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The pocket
universes other than our own are believed to be completely unobservable, so one
can question whether it makes any scientific sense to talk about them. I would argue that it is valid science,
because we are pursuing the consequences of a theory for which we already have
other evidence. Of course the theory of
inflation has to rest on the evidence that we can observe, but once we are
persuaded by these observations, then I think that we should also believe the
other implications, even if they involve statements that cannot be directly
confirmed.
If one accepts
the existence of the other pocket universes, then one can still question
whether they have any relevance to the pursuit of science. I will argue that, even though these other
universes are unobservable, their existence nonetheless has consequences for
the way that we evaluate theories and extract consequences from them.
One question
for which eternal inflation has relevance is the question of the ultimate beginning
of the universe - what can be learn about it, and how can we learn it? In the
following talk, Neil Turok will describe his work with Stephen Hawking  and others on the origin of the universe as a quantum event. He will argue that hypotheses about the form
of the initial wave function lead to statistical consequences for our universe
that can in principle be directly tested.
However, if eternal inflation is a valid description of the universe (as
I think it is), then I would expect that all such hypotheses about the ultimate
beginning of the universe would become totally divorced from any observable
consequences. Since our own pocket
universe would be equally likely to lie anywhere on the infinite tree of universes
produced by eternal inflation, we would expect to find ourselves arbitrarily
far from the beginning. The infinite
inflating network would presumably approach some kind of a steady state, losing
all memory of how it started, so the statistical predictions for our universe
would be determined by the properties of this steady state configuration,
independent of hypotheses about the ultimate beginning. In my opinion theories of the ultimate
origin would remain intellectually interesting, and with an improved
understanding of the fundamental laws of physics, such theories might even
eventually become compelling. But I
expect that any detailed consequences of such a theory would be completely
washed out by the eternal evolution of the universe. Thus, there would be no way of relating the properties of the
ultimate origin to anything that we might observe in today's universe.
Although I
believe that the inflating network would approach a steady state, I should
admit that attempts to pursue this idea quantitatively have run into several
technical problems. First, the
evolution of eternally inflating universes leads to physics that we do not
understand. In particular, quantum
fluctuations tend to drive the repulsive-gravity material to higher and higher
energy densities, where the poorly understood effects of quantum gravity become
more and more important. Second, even if we impose enough assumptions
so that the evolution of the eternally inflating universe can be described, we
still do not know how to define probabilities on the infinite set of pocket
universes that is produced. The problem is akin to asking what fraction
of the integers are odd. Most people would presumably say that the
answer is 1/2, since the integers alternate between odd and even. However, the ambiguity of the answer can be
seen if one imagines other orderings for the integers. One could, if one wished, order the integers
as 1,3, 2, 5,7, 4, 9,11,
6 , ... , always writing two odd integers followed by one even
integer. This list includes each
integer exactly once, but from this list one would conclude that 2/3 of the
integers are odd. Thus, the answer
seems to depend on the ordering. For
eternally inflating universes, however, there is no natural ordering to the
regions of spacetime that comprise the entire universe. There are well-founded proposals for
defining probabilities,but at least in my opinion there is no definitive and compelling argument.
A second
implication of eternal inflation is that the probability for inflation to
start - the question of how likely it is for an initial speck of
repulsive-gravity material to form - becomes essentially irrelevant. Inflation only needs to begin once, in all
of eternity. As long as the probability
is nonzero, it does not seem relevant, and perhaps it is not even meaningful,
to ask if the probability is large or small.
If it is possible, then it will eventually happen, and when it does it
produces literally an infinite number of universes. Unless one has in mind some competing process, which could also produce
an infinite number of universes (or at least an infinite space-time volume),
then the probability for inflation to start has no significance.
The third and
final implication of eternal inflation that I would like to discuss pertains to
the comparison of theories. I would
argue that once one accepts eternal inflation as a logical possibility, then
there is no contest in comparing an eternally inflating version of inflation
with any theory that is not eternal.
Consider the
analogy of going into the woods and finding some rare species of rabbit that
has never before been seen. You could
either assume that the rabbit was created by a unique cosmic event involving
the improbable collision of a huge number of molecules, or you could assume that
the rabbit was the result of the normal process of rabbit reproduction, even
though there are no visible candidates for the rabbit's parents. I think we would all consider the latter
possibility to be far more plausible.
Once we become convinced that universes can eternally reproduce, then
the situation becomes very similar, and the same logic should apply. It seems far more plausible that our
universe was the result of universe reproduction than that it was created by a
unique cosmic event.
Contributed by: Dr. Alan Guth
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