These days most people know that the universe began with a bang-a big bang. But as soon as they try to imagine this event, an awkward question presents itself: what happened before the big bang? Since the idea of something happening without any prior cause is alien to us, the question seems to demand an answer. Yet many cosmologists insist that there is no answer to the question of what happened before the big bang, not because the origin of the universe must forever lie beyond the scope of human inquiry, but because the question itself is meaningless.
It is a subject charged with emotion. Books and lectures on the big bang often provoke a passionate response. Religious people see the scientific attempt to explain how the universe came into existence as a move to finally abolish God the creator. Atheists are also alarmed, because the notion of the universe coming into being from nothing looks suspiciously like the creation ex nihilo of Christianity.
Today, few cosmologists doubt that the universe did have an origin at a finite moment in the past. The alternative-that the universe has always existed in one form or another-runs into a basic paradox. The sun and stars cannot keep burning forever: sooner or later they will run out of fuel and die. The same is true of all irreversible physical processes; the stock of energy available in the universe to drive them is finite, and cannot last for eternity. This is an example of the so-called second law of thermodynamics, which, applied to the entire cosmos, predicts that it is stuck on a one-way slide of degeneration and decay towards a final state of maximum entropy, or disorder. As this final state has not yet been reached, it follows that the universe cannot have existed for an infinite time.
Fortunately we do not have to rely on theoretical reasoning alone to deduce that the universe had a definite origin. Direct evidence for the big bang comes from three different sorts of observations. The most straightforward reason for believing that a gigantic explosion started everything off is that the universe is still expanding today: the galaxies are rushing away from each other. By running the movie backwards it is possible to estimate that the big bang occurred between ten and 15 billion years ago. The fact that the Earth is known to be 4.5 billion years old from radioactive dating adds weight to the big bang scenario.
The second bit of evidence is that the entire cosmos is bathed in a distinctive form of heat radiation, neatly explained as the fading afterglow of the hot big bang. The third strand of evidence is less direct, but still persuasive. It concerns the relative abundance of the chemical elements, which can be correctly accounted for in terms of nuclear processes in the hot dense phase that followed the big bang.
There was no "before big bang"
Most people are happy to accept the scientific account of the universe following its origin in a big bang. But attempts to delve into the originating event itself set alarm bells ringing. "Who cares about half a second after the big bang," as one columnist wrote. "What about half a second before?"
What indeed? Critics like to throw this point at scientists who seem too triumphalist about their ability to demystify nature. Yet the answer is that in standard big bang theory there was no such moment as "half a second before."
To explain what I mean by this mysterious-sounding statement, I must first clear away a common misconception about the nature of the big bang. Contrary to popular belief, it was not the explosion of a compressed lump of matter in a pre-existing void. Whenever I lecture on this topic, bemused members of the audience always ask the same questions: What caused the big bang to happen? Where is the centre of the explosion? Where is the edge of the universe? These seemingly pertinent questions are in fact based on an entirely false picture of the expanding universe.
The best way of grasping the correct situation is to imagine that the universe expands not because the galaxies are all rushing away from a common centre, but because the space between the galaxies is expanding or swelling. The idea that space can stretch, or be warped, is a prediction of Einstein's general theory of relativity and has been well enough tested for all serious cosmologists to accept it-even the few who still contest the big bang. According to general relativity, space and time are not a static arena in which the universe happens, but an integral aspect of its gravitational field. This field manifests itself as a warping, or curvature, of spacetime. In the case of interest to us here, the warping can take the form of a progressive stretching of space.
A helpful analogy to the expanding universe is a balloon with paper spots stuck to the surface. As the balloon is inflated, so the spots, which play the role of galaxies, move apart from each other. In this analogy, it is important to understand that the surface of the balloon represents curved or warped space. The region inside or outside the balloon does not correspond to anything in the real universe.
Armed with this imagery, we are ready to tackle the problem of the origin of the universe. Imagine playing the cosmic movie backwards again. The balloon shrinks rather than expands. If it were perfectly spherical (and the rubber infinitely thin), then at a certain time in the past, the entire balloon would shrivel to a speck. The balloon can't shrink any more. This corresponds to the beginning.
Returning to a normal forwards-in-time description, what has just been described is an origin in which space itself comes into existence from nothing at the big bang and expands to form a larger and larger volume. Notice that the universe doesn't expand into anything: space is itself created as the universe swells. The matter and energy content of the universe likewise originates at or near the beginning. According to this picture, there is no centre and no edge, just as the surface of a balloon has no centre or edge. It isn't possible to get outside the universe and look down on an assemblage of galaxies flying apart.
If you are finding it hard to visualise space being finite but having no boundary, don't worry. Our ancestors were equally baffled about the earth. They supposed that it either extended forever in all directions, or there was an edge somewhere that you might fall off. In fact, as we now know, the earth doesn't extend forever, but nor does it have an edge.
Again, it is important to understand that the speck from which space emerges is not located "in" anything. It is not an object surrounded by emptiness. It is the origin of space -starting out infinitely compressed. In particular, the speck doesn't sit there for an infinite duration and then expand. The speck appears instantaneously from nothing; it does not endure through time. Indeed, according to the theory of relativity, there is no possibility of the speck existing through time because time itself begins there. This is perhaps the most crucial aspect of the big bang theory.
The notion that the physical universe came into existence with time and not in time has a long history. It can be traced back to St Augustine in the fifth century, and as far back as the pre-Christian philosopher Seneca. But it took Einstein's theory of relativity to give the idea scientific respectability. The key feature of the theory of relativity is that space and time are part of the physical universe and not merely an assumed background arena in which the universe happens. Hence any attempt to explain the origin of the physical universe must perforce involve an explanation of how space and time came into existence too.
Significantly, the theory of relativity permits space and time to possess a variety of boundaries or edges, technically known as singularities. One type of singularity is the end point of the gravitational collapse of a star into a black hole. Another corresponds to a past boundary, or origin, of space and time, in a big bang. In the latter case, as one passes back in time, the universe becomes more and more compressed and the curvature or warping of spacetime becomes more extreme until it becomes infinite at the singularity. Thus the past boundary to the universe can be viewed as the point where the gravitational field, and hence the spacetime curvature, is infinite, and cannot be continued. Very roughly, it resembles the apex of a cone, where the surface of the cone tapers to an infinitely sharp point and ceases. Thus, although the universe does not have an edge in space, it does appear to have at least one edge in time-the originating event.
Once the existence of an origin of time is accepted, it is immediately clear that the question "What happened before the big bang?" is meaningless. There was no such epoch as "before the big bang," because time began with the big bang. Unfortunately, the question is often answered with the bald statement "there was nothing before the big bang" and this has caused many misunderstandings. People interpret "nothing" in this context to mean empty space, but as the foregoing discussion should have made clear, space did not exist either prior to the big bang.
So does "nothing" here mean something more subtle, like pre-space, or some abstract state from which space emerges? No. As Stephen Hawking has remarked, the question "What lies north of the north pole" can also be answered by "nothing," not because there is a mysterious land of nothing there, but because the region referred to does not exist. It is not merely physically, but also logically, non-existent. So too with the epoch before the big bang. There is no such epoch by definition, so questions of what happened are as empty as those concerning the land north of the north pole.
Some people get very upset when told this. They think they have been tricked. They suspect that scientists can't explain the origin of the universe and are resorting to obscure and dubious concepts like the origin of time merely to befuddle their detractors.
The mind-set behind such outraged objection is understandable: our brains are hard-wired for us to think in terms of cause and effect. But normal physical causation takes place within time: effect follows cause in sequence. There is a natural tendency to envisage a chain of causation stretching back in time, either without any beginning, or else terminating in a metaphysical first cause or prime mover-like God. But cosmologists now invite us to contemplate the origin of the universe as having no prior cause in the normal sense, not because it has an abnormal or supernatural prior cause, but because there is simply no prior epoch in which a preceding causative agency can operate.
Quantum fluctuations to the rescue
Nevertheless, it would be wrong to suppose that cosmologists have explained the universe by the expedient of abolishing any preceding epoch, any more than one can be said to have explained the universe by assuming it has always existed. In either case one can still ask why the universe has the form and the features that it does, or why any universe exists at all.
The highest attainment of a physical theory would be to give us some idea of why time (and space) suddenly "switched on" in a big bang, instead of us having to accept it as an unexplained fact. The latest thinking is that this spontaneous origination of time and space is a natural consequence of quantum mechanics. This is the branch of physics that applies to atoms and subatomic particles, and it is characterised by Heisenberg's famous uncertainty principle, according to which sudden and unpredictable fluctuations occur in all observable quantities.
Quantum fluctuations are not caused by anything-they are genuinely spontaneous and intrinsic to nature at its deepest level. For example, a collection of uranium atoms will suffer radioactive decay due to quantum processes in their nuclei. This decay will proceed with a definite half-life. But in spite of this it is not possible, even in principle, to predict when it will happen to a given nucleus. If you ask, having seen a particular nucleus decay, why the event happened at that moment rather than some other, well, there is no deeper reason-no underlying set of causes-that explains it. The decay "just happens."
The key step as far as the origin of the universe is concerned is now to apply quantum mechanics, not just to matter, but to space and time as well. Recall that spacetime is an aspect of gravitation, so that this project entails an application of quantum theory to the gravitational field. The quantum theory of fields is a well-tested branch of physics, though it has to be said that there are special technical problems associated with the gravitational case that have yet to be satisfactorily resolved. The quantum theory of the origin of the universe therefore rests on shaky ground.
In spite of these technical obstacles, one may say quite generally that once space and time are made subject to quantum principles, the possibility arises of space and time "switching on," or popping into existence, without the need for prior causation, in accordance with the laws of quantum physics.
The details of the "switching on" of time remain subtle and contentious; nevertheless, I can sketch the basic idea. In his theory of relativity Einstein showed that space and time are closely interwoven; one must really think in terms of four-dimensional spacetime, rather than three-dimensional space and one-dimensional time separately. In spite of this, space is still space, and time is still time. Quantum physics, however, introduces a new feature.
The separate and distinct identities of space and time can become smeared or blurred on an ultra-microscopic scale when subjected to the uncertainty principle. Crudely, for brief durations, time can behave like space and vice versa. In the theory developed by James Hartle of the University of California at Santa Barbara and Stephen Hawking of the University of Cambridge, this quantum smearing implies that, closer and closer to the origin, time is more and more likely to adopt the properties of space, and less and less likely to have the properties of time. This transition isn't sudden, but blurred by the uncertainty of quantum physics. Thus time does not "switch on" abruptly in Hartle and Hawking's theory, but it emerges continuously from space over a very short duration. Thus there is no specific first moment at which time starts, but neither does time extend backwards for all eternity. So quantum physics permits the seemingly paradoxical conclusion that time is finite in the past, even though it is impossible to pinpoint an actual beginning of time.
Of course, a full theory of the origin of the universe needs to explain far more than the mere coming-into-being of space and time. It must also explain such additional features as the origin of energy and matter, the large-scale structure of universe and the observed rate of expansion. Over the past 20 years much progress has been made on these topics too, also using the quantum theory of fields.
These theories have suggested to some a more elaborate scenario, in which the region we have traditionally called "the universe" is but a small "bubble" of space within a vast assemblage of expanding regions, often dubbed the multiverse. And a brand new theory-known as the Ekpyrotic model-has even proposed that big bang stemmed from a collision of two universes separated by a "fifth dimension." But fascinating though such theories may be, in what follows I shall restrict myself to the conventional picture that the observed universe is the only one that exists.
One must resist the temptation to imagine that the laws of physics, and the quantum state that represents the universe, somehow exist before the universe. They don't, any more than there is anything north of the north pole. The laws of physics don't exist in space and time at all. Like mathematics, the laws of physics have an abstract existence. They describe the world, they are not "in" it, (although some people disagree profoundly with this view).
However, this does not mean that the laws of physics came into existence with the universe. If they did-if the entire package of physical universe plus laws just popped into being from nothing-then we could not appeal to the laws to explain the origin of the universe. So to have any chance of understanding scientifically how the universe came into existence, we have to assume that the laws themselves have an abstract, timeless, eternal character. The alternative is to shroud the origin in mystery and give up.
It might be objected that we haven't finished the job by taking the laws of physics as given. Where did they come from? And why those laws rather than some other set? This is a valid objection. Granted, we must eschew the traditional causal chain and focus instead on an explanatory chain, but inevitably we now confront the logical equivalent of the first cause-the beginning of the chain of explanation. What are we to make of that?
It is the job of physics to explain the world based on lawlike principles. Questions about the nature of the laws themselves belong to metaphysics. Scientists adopt differing attitudes to the metaphysical problem of how to explain the laws of physics. Some shrug and say we must just accept the laws as a brute fact. Others suggest that the laws must be what they are from logical necessity. Yet others propose that there exist many worlds each with differing laws, and that only a small subset of these universes possess the special laws needed if life and reflective beings like ourselves are to emerge.
Almost all physicists who work on fundamental problems seem to accept the reality of the laws of physics. And if this is done, then we can say that the laws of physics are logically prior to the universe they describe. That is, the laws of physics stand at the base of a rational explanatory chain, in the same way that the axioms of Euclid stand at the base of the logical scheme we call geometry. Of course, you cannot prove that the laws of physics have to be the starting point of an explanatory scheme, but an attempt to explain the world rationally needs a starting point and for most scientists the laws of physics seem a satisfactory choice.
In the same way, one need not accept Euclid's axioms as the starting point of geometry; a set of theorems like those of Pythagoras would do equally well. But the purpose of science (and mathematics) is to explain the world in as simple and economic a fashion as possible. Euclid's axioms and the laws of physics are such an attempt.
In fact, it is possible to quantify the degree of compactness and utility of these explanatory schemes using a branch of mathematics called algorithmic information theory. Obviously, a law of physics is a more compact description of the world than the phenomena that it describes. For example, compare the succinctness of Newton's laws with the complexity of a set of astronomical tables for the positions of the planets. As physics advances, so the unification and generalisation of laws further reduces the overall algorithmic complexity of our description of the universe. It is usual in science to regard the more compact and embracing description as somehow the more fundamental.
There are those people who are not content to accept the laws of physics as axiomatic, and who seek to go beyond. They see this topic as an appropriate place for discussions of meaning or purpose in the universe. For example, one can investigate mathematically whether other logically self-consistent sets of laws exist. One can also determine whether there is anything unusual or special about the set that characterises the observed universe as opposed to other possible universes. Perhaps the observed laws are in a sense an optimal set, producing maximal richness and diversity of physical forms-who knows? It may even turn out that the existence of life and mind relates in some way to this specialness. These are open questions, but they form a more fruitful meeting ground of science and theology than dwelling on the discredited notion of what happened before the big bang.
