Origin of the Universe

From the text of "Origin of the Universe" (1988)

by the theoretical physicist S.W. Hawking

The problem of the origin of the universe, is a bit like the old question: Which came first, the chicken, or the egg. In other words, what agency created the universe. And what created that agency. Or perhaps, the universe, or the agency that created it, existed forever, and didn't need to be created. Up to recently, scientists have tended to shy away from such questions, feeling that they belonged to metaphysics or religion, rather than to science. However, in the last few years, it has emerged that the Laws of Science may hold even at the beginning of the universe. In that case, the universe could be self contained, and determined completely by the Laws of Science.

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The discovery of the expansion of the universe, completely changed the discussion about its origin. If you take the present motion of the galaxies, and run it back in time, it seems that they should all have been on top of each other, at some moment, between ten and twenty thousand million years ago. At this time, which is called the Big Bang, the density of the universe, and the curvature of spacetime, would have been infinite. Under such conditions, all the known laws of science would break down. This is a disaster for science. It would mean that science alone, could not predict how the universe began. All that science could say is that: The universe is as it is now, because it was as it was then. But Science could not explain why it was, as it was, just after the Big Bang.

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I was interested in the question of whether there had been a Big Bang singularity, because that was crucial to an understanding of the origin of the universe. Together with Roger Penrose, I developed a new set of mathematical techniques, for dealing with this and similar problems. We showed that if General Relativity was correct, any reasonable model of the universe must start with a singularity. This would mean that science could predict that the universe must have had a beginning, but that it could not predict how the universe should begin: for that one would have to appeal to God.

It has been interesting to watch the change in the climate of opinion on singularities. When I was a graduate student, almost no one took singularities seriously. Now, as a result of the singularity theorems, nearly everyone believes that the universe began with a singularity. In the meantime, however, I have changed my mind: I still believe that the universe had a beginning, but that it was not a singularity.

The General Theory of Relativity, is what is called a classical theory. That is, it does not take into account the fact that particles do not have precisely defined positions and velocities, but are smeared out over a small region by the Uncertainty Principle of quantum mechanics. This does not matter in normal situations, because the radius of curvature of spacetime, is very large compared to the uncertainty in the position of a particle. However, the singularity theorems indicate that spacetime will be highly distorted, with a small radius of curvature, at the beginning of the present expansion phase of the universe. In this situation, the uncertainty principle will be very important. Thus, General Relativity brings about its own downfall, by predicting singularities. In order to discuss the beginning of the universe, we need a theory which combines General Relativity with quantum mechanics.

We do not yet know the exact form of the correct theory of quantum gravity. The best candidate we have at the moment, is the theory of Superstrings, but there are still a number of unresolved difficulties. However, there are certain features that we expect to be present, in any viable theory. One is Einstein's idea, that the effects of gravity can be represented by a spacetime, that is curved or distorted by the matter and energy in it. Objects try to follow the nearest thing to a straight line, in this curved space. However, because it is curved, their paths appear to be bent, as if by a gravitational field.

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In the case of quantum gravity, Feynman's idea of a "Sum over Histories" would involve summing over different possible histories for the universe. That is, different curved space times. One has to specify what class of possible curved spaces should be included in the Sum over Histories. The choice of this class of spaces, determines what state the universe is in. If the class of curved spaces that defines the state of the universe, included spaces with singularities, the probabilities of such spaces would not be determined by the theory. Instead, they would have to be assigned in some arbitrary way. What this means, is that science could not predict the probabilities of such singular histories for spacetime. Thus, it could not predict how the universe should behave. However, it is possible that the universe is in a state defined by a sum that includes only non singular curved spaces. In this case, the laws of science would determine the universe completely: one would not have to appeal to some agency external to the universe, to determine how it began. In a way, the proposal that the state of the universe is determined by a sum over non singular histories only, is like the drunk looking for his key under the lamp post: it may not be where he lost it, but it is the only place in which he might find it. Similarly, the universe may not be in the state defined by a sum over non singular histories, but it is the only state in which science could predict how the universe should be.

In 1983, Jim Hartle and I, proposed that the state of the universe should be given by a Sum over a certain class of Histories. This class consisted of curved spaces, without singularities, and which were of finite size, but which did not have boundaries or edges. They would be like the surface of the Earth, but with two more dimensions. The surface of the Earth has a finite area, but it doesn't have any singularities, boundaries or edges. I have tested this by experiment. I went round the world, and I didn't fall off.

The proposal that Hartle and I made, can be paraphrased as: The boundary condition of the universe is, that it has no boundary. It is only if the universe is in this "no boundary" state, that the laws of science, on their own, determine the probabilities of each possible history. Thus, it is only in this case that the known laws would determine how the universe should behave. If the universe is in any other state, the class of curved spaces, in the "Sum over Histories", will include spaces with singularities. In order to determine the probabilities of such singular histories, one would have to invoke some principle other than the known laws of science. This principle would be something external to our universe. We could not deduce it from within the universe. On the other hand, if the universe is in the "no boundary" state, we could, in principle, determine completely how the universe should behave, up to the limits set by the Uncertainty Principle.

It would clearly be nice for science if the universe were in the "no boundary" state, but how can we tell whether it is? The answer is, that the no boundary proposal makes definite predictions, for how the universe should behave. If these predictions were not to agree with observation, we could conclude that the universe is not in the "no boundary" state. Thus, the "no boundary" proposal is a good scientific theory, in the sense defined by the philosopher, Karl Popper: it can be falsified by observation.

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So what does the no boundary proposal, predict for the universe. The first point to make, is that because all the possible histories for the universe are finite in extent, any quantity that one uses as a measure of time, will have a greatest and a least value. So the universe will have a beginning, and an end. However, the beginning will not be a singularity. Instead, it will be a bit like the North Pole of the Earth. If one takes degrees of latitude on the surface of the Earth to be the anallogue of time, one could say that the surface of the Earth began at the North Pole. Yet the North Pole is a perfectly ordinary point on the Earth. There's nothing special about it, and the same laws hold at the North Pole, as at other places on the Earth. Similarly, the event that we might choose to label, as "the beginning of the universe", would be an ordinary point of spacetime, much like any other, the laws of science would hold at the beginning, as elsewhere.

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The no boundary proposal, has profound implications for the role of God in the affairs of the universe. It is now generally accepted, that the universe evolves according to well defined laws. These laws may have been ordained by God, but it seems that He does not intervene in the universe, to break the laws. However, until recently, it was thought that these laws did not apply to the beginning of the universe. It would be up to God to wind up the clockwork, and set the universe going, in any way He wanted. Thus, the present state of the universe, would be the result of God's choice of the initial conditions. The situation would be very different, however, if something like the no boundary proposal were correct. In that case, the laws of physics would hold, even at the beginning of the universe. So God would not have the freedom to choose the initial conditions. Of course, God would still be free to choose the laws that the universe obeyed. However, this may not be much of a choice. There may only be a small number of laws, which are self consistent, and which lead to complicated beings, like ourselves, who can ask the question: What is the nature of God? Even if there is only one, unique set of possible laws, it is only a set of equations. What is it that breathes fire into the equations, and makes a universe for them to govern. Is the ultimate unified theory so compelling, that it brings about its own existence. Although Science may solve the problem of ~how the universe began, it can not answer the question: why does the universe bother to exist? Maybe only God can answer that.