Some people dislike the notion that the Universe had a beginning. Why can't it have existed for ever? The answer is simple. There are many physical processes that are irreversible; if the Universe were infinitely old, these processes would all have run their course. The Universe would already have reached its final state.
An example will make this clear. The Sun cannot keep burning for ever. After a few billion years it will run out of fuel and die. So, too, will all stars. Though new stars are still forming, the stock of raw material is finite, and eventually it will be exhausted. So if the present state of the Universe cannot endure for eternity, it cannot have existed for eternity.
Cosmologists are confident they have identified the origin of the Universe in the famous big bang, evidence for which comes from three key observations. In the late 1920s the US astronomer Edwin Hubble found that the Universe is expanding: the galaxies are rushing away from each other. Running the cosmic movie backwards suggests that all matter emerged from the same place between 10 and 20 billion years ago.
By the 1950s scientists realised that, if the Universe began in a highly compressed state, it should have been intensely hot initially. More-over, a relic of this primeval heat ought to survive today, bathing the entire cosmos in a soft glow. This heat radiation was duly detected, in 1965. Sure enough, it is as if the whole Universe is immersed in a gigantic microwave oven.
By scrutinising the cosmic background heat radiation, astronomers are effectively observing the fading glow of the Universe's fiery birth. The radiation provides a snapshot of the Uni-verse about 300,000 years after the big bang. A simple extrapolation enables us to probe back further, and deduce much about the first few minutes and even seconds after the beginning.
The temperature at the end of the first second was a staggering 10 billion degrees - too hot for composite atomic nuclei to exist. The cosmic material would have been reduced to a soup, or plasma, of subatomic particles. Though this condition seems extreme, it is well within the range of laboratory physics to reproduce. Indeed, subatomic particle accelerators can simulate conditions that prevailed at a mere one trillionth of a second after big bang, when the temperature was 10,000 trillion degrees.
From such laboratory studies, cosmologists have computed the likely nuclear reactions that would have taken place in the primeval plasma. It turns out that about a quarter of the material should have been converted into the element helium, the rest mainly remaining as hydro- gen. Astronomers have checked, and found the predicted proportions to be entirely correct.
These concordances between theory and observation are impressive, and have convinced cosmologists that the idea of a hot big-bang origin for the Universe is correct. But people still feel compelled to ask: what happened before the big bang? What actually caused it? Where did all the matter and energy come from?
Unfortunately, many popular accounts give a grossly misleading picture of the nature of the big bang. It is often depicted as the explosion of a compressed lump of matter in a pre-existing void. But no physical theory could explain why, after an infinite duration of emptiness and inactivity, a big bang should suddenly occur at some arbitrary moment in time.
In fact, it has been clear since the work of the Russian physicist Alexander Friedmann in the 1920s that the big bang represents the origin of time itself - and, indeed, space. The big bang did not happen at a particular moment in time; it was the beginning of time. There was no epoch "before" the big bang for us to discuss.
This idea may seem baffling, but it is scarcely new. Already in the fifth century, St Augustine proclaimed that "the world was made with time and not in time". Augustine's proposal finds support from Albert Einstein. Before the theory of relativity, scientists thought of time and space as simply there. But Einstein showed that they are integral parts of the physical Universe, like matter. As with matter, time and space can be affected by physical processes; for example, they can be warped by gravitation.
Clearly, if time and space are part of the physical Universe, then any account of the origin of the Universe must include the origin of time and space too. This is easily said, but it is hard to envisage time and space coming into being from nothing. Consequently, when cosmologists say there was "nothing" before the big bang, people suspect verbal trickery, as if whatever it was that existed before is sneakily being called "nothing" to conceal ignorance.
However, that is not the sense in which the word nothing is being used. Stephen Hawking has remarked that the answer to the question "what lies north of the North Pole?" is also nothing. That doesn't mean there is a mysterious land called Nothing beyond the North Pole. It means the region concerned simply does not exist; it is not defined. The question is meaningless. In the same way, the question of what happened before the big bang is meaningless because it refers to a non-existent epoch.
Nevertheless, we are so steeped in the notion of cause and effect that we feel cheated when told that the Universe just popped into being, complete with its own space and time, like a rabbit from a cosmic magician's hat. Even if we are forbidden to ask what caused the big bang, in the usual sense of causation, we can still ask for an explanation. Why should a Universe come into existence in that manner?
Until a few years ago, scientists had no answer to this ultimate question. One simply had to accept, as a brute fact, that the Universe originated in a big bang. The job of the scientist was to describe the Universe after it had come into existence. The sudden "switching on" of time at the beginning was regarded as a singular occurrence about which science had nothing particularly useful to say. This hands-off state of affairs was transformed with the realisation that quantum physics provides a loophole to evade the normal strictures of cause and effect. Quantum mechanics is the branch of physics that describes the micro-world of atoms and subatomic particles. The idea of applying quantum mechanics to the Universe as a whole therefore seems bizarre. However, if the Universe was once exceedingly compressed, there must have been a time when quantum effects were of cosmic importance.
At the heart of quantum mechanics lies Heisenberg's uncertainty principle. Roughly speaking, this states that all physical quantities are subject to unpredictable and uncaused changes. For example, unlike the proverbial billiard ball, a moving atom does not follow a well-defined path through space. Quantum uncertainty implies that the atom's trajectory is fuzzy and jittery, so that it is generally impossible to know precisely where the atom is located and how fast it is moving. Nor is there any reason why a particular fluctuation in position or motion occurs as it does. It is not caused by anything else; it "just happens".
Another example of the uncertainty principle concerns the decay of a radioactive nucleus. Observe a given nucleus for long enough and you will see it decay, but there is no answer to the question of why it decayed at that moment rather than some other. The decay is genuinely spontaneous. It does not occur at that moment for any reason. Once again, the event "just happens". This principle is fundamental to modern physics, and has been thoroughly tested.
Physicists also observe subatomic particles suddenly coming into existence via quantum uncertainty. It is then a small conceptual step - albeit a huge leap in scale - to argue that the Universe of time and space likewise originated spontaneously from nothing as a result of quantum uncertainty. Expressed differently, the claim that the Universe originated spon-taneously from nothing does not, after all, contradict the laws of physics. There need be nothing singular or supernatural about such an event, once the properties of quantum mechanics are taken into account.
Of course, it is another matter entirely to construct a plausible physical and mathematical treatment of this cosmic origination process. However, a number of cosmologists have attempted to do just that, most notably James Hartle of the University of California at Santa Barbara and Stephen Hawking of the Univer-sity of Cambridge.
The significance of Hartle and Hawking's work lies not so much in the detailed physics, which is very provisional, but in their discovery that two apparently contradictory things about time may be true at once. At first sight it seems that if time has not always existed, there must have been a first moment of time. But Hartle and Hawking showed that it is possible for time to be bounded in the past without there being a specific first moment!
This paradoxical possibility arises because of the intimate association of time and space. Einstein's theory of relativity obliges us to abandon treating time and space as separate entities. Observations confirm that time and three dimensional space are physically linked in a way that compels us to deal with a unified four-dimensional spacetime. Yet Einstein never went so far as to claim that time is a dimension of space, only that time is related to space.
Quantum mechanics offers a new twist to this interweaving of time and space. Quantum uncertainty, when applied to spacetime, implies that time and space can be uncertain as to which is which. For the briefest duration, time can be "spatialised", ie it can become truly another, fourth, dimension of space, to be added to the three observable ones. In other words, the separate identities of time and space can be smeared away by quantum uncertainty.
Applied to the origin of the Universe, quantum smearing means time can emerge with a separate identity from space, continuously, without having to "switch on" abruptly, as cosmologists previously thought. This process of the continuous emergence of time takes place very rapidly on a human scale, occupying no more than a million-trillion-trillion-trillionth of a second. Crucially, there was no well-defined beginning of time, no specific first moment.
So far I have described how it is physically possible for time and space to originate from nothing in accordance with the laws of quantum physics. But the Universe amounts to much more. What about all the matter that makes up the billions of galaxies we can see through our largest telescopes? Where did that come from?
Once again, Einstein's theory of relativity points the way. The famous equation e=mc2 implies that matter is a form of energy. If enough energy is concentrated, matter can be created. This is no idle speculation. Physicists routinely create matter in the lab from energy. An efficient method is to collide two subatomic particles together at high speed. The energy of impact can then create dozens of new particles.
The big bang was the source of prolific energy, enough to make all the matter that constitutes the stars and gases of the galaxies, plus the heat radiation that bathes the cosmos. But we are bound to ask, where did all that cosmic energy come from in the first place?
Totting up the energy of the Universe is a straightforward exercise, except that not all the contributions are positive. Importantly, gravitational energy actually counts as an energy deficit. Using a monetary analogy, the energy of matter represents savings, but gravitational energy represents a debt.
To take a practical illustration, positive energy, such as that stored in a battery, can be used to perform useful work, eg to power a motor. But gravitational energy requires the expenditure of work to overcome it. Thus to pluck the Earth out of the solar system, to which it is bound by the Sun's gravitational force, would require a huge input of energy to work against the Sun's attraction. Conversely, dropping the Earth toward the Sun would release energy.
Gravitation is a universal force: every object in the Universe pulls on every other object. A rough calculation of the (negative) energy of all this cosmic attraction reveals a remarkable result. Though enormous, it turns out to be very close to the same enormous (positive) energy contained in the material of all the stars. In other words, when the energy of all the matter in the stars is added to the gravitational energy of this same material, the answer comes out to be about zero.
Actually, this is not quite true. The gravitational energy of the stars is only a few per cent of the matter energy. However, astronomers are convinced that the stars represent only a small fraction of all the cosmic matter that exists. They have good evidence that substantial quantities of unseen, or dark, matter lurk in the depths of space. Taking the dark matter into account, it is plausible that the total energy of the Universe is precisely zero!
If this sum is correct, it carries a startling implication: a particle of matter can come into existence without the need for any additional energy. The energy locked up in the material content of the particle is exactly offset by its gravitational interaction with the rest of the Universe. Thus, matter can appear in empty space without actually violating the law of energy conservation. Once again, merely identifying a possibility is not the same as producing a detailed physical theory. However, unlike the situation for the quantum origin of the Universe, there is a considerable body of theory about the origin of matter. This theory goes under the beguiling name of the "inflationary Universe scenario", or simply "inflation".
Conceived in the early 1980s, inflation remains the favoured version of the big bang theory. Boiled down to its essentials, the inflationary scenario goes something like this. Shortly after the Universe originated in a quantum process, and before ordinary matter came to exist, space was filled with an exotic type of energy field. This field had the property of producing a gravitational repulsion - antigravity if you like - that caused the Universe to expand faster and faster, so that it jumped in size (inflated) by a huge amount in a split second.
To understand how this process explains the creation of matter, consider what happens when an ordinary gas expands. As the volume of the gas grows, so the energy of the gas falls because the gas pressure does work. This is the principle behind the car motor: the pressure of the expanding petrol vapour does work turning the engine. The expanding Universe is currently doing something similar. As it expands, so the energy of the heat radiation falls.
However, back in the early, inflationary phase, something very different occurred because the pressure of the field was negative at that time. Negative pressure is another way of describing tension. For example, a block of rubber stretched in all directions exerts a tension, or negative pressure. The idea that a fundamental field can have a negative pressure may seem weird, but it is actually quite familiar to physicists. Indeed, Einstein himself proposed just such a field as long ago as 1917.
If the negative pressure of the field is strong enough, it will produce two crucial effects. The first is a negative gravitational force, or anti- gravity. It is precisely this repulsive force that drives the inflationary expansion. Secondly, when something with negative pressure expands, it doesn't lose energy (as in the case of the petrol vapour in the car engine), it actually gains energy. So this amazing primordial field not only makes the Universe expand faster and faster, it pays itself energy for doing so.
When the inflationary scenario was first investigated by Alan Guth of the Massachusetts Institute of Technology, he quickly spotted that the total field energy in a given region of inflating Universe would go on rising with the expansion. But, as explained, the overall energy of the Universe, taking into account the gravitational energy, remains zero throughout.
Guth understood that the inflationary phase was unstable, and would soon come to an abrupt halt. When it did, the invisible field energy it had accumulated would be released in a surge of creative activity, generating the searing heat and violent burst of matter production that we have come to associate with the big bang.
The picture we now have for the origin of the Universe is thus a curious one. First, time and space come into existence spontaneously. Then the Universe embarks on a brief phase of frenetic expansion, during which vast amounts of free energy accumulate in the primordial field. Finally the accelerating expansion ceases, releasing field energy to make heat and matter.
The rest, as they say, is history. The fiery plasma gradually cooled to the point where stars could form. These stars synthesised the remaining chemical elements in their cores. Then, over the aeons, planets, rocks, clouds, crystals and life emerged, and eventually intelligent beings who look back on the great cosmic story and wonder what, if anything, it all means.
Many a philosopher has declared that nothing can come of nothing, or, to put it colloquially, that there is no such thing as a free lunch. But the Universe, it seems, is the ultimate free lunch. Everything can come of nothing.
Early theologians believed that God created the Universe from nothing. Has science rediscovered this basic concept? The answer is both yes and no. Cosmologists believe that given the laws of physics, the Universe does not need a creator. It can come into existence spontaneously and uncaused. But the laws of physics must be assumed already to have some sort of independent existence. If the laws themselves came into existence only with the physical Universe, they obviously couldn't explain the origin of that Universe.
It is tempting to say that the laws of physics were there before the big bang, but that is an abuse of language. As I have explained, there was no time before the big bang. However, these laws may still be considered to exist in the logical sense of prior - that is, the laws are somehow more fundamental than the Universe they describe, in the same sense that the basic rules of geometry came before their application: Euclid's axioms are more fundamental than Pythagoras's theorem.
So science has done away with the need for a button-pushing creator who lives for eternity before making a Universe at a certain moment in time. Yet the laws that permit a Universe to create itself are even more impressive than a cosmic magician. If there is a meaning or purpose beneath physical existence, then it is to those laws rather than to the big bang that we should direct our attention. I believe St Augus-tine understood this, 15 centuries ago.
! Paul Davies is Professor of Physics at the University of Adelaide.
THE LIFE AND DEATH OF THE STARS
A FREE POSTER, 'The Life and Death of the Stars', is enclosed in this week's Independent on Sunday. The four-part series, 'The Universal Question', which began in The Independent yesterday, continues tomorrow and Tuesday.