The Quantum Structure of Space and Time (293 pages)

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250 The Quantum Structure of Space and Time


6.3.3 Andrei Linde: Eternal inflation in stringy landscape and the
anthropic principle


In the beginning of the go’s, when the inflationary theory was first proposed, one of
its main goals was to explain the amazing uniformity of our universe. Observations
told us that the universe looks the same everywhere and that the physical laws in
all of its parts are the same as in the vicinity of the solar system. We were looking
for a unique and beautiful theory that would unambiguously predict all properties
of our universe, including the observed values of all parameters of all elementary
particles, not leaving any room for pure chance.
However, most of the parameters of elementary particles look more like a collec-
tion of random numbers than a unique manifestation of some hidden harmony of
Nature. But there was one important property shared by many of these parameters:
Changing them in any substantial way would lead to the universe where we could
not exist. This fact is the foundation of the cosmological anthropic principle [l].


This principle is based on a simple fact: We can observe the universe with a given

set of properties only if these properties are compatible with our very existence.
Whereas this fact is certainly correct, many scientists are still ashamed of using
the anthropic principle. It is often associated with the idea that the universe was
created many times until the final success. It was not clear who did it and why
was it necessary to make the universe suitable for our existence. There were some
attempts to relate the anthropic principle to the many-world interpretation of quan-
tum mechanics, or to quantum cosmology, but these attempts looked esoteric, and
they did not explain why all parts of the universe have similar properties. Indeed, it
seemed to be much simpler to have conditions required for our existence in a small
vicinity of the solar system rather than in the whole universe.
Fortunately, most of the problems associated with the anthropic principle were
resolved more than 20 years ago with the invention of inflationary cosmology. First


of all, in the context of inflationary cosmology, nice conditions in a small vicinity

of the solar system imply similar conditions in the observable part of the universe,
thus removing the most difficult objection against the anthropic principle. Also, in
the context of chaotic inflation [2] there is no need to assume that initial conditions
were the same in all parts of the universe. If initial conditions were different in

different parts of the universe (or in different universes), then a generic inflationary

universe should consist of many exponentially large regions containing matter in all
of its possible states, with scalar fields rolled down to all possible minima of their
energy density, and with space with all of its possible types of compactification.
This observation provided the first scientific justification of the anthropic principle

The situation becomes even more interesting when one takes into account quan-

tum fluctuations produced during inflation. It was shown in [5] that even if the

universe started with the same initial conditions everywhere, e.g. in the SU(5)-
symmetric minimum of the SU(5) SUSY, inflationary fluctuations lead to jumps of

[3, 41.
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