Page 65 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 8
laws of science or as support for the strong anthropic principle.

There are a number of objections that one can raise to the strong anthropic principle as an explanation of the
observed state of the universe. First, in what sense can all these different universes be said to exist? If they are
really separate from each other, what happens in another universe can have no observable consequences in our
own universe. We should therefore use the principle of economy and cut them out of the theory. If, on the other
hand, they are just different regions of a single universe, the laws of science would have to be the same in each
region, because otherwise one could not move continuously from one region to another. In this case the only
difference between the regions would be their initial configurations and so the strong anthropic principle would
reduce to the weak one.

A second objection to the strong anthropic principle is that it runs against the tide of the whole history of science. We
have developed from the geocentric cosmologies of Ptolemy and his forebears, through the heliocentric cosmology
of Copernicus and Galileo, to the modern picture in which the earth is a medium-sized planet orbiting around an
average star in the outer suburbs of an ordinary spiral galaxy, which is itself only one of about a million million
galaxies in the observable universe. Yet the strong anthropic principle would claim that this whole vast construction
exists simply for our sake. This is very hard to believe. Our Solar System is certainly a prerequisite for our existence,
hand one might extend this to the whole of our galaxy to allow for an earlier generation of stars that created the
heavier elements. But there does not seem to be any need for all those other galaxies, nor for the universe to be so
uniform and similar in every direction on the large scale.

One would feel happier about the anthropic principle, at least in its weak version, if one could show that quite a
number of different initial configurations for the universe would have evolved to produce a universe like the one we
observe. If this is the case, a universe that developed from some sort of random initial conditions should contain a
number of regions that are smooth and uniform and are suitable for the evolution of intelligent life. On the other hand,
if the initial state of the universe had to be chosen extremely carefully to lead to something like what we see around
us, the universe would be unlikely to contain any region in which life would appear. In the hot big bang model
described above, there was not enough time in the early universe for heat to have flowed from one region to another.
This means that the initial state of the universe would have to have had exactly the same temperature everywhere in
order to account for the fact that the microwave back-ground has the same temperature in every direction we look.
The initial rate of expansion also would have had to be chosen very precisely for the rate of expansion still to be so
close to the critical rate needed to avoid recollapse. This means that the initial state of the universe must have been
very carefully chosen indeed if the hot big bang model was correct right back to the beginning of time. It would be
very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended
to create beings like us.

In an attempt to find a model of the universe in which many different initial configurations could have evolved to
something like the present universe, a scientist at the Massachusetts Institute of Technology, Alan Guth, suggested
that the early universe might have gone through a period of very rapid expansion. This expansion is said to be
“inflationary,” meaning that the universe at one time expanded at an increasing rate rather than the decreasing rate
that it does today. According to Guth, the radius of the universe increased by a million million million million million (1
with thirty zeros after it) times in only a tiny fraction of a second.

Guth suggested that the universe started out from the big bang in a very hot, but rather chaotic, state. These high
temperatures would have meant that the particles in the universe would be moving very fast and would have high
energies. As we discussed earlier, one would expect that at such high temperatures the strong and weak nuclear
forces and the electromagnetic force would all be unified into a single force. As the universe expanded, it would cool,
and particle energies would go down. Eventually there would be what is called a phase transition and the symmetry
between the forces would be broken: the strong force would become different from the weak and electromagnetic
forces. One common example of a phase transition is the freezing of water when you cool it down. Liquid water is
symmetrical, the same at every point and in every direction. However, when ice crystals form, they will have definite
positions and will be lined up in some direction. This breaks water’s symmetry.

In the case of water, if one is careful, one can “supercool” it: that is, one can reduce the temperature below the
freezing point (OºC) without ice forming. Guth suggested that the universe might behave in a similar way: the
temperature might drop below the critical value without the symmetry between the forces being broken. If this
happened, the universe would be in an unstable state, with more energy than if the symmetry had been broken. This
special extra energy can be shown to have an antigravitational effect: it would have acted just like the cosmological
constant that Einstein introduced into general relativity when he was trying to construct a static model of the
universe. Since the universe would already be expanding just as in the hot big bang model, the repulsive effect of

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