Page 63 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 8
atmosphere from the emission of gases from the rocks. This early atmosphere was not one in which we could have
survived. It contained no oxygen, but a lot of other gases that are poisonous to us, such as hydrogen sulfide (the gas
that gives rotten eggs their smell). There are, however, other primitive forms of life that can flourish under such
conditions. It is thought that they developed in the oceans, possibly as a result of chance combinations of atoms into
large structures, called macromolecules, which were capable of assembling other atoms in the ocean into similar
structures. They would thus have reproduced themselves and multiplied. In some cases there would be errors in the
reproduction. Mostly these errors would have been such that the new macromolecule could not reproduce itself and
eventually would have been destroyed. However, a few of the errors would have produced new macromolecules that
were even better at reproducing themselves. They would have therefore had an advantage and would have tended
to replace the original macromolecules. In this way a process of evolution was started that led to the development of
more and more complicated, self-reproducing organisms. The first primitive forms of life consumed various materials,
including hydrogen sulfide, and released oxygen. This gradually changed the atmosphere to the composition that it
has today, and allowed the development of higher forms of life such as fish, reptiles, mammals, and ultimately the
human race.

This picture of a universe that started off very hot and cooled as it expanded is in agreement with all the
observational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered:

1. Why was the early universe so hot?

2. Why is the universe so uniform on a large scale? Why does it look the same at all points of space and in all
directions? In particular, why is the temperature of the microwave back-ground radiation so nearly the same when we
look in different directions? It is a bit like asking a number of students an exam question. If they all give exactly the
same answer, you can be pretty sure they have communicated with each other. Yet, in the model described above,
there would not have been time since the big bang for light to get from one distant region to another, even though the
regions were close together in the early universe. According to the theory of relativity, if light cannot get from one
region to another, no other information can. So there would be no way in which different regions in the early universe
could have come to have the same temperature as each other, unless for some unexplained reason they happened
to start out with the same temperature.

3. Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse
from those that go on expanding forever, that even now, ten thousand million years later, it is still expanding at nearly
the critical rate? If the rate of expansion one second after the big bang had been smaller by even one part in a
hundred thousand million million, the universe would have recollapsed before it ever reached its present size.

4. Despite the fact that the universe is so uniform and homogeneous on a large scale, it contains local irregularities,
such as stars and galaxies. These are thought to have developed from small differences in the density of the early
universe from one region to another. What was the origin of these density fluctuations?

The general theory of relativity, on its own, cannot explain these features or answer these questions because of its
prediction that the universe started off with infinite density at the big bang singularity. At the singularity, general
relativity and all other physical laws would break down: one couldn’t predict what would come out of the singularity.
As explained before, this means that one might as well cut the big bang, and any events before it, out of the theory,
because they can have no effect on what we observe. Space-time would have a boundary – a beginning at the big
bang.

Science seems to have uncovered a set of laws that, within the limits set by the uncertainty principle, tell us how the
universe will develop with time, if we know its state at any one time. These laws may have originally been decreed by
God, but it appears that he has since left the universe to evolve according to them and does not now intervene in it.
But how did he choose the initial state or configuration of the universe? What were the “boundary conditions” at the
beginning of time?

One possible answer is to say that God chose the initial configuration of the universe for reasons that we cannot
hope to understand. This would certainly have been within the power of an omnipotent being, but if he had started it
off in such an incomprehensible way, why did he choose to let it evolve according to laws that we could understand?
The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, but
that they reflect a certain underlying order, which may or may not be divinely inspired. It would be only natural to
suppose that this order should apply not only to the laws, but also to the conditions at the boundary of space-time
that specify the initial state of the universe. There may be a large number of models of the universe with different
initial conditions that all obey the laws. There ought to be some principle that picks out one initial state, and hence

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