Dear Colleagues,Science & Ultimate Reality
When I was a student in the 1960s the ultimate origin of the universe was
widely regarded as lying beyond the scope of physical science. To be sure,
cosmological theory could be applied to the early moments of the universe
following the big bang, but the initiating event itself seemed to be
decisively off-limits - an event without a well-defined cause. The so-called
singularity theorems of Penrose, Hawking and others suggested that the big
bang was an edge or boundary to space-time at which the gravitational field
and the density of matter were infinite, and physical theory broke down.
This meant that there was very little that could be said about how the
universe came to exist from nothing, or why it emerged with the properties
it has.
John Wheeler drew the analogy between the instability of the classical atom,
resulting in the emission of an infinite quantity of radiation, and the
infinite spacetime curvature of gravitational collapse. He conjectured that
just as quantum mechanics had saved classical mechanics from diverging
quantities, and predicted a stable, finite atom, so might quantum mechanics
ameliorate the big bang singularity - smearing it with Heisenberg
uncertainty. But could one take seriously the application of quantum
mechanics, a theory of the subatomic realm, to cosmology, the largest system
that exists? Wheeler, with typical audacity, believed so, and with Bryce
DeWitt produced a sort of Schrödinger equation for the cosmos. Thus was the
subject of quantum cosmology born.
Quantum cosmology received a fillip in the early 1980s from Alan Guth's
inflationary universe scenario, which postulates that the universe suddenly
jumped in size by an enormous factor shortly after the big bang. The point
here is that if Guth is right then although the universe is now very big, it
was once very small - small enough for quantum effects to have played a
formative role.
There remain problems about the formalism of quantum cosmology, both
concerning the interpretation of the theory and certain mathematical
oddities. Nevertheless many calculations have yielded reasonable
predictions. For example, some cosmologists now think that quantum
fluctuations in the inflationary era created the small primordial
irregularities in the early universe that served as the seeds for its
large-scale structure. If so, then the slight temperature variations
detected in the cosmic background heat radiation - the fading afterglow of
the big bang - are none other than quantum fluctuations from the dawn of
creation inflated and writ large in the sky.
Andrei Linde is a cosmologist who has been involved in the quantum cosmology
program since the early days of inflation. Indeed, he developed his own
variant of the inflationary theory, termed chaotic inflation, that has been
lucidly explained in his expository articles and books. One fascinating
prediction of chaotic inflation is that what we call "the universe" might be
merely a "Hubble bubble" within a multiverse of vast proportions. There
could be other "bubbles" out there, way beyond the scope of even our most
powerful instruments, distributed in their infinite profusion.
If "our universe" is indeed but a minute component of a far more extensive
and complex system, the philosophical consequences are profound. Since
Copernicus, scientists have clung to the notion that there is nothing
special about our location in the universe. We inhabit a typical planet
around a typical star in a typical galaxy. But the multiverse theory
suggests that, on a super-Hubble scale, our "bubble" might be far from
typical. It might represent a rare oasis of habitability in a desert that is
generally hostile to life. If so, perhaps many of the felicitous features we
observe, including the fact that the physics of "our universe" seems so
bio-friendly, might actually be the product of a cosmic selection effect. We
live in such a special bubble only because most of the multiverse is
unfriendly to life. This idea generally goes under the name "anthropic
principle," a misnomer since no special status is accorded to homo sapiens
as such. Alternatively, it may be that bio-friendly regions of the universe
are also those that grow very large and so occupy the lion's share of space.
Again, it would be no surprise to find ourselves living in one such region.
But these ideas remain highly speculative and need further mathematical
development. Linde is at the forefront of those who seek to place the
"anthropic principle" on a more rigorous footing.
A summary of Linde's paper follows, and your comments are awaited with
interest.
Paul Davies
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Title: Quantum Cosmology, Inflation, and Anthropic Principle
Author: Andrei Linde
Summary
Everybody knows that quantum mechanics is important in application to
microscopically small particles. Does it make any sense to apply it to our
universe, which is the largest possible system? This question was analysed
by Wheeler and De Witt in the end of the 60's, but only now can we fully
appreciate the true significance of their work. Thirty years ago one could
argue that quantum cosmology is a purely academic subject. Perhaps it was
important for the understanding of the moment the universe was created, but
we cannot completely describe this moment anyway. And when the universe was
born, it was supposed to be so large that for all practical purposes one
could neglect its quantum nature. Indeed, according to the standard big bang
theory, the initial size of the universe was at least thirty orders of
magnitude greater than the so-called Planck length, at which the quantum
nature of space-time becomes important.
The situation changed in the beginning of the 80's with the invention of
inflationary cosmology. This theory provided answers to some of the
questions that for a long time seemed naive and metaphysical: Why is our
universe so large? Why is it so uniform? Why did its different parts begin
their expansion simultaneously? The simplest version of this theory, chaotic
inflation, suggests that the whole universe could emerge from a domain of
space of a Planckian length. This tiny part of space grows up to its present
enormous size due to the process of inflation, exponentially rapid expansion
in a vacuum-like state. In this scenario one does not need to make any
assumptions about initial homogeneity of the universe on a very large scale:
homogeneity on the Planck scale is sufficient. According to this theory, all
galaxies that we see now have been produced by quantum fluctuations
generated during inflation. Some of these quantum fluctuations are so
powerful that they can produce new parts of the inflationary universe. The
process of self-reproduction of the inflationary universe may continue even
now. Instead of looking like a spherically symmetric balloon expanding in
accordance to the laws of the classical Einstein theory, our universe is an
eternally growing fractal produced by quantum effects.
We are coming to a rather paradoxical conclusion. One can use the classical
theory of gravity to describe the motion of planets, but without taking
quantum effects into account one cannot understand the formation of much
larger objects, such as galaxies. More importantly, the process of formation
of the universe is not over yet. One can understand the global structure of
the self-reproducing inflationary universe only in the context of quantum
cosmology.
Quantum fluctuations divide the inflationary universe into many
exponentially large domains (mini-universes) where the laws of low-energy
physics may be different. `Mutation' of the laws of physics is possible in
many versions of inflationary cosmology, but it is most efficient in the
context of chaotic inflation. We can only live in those parts of the
universe that have properties compatible with our existence. This
observation provides a simple justification of the anthropic principle in
inflationary cosmology.
One may consider this as a de-mystification of the anthropic principle. Now
we have no need to assume that somebody creates one universe after another
for our benefit: The universe repeatedly re-creates itself in all of its
possible forms. And we have no problem explaining why it was necessary to
work so hard to create order in all parts of the universe whereas for our
existence it would be sufficient to have good conditions in a small vicinity
of the solar system. Indeed, quantum effects during inflation may divide the
universe into many parts with different properties, but within each of these
parts inflation makes the universe very uniform on exponentially large
scale.
We may try to take a second step and make the anthropic principle more
quantitative. Consider domains of different types. The total volume of some
domains may grows much faster than the total volume of all other parts of
the universe. So one may argue that we have a better chance to live in parts
of the universe that grow faster and, as a result, have greater volume. I
called it the Darwinian approach to cosmology. Under some other assumptions
all parts of the universe may grow at the same rate, but nevertheless some
of them may have much greater volume than others. In order to find which
approach is more appropriate we need to learn how to formulate questions in
the context of quantum cosmology, and which questions are most relevant for
the evaluation of the probability of emergence of life.
This brings back old issues of the interpretation of quantum mechanics and
speculations about the possible role of an observer. One may wonder, as in
the Schrödinger cat paradox, whether the parts of the universe created by
quantum effects really exist now even though they are so far away that we
cannot see them, or the final choice between various options occurs only
after one makes an observation. One may even go beyond the interpretation of
quantum mechanics and ask whether the concept of a self-observing universe
introduced many years ago by Wheeler has only an allegorical meaning, or one
should consider consciousness as a part of nature that may have existence of
its own, rather than being just a tool used to describe the independently
existing reality. These questions may sound naive and metaphysical, but the
history of the development of inflationary cosmology shows that
investigation of apparently metaphysical questions can sometimes be quite
productive.
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