Page 61 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 8
CHAPTER 8

THE ORIGIN AND FATE OF THE UNIVERSE

Einstein’s general theory of relativity, on its own, predicted that space-time began at the big bang singularity and
would come to an end either at the big crunch singularity (if the whole universe recollapsed), or at a singularity inside
a black hole (if a local region, such as a star, were to collapse). Any matter that fell into the hole would be destroyed
at the singularity, and only the gravitational effect of its mass would continue to be felt outside. On the other hand,
when quantum effects were taken into account, it seemed that the mass or energy of the matter would eventually be
returned to the rest of the universe, and that the black hole, along with any singularity inside it, would evaporate
away and finally disappear. Could quantum mechanics have an equally dramatic effect on the big bang and big
crunch singularities? What really happens during the very early or late stages of the universe, when gravitational
fields are so strong that quantum effects cannot be ignored? Does the universe in fact have a beginning or an end?
And if so, what are they like?

Throughout the 1970s I had been mainly studying black holes, but in 1981 my interest in questions about the origin
and fate of the universe was reawakened when I attended a conference on cosmology organized by the Jesuits in
the Vatican. The Catholic Church had made a bad mistake with Galileo when it tried to lay down the law on a
question of science, declaring that the sun went round the earth. Now, centuries later, it had decided to invite a
number of experts to advise it on cosmology. At the end of the conference the participants were granted an audience
with the Pope. He told us that it was all right to study the evolution of the universe after the big bang, but we should
not inquire into the big bang itself because that was the moment of Creation and therefore the work of God. I was
glad then that he did not know the subject of the talk I had just given at the conference – the possibility that
space-time was finite but had no boundary, which means that it had no beginning, no moment of Creation. I had no
desire to share the fate of Galileo, with whom I feel a strong sense of identity, partly because of the coincidence of
having been born exactly 300 years after his death!
In order to explain the ideas that I and other people have had about how quantum mechanics may affect the origin
and fate of the universe, it is necessary first to understand the generally accepted history of the universe, according
to what is known as the “hot big bang model.” This assumes that the universe is described by a Friedmann model,
right back to the big bang. In such models one finds that as the universe expands, any matter or radiation in it gets
cooler. (When the universe doubles in size, its temperature falls by half.) Since temperature is simply a measure of
the average energy – or speed – of the particles, this cooling of the universe would have a major effect on the matter
in it. At very high temperatures, particles would be moving around so fast that they could escape any attraction
toward each other due to nuclear or electromagnetic forces, but as they cooled off one would expect particles that
attract each other to start to clump together. Moreover, even the types of particles that exist in the universe would
depend on the temperature. At high enough temperatures, particles have so much energy that whenever they collide
many different particle/antiparticle pairs would be produced – and although some of these particles would annihilate
on hitting antiparticles, they would be produced more rap-idly than they could annihilate. At lower temperatures,
however, when colliding particles have less energy, particle/antiparticle pairs would be produced less quickly – and
annihilation would become faster than production.

At the big bang itself the universe is thought to have had zero size, and so to have been infinitely hot. But as the
universe expanded, the temperature of the radiation decreased. One second after the big bang, it would have fallen
to about ten thousand million degrees. This is about a thousand times the temperature at the center of the sun, but
temperatures as high as this are reached in H-bomb explosions. At this time the universe would have contained
mostly photons, electrons, and neutrinos (extremely light particles that are affected only by the weak force and
gravity) and their antiparticles, together with some protons and neutrons. As the universe continued to expand and
the temperature to drop, the rate at which electron/antielectron pairs were being produced in collisions would have
fallen below the rate at which they were being destroyed by annihilation. So most of the electrons and antielectrons
would have annihilated with each other to produce more photons, leaving only a few electrons left over. The
neutrinos and antineutrinos, however, would not have annihilated with each other, because these particles interact
with themselves and with other particles only very weakly. So they should still be around today. If we could observe
them, it would provide a good test of this picture of a very hot early stage of the universe. Unfortunately, their
energies nowadays would be too low for us to observe them directly. However, if neutrinos are not massless, but
have a small mass of their own, as suggested by some recent experiments, we might be able to detect them
indirectly: they could be a form of “dark matter,” like that mentioned earlier, with sufficient gravitational attraction to
stop the expansion of the universe and cause it to collapse again.

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