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Copyright © Interdisciplinary Encyclopedia of Religion and Science.
No part of this article may be reproduced, stored in a retrievial system or transmitted without the prior permission of the Editors.
To refer to the content of this article, quote: INTERS – Interdisciplinary Encyclopedia of Religion and Science, edited by G. Tanzella-Nitti, P. Larrey and A. Strumia, http://www.inters.org
 Cosmology
William R. Stoeger
I. Introduction - II. Our knowledge on the physical
universe. 1. The size and age of the universe. 2. The evolution
of the universe. 3. The matter in the universe: Stars, Galaxies
and Dark Matter. 4. The matter and radiation in the universe: the
background radiation. - III. The dynamics of the first stages
of the universe: the inflation - IV. Key Obsevations of contemporary
cosmology. 1. The background radiation and the indication of
a “Big Bang”. 2. The primordial abundances of the light
chemical elements. 3. The systematic redshifts of distant galaxies
and the expansion of the universe. - V. The assumptions of contemporary
cosmology and the models of universe. 1. The conceptual presupposition
of cosmology. 2. The standard Friedmann-Lemaître-Robertson-Walker
(FLRW) models. - VI. The questions on the origin and the future
of the universe. 1. Is the Big Bang the origin of the universe?
2. Science, philosophy and theology on the destiny of the universe.
3. Further theological reflections on cosmology.
The word «cosmology» has become more and more frequent
in scientific literature. It designates the area of physics and
astronomy which investigates the observable universe as a single
object of study, its history, its structure, its dynamics and the
processes which are or have been important in its evolution. Some
specialists would also include the origin and the destiny of the
universe in the subject matter of cosmology - and to a limited extent
these are issues for the cosmologist. However, as we shall see,
the ultimate destiny of the universe — and, even more so, its
ultimate origin — can not really be adequately treated
by cosmology as such.
I. Introduction
This article will discuss the basic understanding contemporary
scientific or physical cosmology has of the observable universe
— of its history beginning with the Big Bang and of its
structure and the key processes which have fashioned, or are fashioning,
its evolution — and the issues which it directly raises
in dialogue with theology. It is important to note that there are
two other meanings of the term «cosmology», which are
related to, but also very different from, physical cosmology. «Cosmology»
has also been used to designate the philosophical treatment of the
material world — of time and space, of change, of place,
etc. — philosophy of nature. We should really call this
«philosophical cosmology». The questions it tries to
answer require input from the natural sciences, including physics,
astronomy and physical cosmology, but its methods and scope go beyond
those of scientific cosmology to consider both the presuppositions
of the natural sciences and the philosophical conclusions based
on their findings.
A third meaning of «cosmology» is found within the studies of
cultures — cultural anthropology. In this context, a
«cosmology» is a coherent collection of stories, images, rituals,
explanations in a culture or society which describe in an imaginative way the origin of
the world, of human kind, of natural phenomena, of social institutions, etc. These mythic
elements endow cultural and social institutions and events — indeed the life of
the cultural and society itself — with meaning and significance. They provide
the basis for ethical and social behavior — and for understanding the ultimate
meaning of life (cf. Bolle, 1987). The remainder of this article will focus on physical,
or scientific, cosmology, and on its relationships with issues in the theology of
creation.
Fundamentally, physical cosmology, (which I shall hereafter refer
to simply as «cosmology»), using physics and mathematics,
constructs a detailed model of the observable universe, on the basis
of evidence gathered from astronomical observations and physics
experiments. As more and more information concerning the history,
structure and behavior of the universe becomes available, the model
undergoes modifications, additions and fine-tuning. What does cosmology
tell us about the universe now as we move into the third millennium?
I. Our knowledge on the physical universe
1. The Size and Age of the Universe. The universe is incredibly
vast. To appreciate how vast it is, it is helpful to recall that
our own Galaxy, the Milky Way, is about 100,000 light years in diameter
and consists of about 100 billion stars, of which our Sun is just
one. There are at least 100 billion other galaxies in the observable
universe. And the most distant objects we are detecting, quasars
and primordial galaxies, are of the order of 10 billion light years
away — the light we are receiving from them now was emitted
that long ago. Thus, the observable universe is between about 12
and 15 billion light years in radius.
Thus, we can see that the universe is also very old, between 12
and 15 billion years since the Big Bang. To relate that to the history
of the Earth and the Solar System, it is helpful to note that a
very firm figure for the Earth's age — from geological evidence
— is 4.8 billion years ( GEOLOGY).
Now, in saying that the observable universe is, say, 15 billion
years old, counting from the Big Bang we should not consider the
Big Bang as the absolute beginning of everything. Cosmology cannot
determine that — it is very possible, even likely, that physical
reality in some completely quantized configuration existed “before”
the Big Bang, even though time as we know it may not have (see below,
VI.1).
2. The Evolution of the Universe. One of the most important
characteristics of the universe as we now understand it is that
it is expanding and cooling. Therefore it is also evolving. It was
much, much different at various stages in the past than it is now
( EVOLUTION,
I). Right after the Big Bang, it was extremely hot and dense, with
a temperature above 1032 °K — so hot and dense,
in fact, that the laws of physics themselves were very, very different,
as was the physical reality they would have described. The four
fundamental interactions of physics — gravitation, electromagnetism,
and the strong and weak nuclear interactions — were almost
certainly unified into one single “superforce”. It was
too hot for there to be space and time as we know them, or any of
the particles and structures we recognize now. Then, as the universe
expanded and gradually cooled, at various threshold temperatures
the fundamental interactions successively split apart, and new things
became possible. Eventually — still much less than a second
after the Big Bang — the temperature of the universe was cool
enough for particles like protons, neutrons and electrons to form.
However, it was not until much, much later that stars formed —
only after more than several tens of millions of years after the
Big Bang. And it is only with stars that we first have the manufacture
of the heavy elements — all the elements heavier than helium
and lithium. Thus, complex molecules, like those which constitute
the bulk of earthly reality around us, and our own bodies, were
only possible after the first generation of stars had died and spread
the heavy elements they had produced throughout their neighborhoods.
Thus, cosmological evolution is the essential precursor to the emergence
of life, and the processes of biological evolution.
Very generally the evolution of the cosmos can be described as
the gradual development from being very hot to being very cold,
from being very, very dense to being nearly empty, from being very
smooth to being very lumpy, from being very simple (just a vast
expanding ball of hot ionized gas) to being very complex (composed
of many systems of superclusters, and clusters, of galaxies, each
of which is full of clusters of stars), from being undifferentiated
to being very highly differentiated. And this complexity and differentiation
is even more impressive on microscopic scales - with the development
of the 92 natural elements, and all the vast array of molecules
they are capable of forming, including DNA and proteins, which carry
the information that is essential for the emergence, development
and maintenance of life and consciousness.
3. The Matter in the Universe: Stars, Galaxies and Dark Matter.
Another significant feature of our universe is its density —
on average it is 10-31 to 10-29 grams per
cubic centimeter at the present time. (Obviously, the farther back
into the past one goes, the higher the density he or she encounters.)
It is empty — but not quite, fortunately for us! In turns out
that this density is very close to the critical density — that
needed to insure that the present expansion will eventually come
to a halt, to be followed by collapse. Recent data indicate that
this density is probably less that critical, and even that the expansion
of the universe may be slightly accelerating. This would mean that
the universe would expand forever. The acceleration of the expansion
would indicate that something like vacuum energy — the energy
of space empty of particles, but not of fields! ( MATTER,
VI) — is presently dominating the dynamics of the universe.
Such vacuum energy (which is often referred to as «the cosmological
constant» Λ, first introduced, but then rejected, by
Einstein) can generate a repulsive pressure-gradient-like force.
Even before the resurgence of Λ, cosmologists and astronomers
realized that less than 5% of all mass/energy in the universe is
luminous. 95% of it is dark, and we know about it only through its
gravitational influences. Furthermore, it is also clear that most
of this dark matter cannot be baryonic — cannot be composed
of protons and neutrons, like all the matter we are made of, and
are familiar with. We know practically nothing else about this overwhelmingly
dominant nonbaryonic matter — it could be in the form of massive
neutrinos, axions, gravitinos, neutralinos, or other similar pervasive
but very elusive and difficult to detect weakly interacting massive
particles (WIMPS). Some of it may very well be in the form of the
vacuum energy we mentioned above.
How is this matter in our universe distributed? Obviously, the
luminous matter is presently distributed in very lumpy fashion on
all small and intermediate length scales. We see planets and stars,
clusters of stars, and these in turn are grouped into galaxies,
with most of the galaxies being found in clusters — with large
expanses of almost empty space, filled with some dust and gas, between
them. These clusters of galaxies are usually part of very large
filamentary structures called superclusters, which encompass voids,
areas relatively empty of galaxies, of the order of 100 million
light years or so across. This is what is sometimes referred to
as the “soap bubble structure” of the universe. There
is considerable evidence that at least some of the nonbaryonic mass/energy
is also distributed in lumpy fashion — in a way which underlies
and perhaps induces or supports the lumpiness of the luminous matter.
4. The Matter and Radiation in the Universe: the Background
Radiation. But, despite this hierarchical lumpiness on so many
scales, the matter of the universe seems to be distributed very
smoothly on the very largest scales — on scales larger than
about 450 light years. If we look out into the universe in different
directions, its texture is similar everywhere — the same type
and roughly the same degree of galactic clustering. Finally, and
most importantly, there is the cosmic microwave background radiation
(CMWBR) at 2.73 °K, which we see at almost this same temperature
in every direction on the sky. It is the ``afterglow'' of the Big
Bang — the primeval fireball as we see it now. The CMWBR originates
from the expanding hot, almost homogeneously distributed gas about
300,000 years after the Big Bang when its temperature had cooled
to about 4,000 °K. At this temperature the free electrons in the
plasma recombine with protons to form neutral hydrogen atoms, and
the universe for the first time becomes transparent to radiation.
This is long before the universe became lumpy — in fact, it
is only after this time that structure can begin to form, as perturbations
in the gas — slight overdensities — begin to grow and
later collapse to form galaxies and clusters of galaxies. Thus,
the CMWBR we detect is coming from the cosmic plasma long before
stars and galaxies existed. The smoothness of this CMWBR reflects
the very smooth distribution of matter at that time, which in turn
indicates that on some very large scale at the present time the
distribution of matter in the universe is on average still very
smooth, and looks the same in all directions. Looking at the universe
is like looking at colored construction paper under a microscope,
and then looking at it from far away — on a bulletin board.
Under the microscope, it is looks very lumpy — composed of
splotches of blue, red, or green. However, that lumpy microscopic
structure melts into a very smooth colored expanse when seen from
a distance — when our eye automatically averages the color
over larger volumes. The observable universe is the same way.
Thus, as we peer out into space in every direction — with microwave eyes —
between all the galaxies, our line of sight eventually encounters the fogbank, at which
the universe becomes opaque to radiation. This is at a distance of between 12 billion and
15 billion light years from us — just 300,000 light years shy of the Big Bang itself.
(It should be remembered that as we see farther out into space, we are also seeing farther
back in time — this is simply due to the finite velocity of light. Thus, the photons
we are receiving right now in the CMWBR are carrying information directly to us concerning
the conditions of the cosmic plasma when the universe was just about 300,000 years old and
only an expanding ball of smoothly distributed hot gas — before there were ever any
stars or galaxies, as mentioned above.) It is from this fogbank, or what is technically
referred to as the last scattering surface, that the microwave photons constituting the
CMWBR emanate. This radiation has what is known as a blackbody, or equilibrium,
spectrum. That indicates that the matter and radiation in the universe were in complete
equilibrium at that time, and in epochs much earlier than that. Furthermore, as already
mentioned briefly, the fact that the black body temperature of this CMWBR is very smooth
— varies only by one part in a 100,000 over the whole sky, once peculiar velocity
effects are neglected — indicates that the density of matter at that time varies only
by one part in 100,000, too. Temperature variations code for density fluctuations.
However, these tiny density fluctuations in the primeval plasma, which were first
confirmed through the positive detection of fluctuations (anisotropies) in the CMWBR
blackbody temperature by the Cosmic Background Explorer (COBE) satellite in 1992, are very
significant. They are considered to be the seeds of later galaxy formation. Without them,
there would be nothing from which galaxies, clusters and superclusters of galaxies, and
therefore stars, could evolve. If the density of the universe is perfectly smooth, then it
remains perfectly smooth, unless some mechanism generates density fluctuations. But, if
there are already slight overdensities and underdensities in the primordial cosmic
material at some point, whatever their origin, then as the universe expands these
overdensities and underdensities can grow to form the rich astronomical structure we now
behold.
The way this happens is very simple — it is due to gravity.
Consider cosmic overdensities. Because there is slightly more matter
in the overdensity, there will be a slightly stronger gravitational
attractive force tending to draw that overdense region together.
Due to this that region will expand less rapidly than the surrounding
universe. Thus, its density will increase further, and its expansion
rate will decrease even further. Eventually, the overdensity, or
perturbation as it is often called, will reach a point where it
stops expanding altogether and begins to collapse under its own
weight (gravity again!). As it collapses it will fragment into thousands
or even millions of knots, each of which themselves will collapse,
forming a cluster of stars or galaxies, depending on how big the
original cloud of gas was to begin with. And, as these regions and
sub-regions collapse, they will begin to spin faster and faster,
due to the conservation of angular momentum (just as a figure skater
spins faster and faster as she draws her arms closer to her body).
Thus, the overdensities discovered by COBE in the primordial cosmic
plasma are essential to our understanding of how our universe came
to be the way it is today.
III. The dynamics of the first stages of the universe: the inflation
However, this raises a more fundamental question: Where did these
overdensities and underdensities — these perturbations —
come from? How can they be explained? We need to account for them.
At the same time, there is another very puzzling coincidence which
needs explanation. This is what is called ``the horizon problem.''
If we look out into the universe in different directions all the
way to the last scattering surface and ask how large causally self-connected
regions on that surface are (that is, regions which are small enough,
so that different points within them could have shared information
— which cannot travel faster than the speed of light —
with one another during the 300,000 years since the Big Bang), we
find that the largest such causally self- connected regions will
be only about one degree across on the sky! — if the universe
was expanding at a “normal” rate since the Big Bang itself.
Thus, the last scattering surface we see, from which the CMWBR originates,
is, from this point of view, tiled with about 50,000 little regions
which are causally isolated from one another. But, if they are causally
isolated from one another, how then can we explain how the temperature
of the material in them is practically the same, as indicated by
the CMWBR measurements?! This is a very serious problem!
It turns out that both of these problems — the origin of perturbations and the
horizon problem, as well as a host of other similar problems — can be solved, if we
postulate a very early period of super-rapid, exponential expansion, referred to as
«inflation». If for a very brief interval, of the order of
``illionths'' of second, the universe expanded by, say, 70 orders of magnitude — that
is, by a factor of 1070 — then both of these serious enigmas disappear. In the first
place, it happens that inflationary expansion generates a spectrum of perturbations in a
well-defined way and freezes them into the primeval plasma until such a time that they can
begin to grow, after the matter in the universe decouples from the radiation. Secondly, if
inflation occurred, then the ancestral region from which our observable universe
originated was much, much smaller just after the Big Bang than it would have been without
inflation. This simply means that with inflation the part of the universe from which our
observable universe originated was causally self-connected beforehand, and
therefore remained causally self-connected thereafter, eliminating the horizon problem.
(It can be demonstrated that, once a region is causally self-connected, it will remain so
throughout an inflationary period.) Thus, practically all cosmologists today postulate
that there was such an inflationary period shortly after the Big Bang, which then ended
before the universe was 10-30 seconds old. There is as yet no direct confirmation that
such an inflationary period actually occurred. However, at the same time, there is no
evidence that is inconsistent with such an epoch, and, what is most important, there is so
far no other viable alternative for solving the perturbation and the horizon problems.
Inflation is closely related to the vacuum energy we discussed earlier
(see above, II.3) — the presence of Λ, the cosmological constant.
The only way in which we can imagine inflation occurring is by the
driving force of a positive vacuum energy, which can be shown to
induce exponential expansion. It is relatively easy to see where
such a vacuum energy might be generated in the very early universe.
Two strong possibilities are: 1.) right at the transition from the
fully quantized configuration in what is called the Planck era,
when physics is completely dominated by the superforce we mentioned
above, to the universe with space, time and gravity as we know them
now (this happened when the temperature slipped below 1032 °K, and
gravity separated from the superforce, leaving the combined nongravitational
interactions united in a single Grand Unified Theory (GUT) force.
These in turn broke apart at somewhat lower temperatures.); and
2.) a little later, when the temperature of the expanding universe
sank beneath 1027 °K, and the strong nuclear interaction separated
from the electroweak interaction (the unification of the electromagnetic
force and the weak nuclear force), thus inducing a great deal of
vacuum energy in this phase transition. In both cases it is fairly
easy to account for the generation of dominant vacuum energy in
small, causally connected regions of space, which under its influence
would then undergo extremely rapid expansion — inflation.
At the same time, however, the universe must be able to exit from this rapidly
expanding state, and in this exit it must somehow be reheated (the exponential expansion
is accompanied by supercooling!) up to some fairly high temperature as it resumes normal
expansion and cooling. This can be accomplished by having the vacuum energy rapidly
dissipate into heat and particles. But this should not happen too rapidly! Before that
occurs the universe has to expand enough to solve the horizon problem, and also to
generate the perturbation spectrum. Providing a completely satisfactory model for the
inflation mechanism is a very difficult, but probably not impossible, task, and has not
yet be accomplished. Besides enabling enough expansion and adequately explaining
reheating, the scalar field which is usually invoked to provide the vacuum energy to drive
inflation, must also be consistent with demands of particle physics and lead to density
perturbations of the observed amplitude at the time of last scattering, when the finishing
touches on the CMWBR were made. Finally, the inflationary period which results must also
lead to a density of matter in the universe which is consistent with the one we presently
observe. It was until recently commonly held that inflationary periods inevitably lead to
a universe with a density extremely close to the “critical density”
(that just above which its expansion would eventually halt). If this were so, then, and if
the density of the universe turns out to be somewhat less than critical, as now seems very
likely, inflation would be ruled out. However, it has now been established that certain
types of inflationary scenarios do not necessary lead to a nearly critical-density
universe.
Thus, it seems that, though an inflationary epoch is quite difficult
to incorporate consistently into an adequate overall cosmological
model of our observable universe and the physics which governs it,
it is still both possible and promising. Of course, as already mentioned,
it is very important to find ways of determining whether or not
inflation did occur, particularly positive evidence for it. If it
turns out that inflation could not have happened, then cosmologists
will be very much in the dark as to how very important features
of the universe — the nearly constant CMWBR temperature in
all directions and the galactic and stellar structure we see —
are ultimately to be explained. No other reasonable alternative
to inflation has appeared on the horizon.
IV. Key Obsevations of contemporary cosmology
1. The background radiation and the indication of a “Big
Bang”. Having discussed in some detail what we know of
the universe, it is time to focus on what evidence we have for saying
all this. In the course of what has already been presented, we have
seen that the CMWBR provides the strongest evidence for the principal
characteristics of the universe as we know it. It is the observational
cornerstone of cosmology. Let us now summarize what the CMWBR tells
us, and may eventually tell us.
The CMWBR has been, and is being, very thoroughly studied and measured
with increasing precision on many different angular scales. Its
very existence as cosmic background radiation is the strongest evidence
we have that there was something like the Big Bang — it assures
us that there was a time when the temperature of the universe as
a whole was more than 4,000 °K (it is now 2.73 °K, the present temperature
of the CMWBR). Thus, together with other corroborating evidence,
it compellingly indicates that, as we go back farther and farther
into the past with our observations, we encounter a succession of
ever hotter, ever denser phases. The theoretical limit of these,
from the simple, standard models we use (Friedmann-Lemaître-Robertson-Walker
(FLRW) models, which are both isotropic and spatially homogeneous
— that is, both spherically symmetric and containing matter
and pressure which are constant at any given time; see below), is
what we call the “Big Bang”. According to these, the temperature
and the density of the universe becomes infinite at the Big Bang.
Cosmologists usually assign this Big Bang or initial singularity
the time t = 0. We shall discuss the Big Bang and what it means
more thoroughly below. Strictly speaking the the Big Bang itself
lies beyond the limits where the models are reliable — they
are adequate until the temperature of 1032 K is reached. Beyond
that point, the physics upon which the models depend — in particular,
Einstein's theory of gravity and of space-time ( RELATIVITY,
THEORY OF)— breaks down. In order to investigate what really
happens at those enormous temperatures, a quantum theory of gravity
has to be used. As yet we do not possess a satisfactory one, though
superstring theories look very promising (see, for example, Greene,
1999).
The next most important feature of the universe and its history the CMWBR convincingly
demonstrates is that there was a time when all the matter in the universe was nearly
homogeneous (smooth) - when there were no stars or galaxies. Along with this is the nearly
incontrovertible evidence that at even earlier times the matter in the universe was
ionized, in perfect equilibrium with the radiation it contains. Therefore the matter was
coupled to (by electron scattering), and opaque to, that radiation. As already mentioned
above, the almost featureless smoothness of the density at the time the CMWBR was last
scattered further indicates that there must be a presently very large length scale on
which the average density is constant — that the universe is almost spatially
homogeneous on that scale.
Finally, the slight perturbations, or fluctuations, of the blackbody
temperature of the CMWBR — as already indicated — signal
the presence of similarly slight fluctuations in the matter density
at that time, providing evidence for the beginning of the formation
of structure in the universe. These temperature fluctuations are
presently being extensively studied on all scales, and their varying
applitudes and patterns promise to constrain very tightly our models
of the universe and the processes important in its evolution. For
instance, the preliminary indication that these perturbations are
scale-invariant — that their strength does not depend on their
size — is at least consistent with their origin in any inflationary
episode. And the placement and strength of relatively small angular
scale (of the order of 1 degree) perturbations due to acoustic oscillations
(sound waves) in the ionized gas at last scattering should help
us determine more precisely the mass-energy density of the universe,
and what percentage of that is due to nonbaryonic matter and to
vacuum energy. Such observations may also help to constrain more
tightly the value of the Hubble parameter H0 which
gives the rate of expansion of the cosmos.
2. The primordial abundances of the light chemical elements.
Moving from the CMWBR, another key piece of evidence in cosmology
is the abundances of helium, deuterium and lithium. Let us concentrate
on helium and deuterium here. The abundance of helium in the universe
is about 24% by weight. But stellar processes can account for very
little of that. And deuterium, which is a fragile isotope of hydrogen
(with a proton and neutron in the hydrogen nucleus, instead of just
a proton), cannot be manufactured by stars, only destroyed. So where
did all the helium and the significant trace of deuterium come from?
According to our knowledge of nuclear physics and what our simplest
adequate cosmological models indicate, conditions (temperature,
density, and neutron-proton ratio) between 1 and 3 minutes after
the Big Bang, when the temperature was between 1011 and 109 °K,
were just right for forming lots of helium (in fact, just about
24% by weight!) and some deuterium and lithium. After the temperature
sinks below about 109 °K, the production of these elements ceases
aSee alsore frozen into the cosmic gas. All the heavier elements,
like carbon, oxygen, phosphorous, copper, iron, chlorine, uranium,
etc., are only produced later — in the cores of stars or in
the supernova explosions which herald their demise.
The primordial abundances of helium and deuterium give further
very compelling evidence then that there was a time when the temperature
of the universe was at 1011 °K. They add weight
to our simple picture of the universe — the Big Bang picture
— expanding, cooling and evolving from a very dense, very hot
initial state. They also strongly support the detailed but very
simple FLRW models of that early stage of the universe, together
with the equilibrium thermodynamics and nuclear physics we employ
to describe the matter and its interactions then. Finally, it is
important to mention that even more careful and detailed measurements
and interpretations of this primordial abundance data, including
that of lithium, give strong evidence that only a very small percentage
of the matter in the universe can be baryonic. Most of it must be
nonbaryonic, as we mentioned before. Unfortunately, those data cannot
constrain what sorts of nonbaryonic particles constitute that dominant
component. That remains one of the great mysteries of physics and
cosmology!
3. The systematic redshifts of distant galaxies and the expansion
of the universe. Finally, there is the first really key cosmologically
significant observation that was made — by E. Hubble and M.
Humason — the systematic redshifts of distant galaxies, which
indicate that the universe is indeed expanding. The farther away
a galaxy is from us, the more its light is redshifted. These redshifts
can be precisely measured by determining at what wavelenths recognizable
lines (of hydrogen or of nitrogen, say) in galaxies' spectra are
found. Then a given redshift is simply the factor by which an observed
wavelenth has shifted from that line's «rest wavelength».
Since these redshifts were first detected, astronomers have exerted
great efforts to obtain the redshifts of galaxies and quasars to
great distances. This redshift mapping has not only confirmed the
expansion of the universe and of the galaxies with it, but has also
provided, together with improved independent distance measurements,
determinations of the Hubble parameter, the rate at which the universe
is expanding, and detailed evidence of the large scale clustering
of galaxies — “the soap-bubble structure” of the
universe, to which we referred earlier (see above, II.3).
It should be emphasized here that the expansion of the universe we
are talking about is not the movement of the galaxies and
quasars away from us within the space that surrounds them
and us, but rather the expansion of space itself. The galaxies remain
relatively fixed in space — though they move a little bit within
space, by what is referred to a their “peculiar motions”.
Space, like a three-dimensional balloon, is expanding, and carrying
the galaxies with it! During inflation, as we have seen, this expansion
of space is exponentially rapid. It can thus involve the separation
of galaxies from one another at many times the speed of light. This
is all right — since Einstein's special relativity only forbids
massive particles moving through space more rapidly than
the velocity of light ( RELATIVITY,
THEORY OF, I). It does not say anything about how rapidly space
itself may expand or inflate.
In considering the three key categories of observations in cosmology
— the CMWBR, the primordial abundances of helium, deuterium
and lithium, and the sytematic redshifts of distant galaxies —
we should immediately notice that they are independent of one another
and strikingly support the same general conclusions — that
the universe is both expanding and cooling, and that the farther
we go back into the past, the hotter and denser it was. There are
also other cosmologically signifcant observations, such as the degree
of clustering in the universe — the strength of clustering
between galaxies and between clusters of galaxies on different scales.
This turns out to be a very important piece of information, as it
constrains models of galaxy formation and sifts out some possible
nonbaryonic dark matter candidates. Galaxy number counts with distance
is another general type of observation, which can help us determine
the density of baryonic matter, as well as the intrinsic (non-cosmological)
evolution of galaxy populations. (One of the great problems in observational
cosmology is separating out the effects of these intrinsic evolutionary
processes from those of the expanding, cooling universe itself).
Even somewhat pervasive and obvious characteristics of the universe
and its behavior — such as the overwhelming dominance of matter
over anti-matter ( MATTER,
III.4) and the one-way direction time assumes (time only goes forward,
not backward — this is often referred to as «the arrow
of time» problem; TIME,
II.4) — require explanation is cosmology or in the physics
associated with it. Both of these problems will probably only find
answers in investigations of the very early universe, in the details
of the processes which dominated the Planck epoch, or the extremely
short period just afterwards — around the time when inflation
may have occurred.
Before moving on, it is helpful to ask: «Is there such a
thing as the observable universe as a single object, which is the
principal focus of cosmology?» Or is there, instead of “a
universe”, just a collection of disparate objects — galaxies
and clusters of galaxies — having very different histories
and therefore essentially unrelated to one another? ( UNIVERSE,
II and IV). From what we have just seen, there is strong evidence
that it does indeed make sense to consider the universe as a single
object of study. In particular the existence and character of the
CMWBR demonstrates that all that we can presently see in the universe
— to the very limits of our technological reach — has
a common history, and is intimately interconnected. Further confirmation
of this is found in the common universal physical and chemical laws,
which seem to hold throughout the universe, and in the common large-scale
features which we see in every direction ( NATURAL
LAWS, II and IV). The universe does give overwhelming evidence of
being a single connected manifold or complex. This is one of contributions
of contemporary cosmology.
V. The assumptions of contemporary cosmology and the models of
universe
Now we turn to consider very briefly the models cosmologists and
physicists construct to describe the universe, and to make predictions
about how it should behave at times, temperatures and densities
which are not directly accessible to our observational capabilities.
But first, we shall discuss the assumptions which they make it developing
these models.
1. The conceptual presupposition of cosmology. Like any
science, cosmology begins with a number of assumptions, which neither
it nor any other science as such can completely justify ( AUTONOMY,
IV.1). Some of these assumptions are explicitly articulated. Others
are not. Among those which are not usually expressed are several
very basic ones which we must make in order to begin any investigation.
These are: that something “out there” exists, rather than
nothing; that effects require causes; that physical reality is ordered
and not entirely chaotic. The justification for these assumptions
is fundamentally that “they work” — they enable us
to get somewhere and are prevasively supported by our pre-scientific
experience and knowledge.
There is also one assumption of a more philosophical tenor which cosmologists
often explicitly mention. It is that the same physical laws hold
uniformly throughout the universe ( NATURAL
LAWS, II.3 and IV.1). The laws of gravitation and of electromagnetism,
for example, which we can demonstrate hold here in the immediate
neighborhood of the Earth and the Solar System, are assumed to hold
in exactly the same way throughout the universe. There is some scientific
justification for this — from careful astronomical observations
we have some evidence — but not enough evidence — that
the laws of atomic physics, for instance, and of electromagnetism
and gravity are same in the vicinity of a distant quasar as on earth.
These observations give some indication that matter behaves the
same way there as it does here.
And now we need to discuss two assumptions which are proper to cosmology and which are
always mentioned. The first of these is «the cosmological principle»
and the second is «the manifold-metric model of space-time».
The cosmological principle simply states that there are no physically privileged
spatial points or locations in the universe. In particular, our position within the
universe is in no way physically privileged — the universe therefore should look very
much the same from any other location at this time in its history. Strictly applied this
assumption implies that the universe is isotropic (spherically symmetric) and spatially
homogeneous (smooth — not lumpy — that is, spherically symmetric about every
point!).
But the universe is not spatially homogeneous! It is lumpy on all small and
intermediate length scales. This implies that the cosmological principle cannot hold
precisely in its exact or strong form. The fact that we are intelligent observers, for
instance, implies that we must be have a moderately overdense, very temperate and stable
— and therefore somewhat privileged — location! At the same time, we would want
to assert that observers on planets in any other galaxy would detect the same CMWBR, the
same systematic redshifts of distant galaxies, the same primordial abundance of elements,
and see the same overall structural texture.
Turning to the manifold-metric model of space time, this simply means that space-time
is treated as a continuous four-dimensional membrane — or a continuous
three-dimensional surface which expands or contracts smoothly in time. On the membrane or
manifold is a metric, or “distance function”, which expresses the
distance from one point to another and which also describes angles. Modelling space-time
this way automatically endows it with certain properties, which it may or may not have in
reality. For instance, we speak of the spatial manifold or membrane expanding at certain
rate, like a balloon, and possessing a certain vacuum energy. The question is: does this
not imply that space-time is already a container or an object in too absolute a sense? At
the same time, however, it seems eminently reasonable to use this model, because it
incorporates the spatial and temporal relations objects and events possess relative to one
another.
Finally, there are two physical assumptions which cosmologists always make. The first
is the assumption of a theory of gravity — almost always Einstein's general
relativity, which has Newton's gravitational theory as its classical limit. Without a
theory of gravity you cannot have cosmology, only a cosmography. Gravity gives a
cosmological model its dynamics and its evolutionary characteristics. It specifies how a
given spatial configuration at one time will evolve into spatial configurations at later
times under the influence of the mass-energy distributed within it. Or, going backward in
time, the gravitational theory tells us what antecedant spatial configurations must have
been like, given a particular spatial configuration now.
The second physical assumption is the fluid approximation for the
matter and the radiation content of the universe. Essentially this
means that both are distributed in such a way that a well-defined
density can be assigned at each point and that a relationship between
density, pressure and temperature holds (given by a so-called «equation
of state»). One needs such an equation of state, or its equivalent,
to determine uniquely a cosmological model. If the fluid approximation
does not hold — if, for instance, matter turns out to be hierarchically
clustered on all length scales — that is, if there is
no very large length scale above which matter becomes smoothly distributed
on average (as preponderant evidence now indicates) — then
the fluid approximation would be invalid. In that case we would
have to use the very unwieldy techniques of what is called kinetic
theory to describe the mass-energy distribution in the universe.
2. The Standard Friedmann-Lemaître-Robertson-Walker (FLRW) Models.
Although we have already mentioned the very important standard
FLRW cosmological models in our extensive discussions so far (see
above, IV.1), it is time to focus our attention on them very briefly.
We want to describe them, comment on their usefulness and adequacy,
indicate what they help to tell us about the universe and its origin
and destiny, and describe how we typically use them as the basis
for dealing with structure formation.
The FLRW models which form the standard theoretical foundation of cosmology are really
just the spherically symmetric (isotropic) and spatially homogeneous solutions to
Einstein's field equations with a perfect fluid equation of state. These solutions
describe spherically symmetric space-times filled with matter which has a constant density
at any given time — that is, it is without any lumps or spatial variations. These
space-times are intrinsically dynamic. They represent three-dimensional spaces either
expanding or contracting, with expanding universes cooling and contracting universes
heating up. There are three classes of FLRW models: a) those which have enough matter in
them (the density of matter is greater than the critical density) so that they expand to a
maximum radius, and then collapse; b) those which have just exactly the critical density,
and thus have flat three-dimensional space-sections and just manage to continue to expand
forever; and c) those which have a density which is below the critical density and thus
also will expand forever. Having the critical density or less means that the gravitational
forces induced by it are not enough to brake the expansion of the universe.
As already indicated, a very important feature of FLRW models is that they possess an
initial singularity, or Big Bang. If we go back in time from any point in an FLRW model,
we find that in some finite period in the past, the universe so described encounters a
singularity — a point at which the density, temperature, and curvature of the
universe goes infinite. This is almost always interpreted as representing the
“initial state”, or origin, of the universe and is conventionally
assigned the time t = 0. It is obvious, too, that these models also indicate two general
possibilities for the final state of the universe, depending on whether or not the density
of mass-energy in the universe is greater than the critical density (cf. Clark, 1997;
Adams and Laughlin, 1997). If the density is just critical or less than critical, the
universe will expand forever, gradually running down and dispersing to the point that no
further star formation, galaxy formation or any other local regenerative processes will
occur. The universe would no longer be life-bearing. This is what is called cosmological
``heat death.'' If, on the other hand, the density of the universe is greater than the
critical density, the gravitational force due to the matter in the universe will
eventually bring the expansion to a halt and induce collapse. In its accelerating
contraction the cosmos would get hotter and hotter as it plunges back towards some
extremely hot and dense state similar to what it was like near the Big Bang. This would
obviously also completely destroy all the complex structures it had produced. Below we
shall discuss more fully what philosophical and theological significance should be
attributed to this cosmic “origin” and “destiny” as
given by these standard FLRW models.
But first we need to evaluate briefly the significance of the FLRW models within the
overall context of contemporary cosmology. How important are these very simple, standard
models in giving us an overall general picture of what the universe is really like? The
fact that the universe is lumpy on all small and intermediate scales may lead us believe
that these FLRW models, which are exactly isotropic and spatially homogeneous, really do
not even approach adequacy in describing the universe as it really is and should not be
trusted. But this would be a seriously flawed conclusion. Though the FLRW models as such
cannot describe these inhomogeneities and their behavior, they do describe very
well the large scale features of the cosmos and its thermal history amazingly well. All of
the key observational evidence, the systematic redshifts of distant galaxies, the
abundances of the elements, the various stages the universe as whole has negotiated since
the Big Bang, and most importantly the cosmic micrwave background radiation, strongly
substantiate the picture provided by these models.
Furthermore, although inflationary epochs as such cannot be explained
by what is given by an FLRW model as such (considerations beyond
those involved with such models are needed), they and the processes
leading to them can be easily incorporated and described within
the FLRW framework — via a dominant cosmological constant,
or vacuum energy. Of course, at very early times and at extremely
high temperatures, immediately after the Big Bang, FLRW models are
completely inadequate, as already indicated and as will be briefly
revisited — quantum gravity effects will dominate and a much
more fundamental quantum treatment of this region is needed. However,
once the universe has emerged from this era (the so-called «Planck
era») cooled to less that 1032 °K — FLRW
models provide a remarkably accurate description of the evolution
and general features of our cosmos. Finally, even with respect to
structure formation itself, which exact FLRW models cannot describe,
thorough mathematical-physical treatments based on FLRW models —
what are called «linearized perturbed FLRW models» —
seem to provide good approximations to the initiation and development
of structure formation on most scales. This is essentially because,
from the anisotropies in the CMWBR, we know that structure formation
began from very small deviations from the background FLRW matter
density of the universe. The growth of such deviations is easily
described by linearized perturbation theory, based on the FLRW background.
Only later stages of the evolution of such structure — the
final collapse of perturbations to form stars and galaxies —
require nonlinear treatment.
VI. The questions on the origin and the future of the universe
1. Is the Big Bang the origin of the universe? From what
we have already seen, the answer to this question has to be “no”.
There are several reasons for this important conclusion. First,
although the initial singularity given by the FLRW models can be
considered “the origin of the model universe” they describe,
we have already emphasized that those models break down precisely
in the region near the singularity. Very different physics —
involving quantum gravity, the possible unification of gravity with
the three nongravitational physical interactions and the application
of these to the very early universe — is needed to describe
this crucial initial phase of the universe's history in any way
even approaching adequacy. The promising preliminary work being
done in this area of quantum cosmology seems to indicate, for instance,
that practically all schemes for treating this cosmic regime require
the “disappearance of time” as we know it: time
is something that only gradually emerges from the quantum configuration
of the early universe as it makes the transition into the space-time
manifold described by Einstein's gravitational theory, general relativity
(cf. Isham, 1988, 1993). Thus, strictly speaking, it is only an
analysis of an adequate quantum description of this very early phase
of the universe which could possibly shed light on its “origin”.
That would have to include some consideration of the fact that,
since time as we know it disappears as we go backwards into this
initial quantum state, there may not have been an origin of the
universe in time, but only an origin of time from some pre-existing,
“timeless” quantum state.
Secondly, given our present understanding of the limitations of
the physical sciences, in particular of physics and cosmology, it
is clear that neither can ever really indicate or model the ultimate
origin of the universe, how it makes the transition from complete
and utter nonexistence — from absolutely nothing — to
existence, and how the laws which govern its behavior ultimately
arose. Cosmology, along with the other sciences, always presuppose
existence and order. They are incapable of ultimately explaining
it. Finally, the question of the ultimate origin of the universe
then is not a question of a beginning in time, which as we now see
may not even be indicated from a quantum cosmological point of view,
but rather the question of the ultimate ground of the universe's
being and order, which must transcend the universe itself, since
it is contigent — it does not contain the ultimate explanation
of its own existence. Quantum cosmological schemes, like those of
Hawking and Hartle, Vilenkin, and Linde, which suggest possible
processes by which the universe as we know it emerged from a vacuum
state, or from some other very simple quantum configuration are
interesting, provocative and important educated suggestions as to
how the universe became as it is, but they should not be confused,
as they sometimes are, with providing an ultimate explanation of
the cosmos and of physical reality cf. Isham, 1988; Zycinski, 1996).
2. Science, philosophy and theology on the destiny of the universe.
Similarly, the destiny of the universe as predicted by our cosmological
models should not be taken as indicating that there is no personal
afterlife, or that theological or philosophical intimations of immortality
are purely illusory. From what we know of cosmology, it is true
that practically any elaboration or improvement of our understanding
of physical and cosmological futures leads inevitably to either
something like heat death or to a fiery big crunch. However, it
must be remembered that in constructing physical theories and cosmologies,
many aspects of reality have been abstracted from, in order to focus
on the purely physical and cosmological in their simplest terms.
Certainly the ultimate death and dissolution that is evident from
these considerations, and from many others in astronomy, biology
and neurophysiology, is strongly confirmed by experience. However,
at the same time there are powerful indications involving universal
personal, ethical, aesthetic and religious experiences which reveal,
or at least indicate, a positive ultimate destiny which is hidden
at the core of, but transcends, limited and seemingly ill-fated
reality. The details of how that is related to and emerges from
our own death and dissolution and that of the universe takes us
beyond where sciences can go ( CREATION,
VI).
3. Further theological reflections on cosmology. From what
we have just seen, we can appreciate that the Big Bang should not
be considered “the creation event”, the beginning of all
that exists outside of God
in time. We may still consider it as a symbol
of “the creation event” as long as we do not identify
it as such. That is, it is a scientifically based expression of
the contingency of the universe and of all reality as we know and
experience it. However, theologically speaking, creation, in its
most radical meaning, does not imply ultimate temporal origination
— although that may be very well be involved — but rather
ultimate dependence on something, that is on the divine, which is
capable of accounting for ultimate existence and order ( CREATION,
III). Strictly speaking, as St. Thomas Aquinas well appreciated,
it is possible that material reality existed from all eternity in
some form or other and is still utterly dependent on God —
that it is still very much God's creation (cf. Summa theologiae,
I, q. 46, a. 2; De Aeternitae mundi).
There are other implications stemming from contemporary scientific
cosmology which can deeply enrich theological reflection, and which
both philosophy and theology must take seriously ( NATURAL
IN THE WORK OF THEOLOGIANS). First, if cosmology reveals the basic
truths concerning the structure and history of the universe, and
the events and processes which contribute to those, then in some
definite sense, from a theological point of view, God has been creatively
acting through those events and those regularities and relationships
in nature. Secondly, we must say that God has endowed nature with
functional and formational integrity (cf. Van Till 1996 and 1999)
— with a certain relative autonomy
of being and process. That is, God is not constantly intervening
in the processes of nature, but instead working through them —
those we understand and those we do not yet understand. This point
of view strongly reinforces Christianity's incarnational emphasis
( JESUS
CHRIST, INCARNATION AND DOCTRINE OF LOGOS, III). And thirdly, all
creation is interconnected and interrelated — we cannot separate
human beings from the from the rest of creation, nor the rest of
creation from human beings ( ANTHROPIC
PRINCIPLE). There is a deep and lasting solidarity of all parts
of creation with each other ( NATURE,
VIII). It is in this solidarity that we realize that death and dissolution
are inevitable, but at the same time that somehow, from what our
human experience and convictions intimate — in a way we do
not yet understand — that death and dissolution are not the
final word (cf. Polkinghorne and Welker, 2000; RESURRECTION).
William R. Stoeger
See also: COSMOS,
OBSERVATION OF; CREATION; MATTER; RELATIVITY, THEORY OF; TIME; UNIVERSE.
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