Page 67 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 8
much more contact with my audience. In the audience was a young Russian, Andrei Linde, from the Lebedev
Institute in Moscow. He said that the difficulty with the bubbles not joining up could be avoided if the bubbles were so
big that our region of the universe is all contained inside a single bubble. In order for this to work, the change from
symmetry to broken symmetry must have taken place very slowly inside the bubble, but this is quite possible
according to grand unified theories. Linde’s idea of a slow breaking of symmetry was very good, but I later realized
that his bubbles would have to have been bigger than the size of the universe at the time! I showed that instead the
symmetry would have broken everywhere at the same time, rather than just inside bubbles. This would lead to a
uniform universe, as we observe. I was very excited by this idea and discussed it with one of my students, Ian Moss.
As a friend of Linde’s, I was rather embarrassed, however, when I was later sent his paper by a scientific journal and
asked whether it was suitable for publication. I replied that there was this flaw about the bubbles being bigger than
the universe, but that the basic idea of a slow breaking of symmetry was very good. I recommended that the paper ¿
published as it was because it would take Linde several months to correct it, since anything he sent to the West
would have to be passed by Soviet censorship, which was neither very skillful nor very quick with scientific papers.
Instead, I wrote a short paper with Ian Moss in the same journal in which we pointed out this problem with the bubble
and showed how it could be resolved.

The day after I got back from Moscow I set out for Philadelphia, where I was due to receive a medal from the
Franklin Institute. My secretary, Judy Fella, had used her not inconsiderable charm to persuade British Airways to
give herself and me free seats on a Concorde as a publicity venture. However, I .was held up on my way to the
airport by heavy rain and I missed the plane. Nevertheless, I got to Philadelphia in the end and received my medal. I
was then asked to give a seminar on the inflationary universe at Drexel University in Philadelphia. I gave the same
seminar about the problems of the inflationary universe, just as in Moscow.

A very similar idea to Linde’s was put forth independently a few months later by Paul Steinhardt and Andreas
Albrecht of the University of Pennsylvania. They are now given joint credit with Linde for what is called “the new
inflationary model,” based on the idea of a slow breaking of symmetry. (The old inflationary model was Guth’s
original suggestion of fast symmetry breaking with the formation of bubbles.)

The new inflationary model was a good attempt to explain why the universe is the way it is. However, I and several
other people showed that, at least in its original form, it predicted much greater variations in the temperature of the
microwave background radiation than are observed. Later work has also cast doubt on whether there could be a
phase transition in the very early universe of the kind required. In my personal opinion, the new inflationary model is
now dead as a scientific theory, although a lot of people do not seem to have heard of its demise and are still writing
papers as if it were viable. A better model, called the chaotic inflationary model, was put forward by Linde in 1983. In
this there is no phase transition or supercooling. Instead, there is a spin 0 field, which, because of quantum
fluctuations, would have large values in some regions of the early universe. The energy of the field in those regions
would behave like a cosmological constant. It would have a repulsive gravitational effect, and thus make those
regions expand in an inflationary manner. As they expanded, the energy of the field in them would slowly decrease
until the inflationary expansion changed to an expansion like that in the hot big bang model. One of these regions
would become what we now see as the observable universe. This model has all the advantages of the earlier
inflationary models, but it does not depend on a dubious phase transition, and it can moreover give a reasonable size
for the fluctuations in the temperature of the microwave background that agrees with observation.

This work on inflationary models showed that the present state of the universe could have arisen from quite a large
number of different initial configurations. This is important, because it shows that the initial state of the part of the
universe that we inhabit did not have to be chosen with great care. So we may, if we wish, use the weak anthropic
principle to explain why the universe looks the way it does now. It cannot be the case, however, that every initial
configuration would have led to a universe like the one we observe. One can show this by considering a very
different state for the universe at the present time, say, a very lumpy and irregular one. One could use the laws of
science to evolve the universe back in time to determine its configuration at earlier times. According to the singularity
theorems of classical general relativity, there would still have been a big bang singularity. If you evolve such a
universe forward in time according to the laws of science, you will end up with the lumpy and irregular state you
started with. Thus there must have been initial configurations that would not have given rise to a universe like the
one we see today. So even the inflationary model does not tell us why the initial configuration was not such as to
produce something very different from what we observe. Must we turn to the anthropic principle for an explanation?
Was it all just a lucky chance? That would seem a counsel of despair, a negation of all our hopes of understanding
the underlying order of the universe.

In order to predict how the universe should have started off, one needs laws that hold at the beginning of time. If the
classical theory of general relativity was correct, the singularity theorems that Roger Penrose and I proved show that

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