurred when
the exploding universe had cooled off to
very nearly
its present value—about two hundred million
years
after the initial explosion. The density of matter and
radiation was then favorable for gravitational contrac-
tion to take over in local regions
and to compress the
gas into huge clouds. This, however, could occur
only
after another process had come into operation—the
natural and unavoidable fragmentation of the expand-
ing gas into local eddies. One can show that a stream
of gas
becomes unstable against such a fragmentation
when the length of the stream
exceeds a certain num-
ber whose value can be
derived from hydrodynamical
theory. In an expanding universe this is bound
to hap-
pen after the expansion has progressed
beyond a given
point. The average size of the turbulent eddies that
are formed during this kind of fragmentation is deter-
mined by the speed and density of the expanding gas.
The details of this fragmentation process were
worked out many years ago by
J. H. Jeans. According
to his calculation, we know that the expanding gas
must
have broken up into fragments having an average size
equal to
that of a typical galaxy. These galaxies in turn
also suffered
fragmentation (on a smaller scale) by the
same process and the oldest stars
were thus formed.
These oldest stars (about 8 billion years old) were
formed at the center of the galaxies; and that is where
we find them now,
although they also constitute the
globular clusters that surround the core
of a galaxy.
Since the very oldest stars were formed almost ex-
clusively from the primordial hydrogen and helium,
at least some
of the heavy elements that we now ob-
serve all
about us in the universe must have been
synthesized in the interiors of
these stars as they
evolved. This, indeed, is the case, for we now
know,
from the theory of stellar interiors, that thermonuclear
processes occur near the center of a star, resulting in
the transmutation
of the light to the heavy elements.
When the oldest stars were first
formed, they con-
tracted very rapidly until
their central temperatures
reached about 10 million degrees, at which point
ther-
monuclear energy was released
with the transformation
of hydrogen to helium; this process kept the stars
in
equilibrium and supplied them with their energy for
the first few
billion years of their lives—in fact, until
about 12% of their
hydrogen had been transformed into
helium.
The core of each star, consisting entirely of helium,
then began to contract
quite rapidly under its own
weight, and the central temperature rose (in a
few
hundred million years) to about 100,000,000 degrees.
At this high
temperature, the helium nuclei in the core
were transformed to
carbon—the first step in the
buildup of the heavy elements. This
led to the forma-
tion of a carbon core which
contracted still further,
resulting in still higher core temperatures. In
fact, the
temperature in the core continued to increase until the
billion degree mark was reached, and the heavy ele-
ments, right up to iron, had been synthesized. But at
that
point a drastic change occurred in the evolution
of the star, for very
little of its nuclear fuel was left
to supply the energy required to
support its own
weight. The star, which by this time had evolved into
a very large and luminous object, collapsed violently
and became a
supernova, ejecting great quantities of
material from its outer regions.
Following the supernova explosion, the hot residual
core (consisting of such
nuclei as iron, calcium, magne-
sium, and free
electrons) continued to contract, finally
becoming a white dwarf of
enormous density. It re-
mains in this stage
when the outward pressure of the
free electrons just balances the
gravitational contrac-
tion. But this is not
so in all cases, and the star must
continue to contract beyond the white
dwarf stage if
it is massive enough—ultimately becoming a very
hot
neutron star, about ten miles in diameter. Although
such stars
have not yet been observed directly, astron-
omers believe that they constitute some of the X-ray
sources now
being observed and are the recently dis-
covered “pulsars.” But even neutron stars are not
the
final stage of stellar evolution, for the theory of relativ-
ity tells us that such stars must
continue to contract
until they disappear from sight.
But what of the material that was ejected from each
star that became a
supernova? This was swirled into
the outer regions of the galaxy, where it
became the
gas and dust that formed the spiral arms that we now
see.
From this gas and dust—consisting not only of the
primordial
hydrogen and helium, but also of such heavy
elements as carbon, oxygen,
sodium, calcium, and
iron—the second generation, and hence
younger stars
such as our sun, were formed. But something else
happened at the same time—planets were also formed.
It can be
shown, as has been done by C. F. von
Weizsäcker, G. F. Kuiper, H.
Urey, H. Alfvén, and
others, that the turbulences that must occur
when a
star like the sun is formed by gravitational contraction,
from
dust and gas, must lead to the formation of planets
at fairly definite
distances from the star. This is in
agreement with the arrangement of the
planets in our
solar system.
We thus see that the cosmological theories that stem
from Einstein's
gravitational field equations agree with
the overall architectural and
dynamical features of the
universe as we now observe them. At the same
time,
these theories show us how the present state of the
universe has
evolved from a highly condensed initial
state, and tell us what to expect
in the future evolution
of the universe. Although many of the details are
still
missing from this forecast, the dominant features are
clearly
indicated, and we have every reason to believe
that we shall soon be able
to answer most of the ques
tions about the universe that seemed so unanswerable
just a few
years ago, for never before in the history
of science have so many capable
scientists been work-
ing on this exciting
problem.
