The Theory Of Everything: The Origin and Fate of the Universe

by Stephen Hawking

And that brings us to the question of what could have caused the stars to have turned on in the first place.

We now know that our galaxy is only one of some hundred thousand million that can be seen using modern telescopes, each galaxy itself containing some hundred thousand million stars. We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in its spiral arms orbit around its center about once every hundred million years. Our sun is just an ordinary, average-sized, yellow star, near the outer edge of one of the spiral arms.

We have certainly come a long way since Aristotle and Ptolemy, when we thought that the Earth was the center of the universe.

Or, in other words, the farther a galaxy was, the faster it was moving away.

On the other hand, if the rocket has more than a certain critical speed–about seven miles a second–gravity will not be strong enough to pull it back, so it will keep going away from the Earth forever.

This should not unduly worry us since by that time, unless we have colonies beyond the solar system, mankind will long since have died out, extinguished along with the death of our sun.

As far as we are concerned, events before the big bang can have no consequences, so they should not form part of a scientific model of the universe.

In other words, one has a singularity contained within a region of space-time known as a black hole.

So it is now generally accepted that the universe must have a beginning.

Chandrasekhar calculated that a cold star of more than about one and a half times the mass of the sun would not be able to support itself against its own gravity. This mass is now known as the Chandrasekhar limit.

In other words, the singularity always lies in his future and never in his past.

So after gravitational collapse a black hole must settle down into a state in which it could be rotating, but not pulsating. Moreover, its size and shape would depend only on its mass and rate of rotation, and not on the nature of the body that had collapsed to form it.

Another way of seeing this is that the event horizon, the boundary of the black hole, is like the edge of a shadow. It is the edge of the light of escape to a great distance, but, equally, it is the edge of the shadow of impending doom.

As the black hole loses mass, the area of its event horizon gets smaller, but this decrease in the entropy of the black hole is more than compensated for by the entropy of the emitted radiation, so the second law is never violated.

Even if the search for primordial black holes proves negative, as it seems it may, it will still give us important information about the very early stages of the universe. If the early universe had been chaotic or irregular, or if the pressure of matter had been low, one would have expected it to produce many more primordial black holes than the limit set by our observations of the gamma ray background. It is only if the early universe was very smooth and uniform, and with a high pressure, that one can explain the absence of observable numbers of primordial black holes.

The only feature of the astronaut that would survive would be his mass or energy.

quantum mechanics allows the universe to have a beginning that is not a singularity. This means that the laws of physics need not break down at the origin of the universe.

The state of the universe and its contents, like ourselves, are completely determined by the laws of physics, up to the limit set by the uncertainty principle. So much for free will.

Eventually, once the temperature had dropped to a few thousand degrees, the electrons and nuclei would no longer have had enough energy to overcome the electromagnetic attraction between them. They would then have started combining to form atoms.

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. On the other hand, if the expansion rate at one second had been larger by the same amount, the universe would have expanded so much that it would be effectively empty now.

At the singularity, general relativity and all other physical laws would break down. One cannot predict what would come out of the singularity.

Space–time would have a boundary— a beginning at the big bang.

one has to take into account the selection principle that we live in a region of the universe that is suitable for intelligent life.

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.

But the inflation we think may have occurred in the size of the universe was much greater even than that—a million million million million million times in only a tiny fraction of a second. Of course, that was before the present government.

As the universe expanded, the matter particles got farther apart. One would be left with an expanding universe that contained hardly any particles. It would still be in the supercooled state, in which the symmetry between the forces is not broken. Any irregularities in the universe would simply have been smoothed out by the expansion,

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.

one has to use a quantum theory of gravity to discuss the very early stages of the universe.

It is not necessary to postulate new laws for singularities, because there need not be any singularities in the quantum theory.

Imaginary time may sound like science fiction, but it is in fact a well–defined mathematical concept.

It may be that Euclidean space-time is the fundamental concept and what we think of as real space-time is just a figment of our imagination.

The surface of the Earth is finite in extent but it doesn’t have a boundary or edge. If you sail off into the sunset, you don’t fall off the edge or run into a singularity.

One could say: “The boundary condition of the universe is that it has no boundary.”

a scientific theory is just a mathematical model we make to describe our observations. It exists only in our minds. So it does not have any meaning to ask: Which is real, “real” or “imaginary” time? It is simply a matter of which is a more useful description.

But if the universe is really completely self-contained, having no boundary or edge, it would be neither created nor destroyed. It would simply be. What place, then, for a creator?

The explanation that is usually given as to why we don’t see broken cups jumping back onto the table is that it is forbidden by the second law of thermodynamics. This says

that disorder or entropy always increases with time.

I shall argue that it is only when they agree with the cosmological arrow that there will be intelligent beings who can ask the question: Why does disorder increase in the same direction of time as that in which the universe expands?

It appears that the universe evolves according to well-defined laws. These laws may or may not be ordained by God, but it seems that we can discover and understand them.

We do not and cannot know the boundary conditions of the universe in the past. However, one could avoid this difficulty if the boundary condition of the universe is that it has no boundary.

A good example of this was Einstein, who said that the cosmological constant, which he introduced when he was trying to make a static model of the universe, was the biggest mistake of his life.

Can God make a stone so heavy that He can’t lift it?

“How much choice did God have in constructing the universe?”

Philosophers reduced the scope of their inquiries so much that Wittgenstein, the most famous philosopher of this century, said, “The sole remaining task for philosophy is the analysis of language.” What a comedown from the great tradition of philosophy from Aristotle to Kant.

Then we shall all be able to take part in the discussion of why the universe exists. If we find the answer to that, it would be the ultimate triumph of human reason. For then we would know the mind of God.