What happened before the Big Bang? There was another Big Bang and, before that, another …
Many and strange are the universes that drift like bubbles in the foam upon
the River of Time.
Arthur C. Clarke, ‘The Wall of Darkness’, The Other Side of the Sky
Everything has been said before but because nobody listens we keep having
to going back and begin all over again.
André Gide
It came out of nowhere like an express train out of the night. Only it wasn’t an express train – it was an entire universe, hurtling towards our own from a higher dimension. Before the collision, our Universe was an empty, aching void. In the immediate aftermath, it ignited, exploding outwards in an unstoppable firestorm of light and matter.
Is this an accurate description of the Big Bang? Is everything we see, out to the very farthest reaches probed by our biggest telescopes, merely the wreckage of a titanic collision between universes? A group of physicists from Britain and America believes the answer is yes. They call this colliding-universe scenario the ‘ekpyrotic universe’, from the Greek for ‘born out of fire’. What is more, they say, the cosmic smash-up that triggered the Big Bang may not have been a unique event. ‘Before the Big Bang, there was another Big Bang and, before that, another one, stretching all the way back through the mists of time,’ says Neil Turok of Cambridge University.
This awe-inspiring vision has arisen out of ‘superstring theory’. Superstring theory, or simply string theory, views the fundamental building blocks out of which everything in the Universe is made not as tiny, point-like ‘particles’ but as impossibly small ‘strings’ of super-dense matter. The strings – which are about 10 trillion trillion times smaller than an atom – vibrate exactly like the strings of a violin. And each note they create corresponds to a distinctly different microscopic particle such as an electron or a quark. The higher the pitch of the note, the more energy in the vibration and the heavier the particle.
In the past century, experimental physicists have discovered a host of ‘fundamental particles’. These are ‘glued’together by four fundamental forces, which in turn are transmitted between the particles by a legion of ‘force-carrying’ particles.* In order to mimic all the myriad properties of such a bewildering zoo of particles, strings must be free to vibrate in a large number of different ways. And this can happen, theorists have discovered, only if strings inhabit a bizarre world with a total of ten dimensions of space and time.
A ten-dimensional universe is a big embarrassment for physicists. After all, the world about us gives every appearance of having just four dimensions: north–south, east–west, up–down and past–future. However, string theorists refuse to be fazed by the apparent contradiction between their theory and reality. They insist that, hidden from our view, are additional space dimensions. Whereas the familiar space dimensions extend across billions of light years of space and billions of years of time, these hidden dimensions are said to be ‘rolled-up’ so incredibly small that they have escaped our notice in all experiments to date.
Why go to the trouble of inventing impossible-to-see strings of matter quivering in impossible-to-detect dimensions? The answer, of course, is that there is a big pay-off.
For one thing, string theory resolves a serious conflict between two ideas which are driving forces of modern physics. The first is ‘atomism’. This is the belief that, although the world around us looks bewilderingly complex, this is merely an elaborate illusion: beneath the skin of reality there is just a handful of simple, indivisible building blocks. The complexity of the everyday world is nothing more than a manifestation of the enormous number of ways these building blocks can be stuck together. Everything is in the combinations.
At one time the fundamental Lego blocks of reality were thought to be atoms. Currently, they are believed to be even smaller motes of matter called quarks and ‘leptons’.
The second important idea driving modern physics is that of ‘unification’. This is the belief that many of the fundamental entities we have discovered are really just different faces of entities yet more fundamental.* Physicists believe, for instance, that nature’s four fundamental forces are merely different aspects of a single ‘superforce’ and that even quarks and leptons are different faces of some yet more fundamental particle.
Atomism and unification have proved to be enormously fruitful ideas. But they are set on an inevitable collision course. The reason is that one day, presumably, we will discover the ultimate building block of matter, the indivisible mote out of which everything else is constructed. By definition, it will have no internal structure. After all, if it were constructed from other things, it could be subdivided further and so could not in any way be considered ultimate. But, if it has no internal structure, how can it have different faces? Clearly, it cannot. Like a full stop, it will look the same from every possible viewpoint. Ultimately, then, atomism and unification are irreconcilable.
String theory offers a way out of this impasse. A string is a fundamental, indivisible entity. On the other hand, it can vibrate in a myriad different ways and consequently have innumerable different faces.
Reconciling atomism and unification, however, is not the only pay-off of string theory. Crucially, the theory holds out the tantalising hope of solving one of the greatest outstanding problems in science: how to unite Einstein’s general theory of relativity – which describes the force of gravity – with quantum theory, which describes the other three fundamental forces of nature.
Because gravity is a weak force which becomes noticeable only when there is a large amount of gravitating matter about, Einstein’s theory predicts the behaviour of big things such as planets circling the Sun and even the whole Universe. Quantum theory’s domain of expertise, on the other hand, is the world of small things like atoms and their constituents. Because the domains of the two theories are so completely different, there is generally no overlap, so each theory can be used entirely independently of the other. However, a serious difficulty arises when people want to understand what was going on in the first moments after the Big Bang, the titanic explosion in which the Universe was born about 13.7 billion years ago. At that remote epoch, the Universe was both very massive – the domain of Einstein’s theory of gravity – and smaller than an atom – the domain of quantum theory. Neither Einstein’s theory nor quantum theory is therefore sufficient on its own to illuminate this remote period. What is desperately needed is a hybrid of the two, an over-arching theory which meshes both together – a ‘quantum theory of gravity’.
To say that devising such a theory is hard is a bit of an understatement. Einstein’s theory of gravity, in common with all non-quantum, or ‘classical’, theories, is a recipe for predicting the future. If a planet is here now, the theory predicts that tomorrow it will have moved over there, following a particular path through space. All these things the theory predicts with 100 per cent certainty. Contrast this with quantum theory, which is a recipe for merely predicting possible futures. For an atom flying through space, all that can be known is its ‘probable’ final position, its ‘probable’ path. The Herculean task faced by physicists is therefore to unite, or ‘unify’, two theories – one that deals with certainty and one that deals with uncertainty.
String theory offers hope. It is inherently a ‘quantum’theory. And one of the myriad possible string vibrations turns out to have all the properties of a ‘graviton’, the hypothetical ‘carrier’ of the gravitational force. Consequently, string theory contains within it a theory of gravity (though not necessarily Einstein’s theory of gravity). This is why many physicists see it as the great hope for a unified theory.*
One-dimensional strings turn out not to be the only entities which can pop up in string theory. Because there are ten space-time dimensions to play with, the theory can support the existence of more complicated objects, with two, three, four or more dimensions. These are called ‘branes’. A string, to use the terminology, is one-brane whereas a more general brane, with p dimensions, is a p-brane – a physicist’s rather weak joke.
The existence of branes raises a remarkable possibility. Perhaps that is all our Universe is. Maybe it is a four-brane – a four-dimensional ‘island universe’ floating in a ten-dimensional space-time. This possibility, in turn, raises another intriguing one. If our Universe is a four-brane, it is unlikely to be the only one. Adrift in the unimaginable ten-dimensional void of string theory there may be other island universes. And, if there are other brane-universes out there, might they occasionally fly close to each other, perhaps even collide?
It is a possibility that has captured the imagination of Turok and his colleague, Paul Steinhardt of Princeton University in New Jersey. ‘If there are other universes out there, then it stands to reason that they might occasionally run into each other,’ says Turok. ‘It might, at long last, explain what the Big Bang was.’
There could of course be countless island universes besides our own lurking out there in the higher-dimensional abyss. However, the simplest scenario is always the easiest to deal with mathematically. Also, nature, for reasons nobody really understands, invariably chooses the simplest option.* Turok, Steinhardt and their colleagues therefore assume that, in the whole multi-dimensional stringy universe, there are just two lonely four-branes – ours and one other.
Four-dimensional objects are of course impossible for us to visualise since we inhabit a fundamentally three-dimensional world. Faced with this difficulty, Turok and Steinhardt visualise the four-branes as two-dimensional objects – like the two slices of bread in a sandwich. Again, for simplicity, they assume that the two slices of bread are infinite in extent, so that they form the ultimate boundaries of the Universe. They also assume they are utterly empty, without matter or light – and it is impossible to get much simpler than that.
Between the two slices of bread, where the sandwich filling would normally go, is the country of the fifth dimension. And it is along this dimension – which we can no more perceive than a blind man can experience the colour ‘blue’ – that the two brane-universes hurtle towards each other.
Moving bodies possess energy merely by virtue of their motion. Wander into the path of a speeding cyclist and you will be left in no doubt about this. Consequently, the fast-approaching branes have tremendous ‘energy of motion’ along the fifth dimension.
As already mentioned, there is a cast-iron rule in physics called the conservation of energy which asserts that energy can never be created or destroyed, only transformed from one form into another. In a light bulb, for instance, electrical energy is converted into an equivalent amount of light energy and heat energy. So, when the branes collide, the conservation of energy ensures that their energy of motion along the fifth dimension is dumped into their four-dimensional interiors as surely as the energy of motion of two colliding express trains is dumped into the twisted wreckage. This energy sets the branes expanding violently. ‘It creates the headlong explosion of space we have come to call the Big Bang,’ says Turok.
The Big Bang, of course, was a lot more than a violent expansion of completely empty space. We, and the matter out of which we are made, are testimony to that. Here, Einstein has something important to say. In 1905, he stunned physicists by his discovery that mass is actually a form of energy – the most concentrated type of energy of all. Consequently, not only can mass be turned into other forms of energy – for instance, the scorching heat of a nuclear fireball – but other forms of energy can be converted into mass. So, the energy of the colliding branes ends up not only in the furious expansion of empty space but also in the creation of mass – a blistering hot fireball of fundamental particles.
Of course, the same thing happens to the other brane – the one that collides with ours. It too experiences a ‘hot’ Big Bang.
In this scenario, the subsequent evolution of our Universe is pretty much the same as is widely accepted. As the space of our brane exploded in size, the expansion rapidly cooled the Big Bang fireball. Eventually, when it was cold enough, galaxies and stars congealed out of the shimmering debris. And, one day, after 13.7 billion years, a group of physicists on the third planet of an unremarkable star in a nondescript galaxy called the Milky Way hit on the idea that a collision between peculiar entities called branes might at last provide an explanation for everything we see around us.
Explaining everything we see around us, however, means explaining a lot more than simply the explosion of the blisteringly hot matter of the Big Bang. There is the small matter of how the Universe got to look the way it does today.
Early on in the history of our Universe, matter was spread extremely smoothly throughout space. We know this from the heat of the Big Bang fireball, which still permeates every pore of space. After all, it was bottled up in the Universe and had nowhere else to go. Greatly cooled by the expansion of space in the past 13.7 billion years, this ‘afterglow of creation’ appears today not as light visible to the naked eye but as invisible microwaves of the kind used by mobile phones and microwave ovens.* One of the most striking features of this ‘cosmic background radiation’ is its smoothness: it arrives at Earth equally from all directions. This tells us something important about the way matter was distributed shortly after the Big Bang. Since it was mixed in with the heat radiation in the Big Bang fireball, it too must have been smeared remarkably smoothly throughout space.
Yet, today, far from being spread evenly throughout space, the matter of the Universe is tied up in ‘galaxies’, great islands of stars, separated by great voids of empty space. One of the key questions in cosmology is: How did the Universe go from being smooth to being lumpy?
A partial answer was provided by NASA’s Cosmic Background Explorer (COBE) satellite. In 1992, it discovered that, although the afterglow of the Big Bang looks perfectly smooth around the sky, in some directions it is ever-so-slightly brighter and in others ever-so-slightly fainter than in others. These slight irregularities in the cosmic background radiation are believed to mark the embryonic ‘seeds’ of great clusters of galaxies in today’s Universe. They mark denser-than-average regions of the early Universe, which, by virtue of their stronger-than-average gravity, were able, as they grew older, to pull in more matter than neighbouring regions. This increased their gravity, enabling them to suck in yet more matter, and so on – a cosmic instance of the rich getting ever richer.
The cosmic background radiation in fact comes to us from an epoch when the Universe was already about 450,000 years old. Since the seeds of galaxy clusters in today’s Universe were already present at that time, it follows that they must have been imprinted on the Universe at an even earlier time. But by what? Turok and Steinhardt’s colliding-universe scenario appears to provide an answer. Everything hinges on the restless churning of the ‘vacuum’.
Usually, we think of the vacuum of space as completely empty. This is not, however, the way modern physics sees it. As already pointed out, quantum theory permits energy to pop into existence at any time out of absolutely nowhere. The proviso is that, within a split-second, it must pop back out of existence. Think again of that teenager who borrows his dad’s car for the night but gets it back in the garage before his dad gets up the next morning and notices its absence. Well, energy is a bit like the borrowed car. In the microscopic world the law of conservation of energy is not such a cast-iron rule. It fails to notice energy popping into existence out of nothing as long as it pops back out of existence quickly enough.
Now, energy, according to Einstein’s general theory of relativity, warps the space-time around it. Usually, of course, we think of gravity as a property of mass but, since mass is a form of energy, this is in perfect accord with general relativity. The upshot is that the energy which is permitted by quantum theory to pop briefly into existence actually warps the space-time around it. If we could examine empty ‘space’ extremely closely with some kind of ‘super-microscope’, we would not see it as smooth and unruffled. Instead, the ‘quantum vacuum’ would be in a state of ceaseless convulsion, seething and churning like a saucepan of boiling water.
This has profound implications for two branes in the last moments before their titanic Big-Bang-triggering collision. The reason is that the branes themselves have enormously powerful gravity. And, when the two slices of bread come so close there is hardly any gap between them, the gravity of one brane plucks the surface of the other. This has the effect of making the churned-up surface of each brane even more churned up.
Look at it another way. Imagine you can view the surface of our brane with that super-microscope. Furthermore, imagine you can freeze the image for an instant. What you will see is a terrain reminiscent of the Himalayas. Clearly, the gravity of the approaching brane has the greatest effect on the tallest peaks since they are closer to its influence. It is these peaks that, like soft toffee, are pulled even higher at the expense of their less lofty counterparts.
This mechanism, according to Turok and Steinhardt, greatly magnified the lumps and bumps on our brane. They were the ultimate ‘seeds’ of clusters of galaxies in today’s Universe. Once the branes collided and matter was conjured into being to fill our brane, that matter would have naturally gravitated towards them. After all, the more warped a region’s space-time the stronger its gravity. The two things are one and the same. If Turok and Steinhardt are right, then the biggest structures in existence in today’s Universe were spawned by regions of space-time smaller than an atom.
But explaining how the matter of our Universe went from being smooth to being lumpy is only one difficulty that must be addressed by a theory of the origin of the Universe. Another is how today’s Universe is so smooth.
But – wait a minute – we’ve agreed that it isn’t smooth, it’s lumpy. Well, that’s perfectly true if you look at individual galaxies and clusters of galaxies. However, if you look across great swathes of space, averaging out all the irregularities, it turns out that the density of matter is remarkably similar from place to place. What is more, the cosmic background radiation is extraordinarily uniform as well – apart, of course, from those very tiny irregularities found by COBE.
As already pointed out, all this poses a problem for the standard picture of the Big Bang. As the Universe expanded and cooled, inevitably the temperature of some bits fell a bit faster than others, the density of some bits dropped a bit faster than others. Since heat flows from hot regions to cold regions, and matter flows from a dense region of gas to a less dense region, these temperature variations and density variations might be expected to equalise out. However, there is a problem.
As noted before, the cosmic background radiation comes from an epoch 450,000 years after the moment of creation. Light at that time could have traversed a maximum of 450,000 light years. But, if you imagine the expansion of the Universe running backwards, like a movie in reverse, you discover that 450,000 years after the moment of creation the entire observable Universe was something like 18 million light years across.
According to Einstein, nothing can travel faster than light. It is the ultimate speed limit. So, as the Universe expanded and cooled, heat could not have spread fast enough from hot regions to cooler regions; matter could not have spread fast enough from dense regions to less dense regions. No influence could have travelled more than 450,000 light years, a tiny fraction of the 18 million light years from one side of the Universe to the other. There was simply insufficient time for the temperature and density of the Universe to equalise.
The unavoidable conclusion is that today’s Universe cannot possibly have the same matter density everywhere; neither can the temperature of the cosmic background radiation be the same everywhere. The trouble is they are. The standard Big Bang theory is therefore in serious conflict with our most basic observations of the Universe. To fix it, something else is needed. A missing ingredient.
As pointed out before, the missing ingredient, according to consensus opinion, is ‘inflation’. According to the theory, the Universe, in its first split-second, underwent an ultra-brief phase of super-fast expansion.
No one knows the fine details of inflation, only that it could have been driven by a bizarre state of the quantum vacuum – one in which gravity blew rather than sucked. The key thing is that, if the Universe did indeed undergo a phase of super-fast expansion, the entire observable Universe could have come from a far smaller volume of space than we would naively expect simply from running the movie of the expansion backwards. And, if it came from a far smaller volume, then heat and matter would have had plenty of time to equalise the temperature and density. The theory once again accords with our observations of the Universe. Inflation has the added advantage that it ‘inflates’, or magnifies, the roiling quantum vacuum – including those seeds of cosmic structure – providing a convenient explanation for the origin of galaxies in the Universe.
A new and elegant way of keeping the Universe uniform, however, is suggested by Turok and Steinhardt’s colliding universe scenario. On the microscopic scale, the approaching branes are like a choppy sea. However, on a larger scale these contortions average out, so that the branes are to all intents and purposes utterly flat. What this means, is that as the branes first touch, they touch everywhere at once. As the energy of motion of the branes is converted into particles and heat, it is everywhere in the Universe converted into exactly the same density of particles at exactly the same temperature. Hey presto, the uniform density and temperature of today’s Universe is explained.
So far, so good, then. The brane-collision scenario can explain what the Big Bang was. It can explain where the galaxies in today’s Universe came from. And it can explain why matter is spread so smoothly throughout space and why the afterglow of the Big Bang is exactly the same everywhere in the Universe. But that still leaves the mystery of why, 13.7 billion years ago, two branes sailing through the ten-dimensional void just happened to slam into each other.
To have generated such a violent collision, some force must have snapped the branes together. Since the only thing between the branes was the vacuum, the only possible culprit is the vacuum. Like a Star Trek tractor beam, it must have exerted an irresistible force of attraction on the branes.
If the idea of the vacuum exerting a force sounds preposterous, think again. Not only can the vacuum do such a thing but exactly this kind of vacuum-generated force turns out to be the dominant influence controlling the evolution of our Universe today!
Evidence for such a vacuum force first came to light in 1998. Two teams of researchers were observing ‘supernovae’ in distant galaxies. One team was led by the American Saul Perlmutter and the other by the Australians Nick Suntzeff and Brian Schmidt. Supernovae are exploding stars which often outshine their parent galaxy and so can be seen at great distances out in the Universe. The kind the two teams were looking at were known as ‘Type Ia supernovae’. They have the property that, when they detonate, they always shine with pretty much the same peak luminosity. So, if you see one that is fainter than another, you know it is farther away.
What the astronomers saw, however, was that the ones that were farther away were fainter than they ought to be, taking into account their apparent distance from the Earth. The only way to explain what they were seeing was that the Universe’s expansion had speeded up since the stars exploded, pushing them farther away than expected and making them appear fainter.
But, as noted earlier, this was contrary to all expectations. In the aftermath of the Big Bang, the galaxies – the building blocks of the Universe – are flying apart from each other like pieces of cosmic shrapnel. The only force acting on them should be their mutual gravity and this ought to be pulling them together and ‘braking’ the expansion of the Universe.
The discovery that the expansion of the Universe is in fact speeding up implied that there is a hitherto unsuspected force operating in the Universe. It is overwhelming gravity and driving the galaxies apart. Its origin can only be in the space between galaxies. Far from being empty, the vacuum must be filled with some kind of weird anti-gravity stuff. It is this ‘dark energy’, mentioned earlier, that is remorselessly driving the galaxies apart.
The first thing to say about dark energy is that nobody understands it. In fact, it is a huge embarrassment. Quantum theory – the very best theory physicists possess – predicts that, if a chunk of vacuum has any energy at all, it should have 1 followed by 123 zeroes more than is observed! This has been described by the Nobel Prize-winner Steven Weinberg of the University of Texas as: the ‘worst failure of an order-of-magnitude estimate in the history of science’.
Though quantum theory cannot explain the amount of energy observed to be in the vacuum, Einstein’s theory of gravity nevertheless provides a means by which the energy of the vacuum can exert a force. It is all down to the ‘source’ of gravity. Newton thought it was mass; Einstein realised it was any form of energy, which includes mass-energy. This, however, is not the complete story. As pointed out before, a close inspection of Einstein’s ‘field equations’ of gravity reveals that the source of gravity is in fact energy – strictly speaking, energy density, the energy in a unit volume of space – plus three times the pressure. It’s this second mathematical term that makes all the difference.
In all normal circumstances, the energy density of matter far exceeds any pressure it exerts on any container in which it is confined, making the pressure term of no consequence whatsoever. The possibility nevertheless exists that our Universe contains a material totally unlike anything we know of – a material with a pressure comparable to its energy density. If the pressure is positive – that is, it pushes outwards like the helium gas in a Mickey Mouse balloon – then its effect is to increase the material’s gravity. If the pressure is negative – that is, it pulls things inwards like the tension in a piece of stretched elastic – then, incredibly, the material’s gravity can in fact reverse.* As already mentioned, gravity can blow instead of suck.
Here then is the recipe for the invisible dark energy which fills our Universe. Space is evidently filled with invisible stuff the like of which we have never before imagined. Everywhere it is trying desperately to shrink. Yet, paradoxically, it is causing the Universe to expand ever faster.
The discovery of the key importance of the vacuum in our Universe has come as a great shock to the scientific community. Nobody – least of all, the physicists who are at a loss to find a plausible explanation – wanted it. To Turok and Steinhardt, however, the vacuum is not some unwelcome entity that throws a spanner in the cosmic works. Far from it. In the colliding-universe scenario, it is an absolutely essential ingredient. It is the vacuum that creates the force of attraction which pulls the branes together in the fifth dimension. It is the vacuum that smacks them together like clashing cymbals to create the Big Bang.
But surely the vacuum in today’s Universe is pushing everything apart, not pulling things together? True enough. Recall, however, that the vacuum has the ability both to blow and to suck – it all depends on how its pressure is related to its energy density. Well, remarkably, it is possible for the vacuum to suck in the fifth dimension while simultaneously blowing within the four-dimensional island of our brane-universe. In one stroke, the vacuum can explain both the force that dragged the branes together and the force which is currently causing the expansion of our Universe to speed up.
But the vacuum can do even more than this. The relationship between its pressure and its energy density need not stay the same for all time. It can change. The vacuum can go from blowing to sucking and vice versa. And this has profound implications for Turok and Steinhardt’s brane-collision scenario. It provides a means by which Big-Bang-generating collisions can happen over and over again.
When the branes are far apart, the vacuum in the fifth dimension sucks, pulling them together. The branes eventually collide and actually pass right through each other. Now, however, when the branes are close together, the vacuum changes from sucking to blowing. It drives the branes away from each other until, when they are far apart again, the vacuum changes from blowing to sucking and the whole process repeats itself.
In effect, the vacuum, which blows continually in our four-dimensional Universe, acts like a spring between the branes in the fifth dimension. When the branes are apart, the spring is stretched and under tension, and pulls the branes together. When the branes are close, the spring is squeezed and compressed, and pushes the branes apart. In this way, the branes come together and collide, move apart, then come together and collide again, over and over again.
If Turok and Steinhardt are right, there was not just one Big Bang but a whole series of Big Bangs, stretching back into the infinite past. This in itself is not a new idea. In its previous incarnation, people called it the ‘oscillating’, or simply ‘bouncing’, universe.
In the bouncing universe, the gravity of all the galaxies tugging on each other eventually slows their fleeing motion to a standstill. The expansion having run out of steam, the Universe embarks on a phase of runaway contractions. In fact, it shrinks all the way down to a ‘Big Crunch’ – a sort of mirror image of the Big Bang in which all matter is crammed into the tiniest of tiny volumes. But this is not the end of the story. Matter is hypothesised to have some residual stiffness, like a hard rubber ball which can be squeezed so far but no farther. At the very last moment, therefore, the Universe rebounds in another Big Bang, which is followed by another Big Crunch and another Big Bang, and so on, ad infinitum.
The idea of such a bouncing universe was once very popular with cosmologists because it provided an answer to the perennially awkward question: what happened before the Big Bang? According to the bouncing-universe scenario, before the Big Bang there was an earlier Big Bang. And, before that, an even earlier one. Far from being a unique event, our Big Bang was merely one Big Bang in a never-ending cycle of Big Bangs and Big Crunches. Despite the undoubted aesthetic appeal of the bouncing universe, however, the theory was scuppered by several serious difficulties.
One arises because, in each Big Bang-Big Crunch cycle, new stars are born. Because these pump heat into space, they make the next Big Bang hotter than its predecessor. And, the hotter the Big Bang, the bigger the Universe grows before its expansion runs out of steam. This may not seem a problem. However, imagine looking backwards in time like some omniscient god. The Big Bang-Big Crunch cycles get progressively cooler and smaller in amplitude until, eventually, they dwindle away to nothing. This is the moment at which they began.
But if the cycles had a beginning – no matter how far back in the ultra-remote past that was – all we have really succeeded in doing is replacing the awkward question – What happened before the Big Bang? – with the equally awkward question – What happened before the first Big Bang? This hardly counts as progress.
An even more serious flaw in the bouncing-universe scenario was found in the 1960s by Stephen Hawking and Roger Penrose at Cambridge University. They proved that, if Einstein’s theory of gravity provides the correct description of the Universe, then the Universe must have begun in a ‘singularity’. This is a region of space where the density, the temperature, and so on, all sky-rocket to infinity. The appearance of a singularity in any theory of physics signals the total breakdown of predictability. It is therefore impossible to determine what happened before the singularity. The very concept of ‘before’has no meaning. In effect, Hawking and Penrose had dropped an opaque curtain across the Big Bang, obscuring for ever the view of earlier times.
If general relativity could not say anything sensible about the period ‘before the Big Bang’, the bouncing universe, with its endless pre-Big Bang cycles, was clearly a non-starter. How, then, at the beginning of the twenty-first century, is it possible for Turok and Steinhardt to revive a cosmological scenario with multiple Big Bangs stretching back into the infinite past? They can do it, it turns out, because their scenario is only superficially similar to the bouncing universe. In fact, it has major differences. And it is these differences that … well … make all the difference.
To emphasise that the brane-collision scenario is not the same as the bouncing universe, Turok and Steinhardt have christened it the ‘cyclic universe’. One profound difference is that it arises not out of Einstein’s theory of gravity but out of string theory, which proponents believe is a more fundamental and accurate description of reality. Since the ‘singularity theorems’ of Hawking and Penrose apply only to Einstein’s theory, Turok and Steinhardt claim that the Universe never goes through a singularity in the cyclic scenario.
From the point of view of general relativity, of course, there is a singularity at the moment the branes touch. This is the Big Bang. However, Turok and Steinhardt maintain that, from the point of view of string theory, the ‘scale factor’, which sets the size of space-time on a brane, remains perfectly finite. In other words, at the very moment Einstein’s theory says the matter on the branes should have a density and temperature which is sky-rocketing to infinity, string theory declares that everything is in fact quite well-behaved.
How can general relativity and string theory look at exactly the same event and one see a singularity and one not? Turok believes the answer is that the singularity here is not actually a real thing. It is merely an artefact of our point of view. ‘Look at the Big Bang the wrong way – from the point of view of general relativity – and you get a singularity,’ he says. ‘Look at it the right way – from the perspective of string theory – and there is no singularity.’
There is a precedent. Another singularity in the theory also turned out to be an artefact of our viewpoint. A year after general relativity’s birth in 1915, the German physicist Karl Schwarzschild was serving in the trenches of the First World War when he discovered within general relativity a description of a non-spinning black hole.* The ‘Schwarzschild solution’ sported a singularity at the ‘event horizon’, the imaginary membrane surrounding a black hole which marks the point of no return for matter spiralling in. This singularity, however, turned out to be simply an artefact of the particular ‘coordinate system’ which Schwarzschild had used. ‘When he changed to another system, the singularity went away,’ says Turok. ‘The event horizon of a black hole is not a place of infinite density. For a big enough black hole, it can even be crossed in safety.’
In the same way, Turok and Steinhardt hope that the singularity in the brane-collision scenario is a mirage which goes away when looked at through the spectacles of string theory. ‘String theorists are currently working to see if they can prove it,’ says Turok. ‘We are hopeful.’
However, the problem of the singularity was not the only problem to beset the bouncing universe. There was the awkward fact that the bounces got bigger and bigger with time, implying that the Big Bang-Big Crunch cycles could not have been going for ever but must have begun at some moment in the remote past. Such a ‘beginning’ to the Universe is aesthetically unsatisfying. It can be avoided in a bounce scenario only if each cycle remains the same size as its predecessor. This, it turns out, is exactly what happens in the cyclic universe.
In the bouncing universe, each Big Bang is bigger than its predecessor because the stars which form in each cycle pump out heat into space, making succeeding bangs hotter. However, in the brane-collision scenario this does not happen for one very good reason – the presence of the vacuum.
The vacuum both sucks and blows along the direction of the fifth dimension, repeatedly pushing the branes apart and pulling them back together. However, this is not what happens within the space of each brane. Here the vacuum does nothing but blow. It is blowing today, as Perlmutter and others discovered in 1998. It is causing the space of our brane-universe to expand ever faster. And it is precisely this accelerated expansion of space that rescues the cyclic universe from the same fate as the bouncing universe.
An unavoidable consequence of this runaway expansion is that all the matter in the Universe – and of course all the heat pumped into space by stars – will eventually be smeared incredibly thinly throughout a tremendous volume of space. In effect, all the stuff in the Universe will be diluted out of existence. The brane will be returned to essentially the state it was in when the brane collision triggered a Big Bang. Consequently, when the next brane collision triggers the next Big Bang, that bang will be precisely as big as its predecessor.
In the cyclic universe, therefore, it really is possible to have a never-ending series of Big Bangs marching backwards into the infinite past and forwards into the infinite future. The awkward question of what happened before the Big Bang finally has an answer. It is Big Bangs all the way back!
Of course, even if the question of what happened before the Big Bang has an answer, there still remains the even more awkward question – Why a Universe with an infinite series of Big Bangs rather than something else? Or, more succinctly, why is there something rather than nothing? This is the grandaddy of all cosmological questions. It may even be beyond the capability of science to answer …*
The bouncing universe not only had Big Bangs but Big Bangs alternating with Big Crunches. And this highlights a major qualitative difference between the bouncing and cyclic scenarios. In the cyclic universe, the oscillations occur in the invisible, and unobservable, fifth dimension, with the branes coming together, colliding, flying apart, then coming together again. There is no oscillation in our four-dimensional Universe. Instead, as the branes collide and re-collide, the space of our brane-universe is repeatedly subjected to bursts of headlong expansion. As each runs out of steam, there is a new bang which starts everything expanding again.
One instant our brane-universe is empty. The next there is a Big Bang and all of space is filled with super-heated matter and light which expands explosively.† The expansion of space eventually cools the matter enough that it can congeal into galaxies and stars and planets. Eventually, after tens, maybe hundreds of billions, of years, the expansion of space dilutes matter and light effectively out of existence. Our brane-universe is empty. The cosmic slate is wiped clean. Suddenly, there is another Big Bang and all of space is filled with super-heated matter and light which expands explosively … In this way, our brane-universe expands for ever, the expansion of space boosted periodically by brane collisions in a hidden fifth dimension.
‘Some say the world will end in fire, Some say in ice,’ wrote the poet Robert Frost. In the cyclic universe, the Universe alternates for ever between phases of ice and fire.
In all this, the vacuum is of crucial importance. Not only is it responsible for repeatedly crashing together the branes like cosmic cymbals in the fifth dimension but it is responsible for the enormous expansion of space necessary to dilute starlight and ensure that Big Bangs do not get bigger with time. By contrast, in the standard picture of cosmology – which consists of the Big Bang plus inflation – the vacuum which is currently speeding up the expansion of the Universe is simply an arbitrary phenomenon which must be bolted onto the model in order that it should accord with reality. What is more, the vacuum has to have two states with vastly different energies – the ultra-low-energy state governing the expansion of today’s Universe and the ultra-high-energy state which drove inflation during the first split-second of the Universe’s existence.
But, isn’t the cyclic universe, with cycles all identical, its endless repetition, mind-numbingly dull? Turok and Steinhardt think not. ‘Just because the cycles repeat does not mean the events in each cycle are identical,’ says Turok. ‘The laws of quantum mechanics, which govern the microscopic world of atoms and their constituents, are inherently unpredictable – they exhibit randomness,’ says Turok. ‘Consequently, the detailed sequence of events in each cycle is different.’
More speculatively, Turok points out that, though our four-dimensional brane-universe would behave the same from one cycle to the next, the extra rolled-up dimensions might vary their sizes. The significance of this is that the fundamental forces of nature are suspected to be merely manifestations of these hidden dimensions. ‘The laws of physics could actually change from cycle to cycle,’ says Turok.
This might at last explain a peculiar, and much-remarked-upon, feature of our Universe. The laws of physics appear to be ‘fine-tuned’ for the existence of stars, galaxies and life. For instance, if the force of gravity were only a few per cent stronger than it is, it would crush and heat up the cores of stars to such an extent that they would burn their hydrogen fuel in less than a billion years. This would be an insufficient time for the evolution of complex life, which on Earth has taken the best part of four billion years. On the other hand, if the force of gravity were only a few per cent weaker than it is, it would be unable to crush and heat up stellar cores sufficiently to even burn their hydrogen fuel. Stars like the Sun would be an impossibility.
This is but one example of the fine-tuning of the laws of physics. If the strength of any of the other fundamental forces, or the masses of fundamental particles, were even slightly different, there would be no stars or planets or life.
There would appear to be only two logical explanations for the fine-tuning we observe in our Universe. Either ‘God’ fine-tuned the Universe for life – though perfectly acceptable, this has the drawback that it prevents any further scientific enquiry. The other possibility is that there is more than one universe – in fact, a huge number of universes – each with different laws of physics. Among this vast ‘multiverse’ of universes are one or more with the laws of physics necessary for the creation of galaxies and stars and life. We have obviously arisen in such a universe, goes the argument. After all, how could we have arisen in any other?
If there is a multiverse, an obvious question raises its head: where are the other universes? Turok and Steinhardt’s scenario suggests an answer: in the other cycles. In other words, the laws of physics are different in every cycle and – surprise, surprise – we have arisen in the only cycle where it was possible for us to arise.
Turok admits to being very excited by all the possibilities of the cyclic universe. ‘The whole thing has just fallen into our laps,’ he says. ‘We didn’t set out to resuscitate the bouncing universe, yet so many things all appear to be slotting into place.’
‘It’s taken my breath away,’ says Steinhardt. ‘I have been both shocked and elated at how we have proceeded from a vague, intuitive notion and constructed a model as compelling and powerful as the cyclic universe.’
So much for the cyclic universe’s advantages – aesthetic and otherwise – does it represent reality? The only way to tell is if there is some measurable property of our Universe for which the cyclic scenario makes a prediction at odds with the standard cosmological model – the Big Bang plus inflation. It turns out there is.
According to Einstein’s theory of gravity, whenever matter is moved violently, it produces gravitational waves – actual ripples in the fabric of space-time which propagate outward from their source in much the same way that concentric ripples spread out from an impacting raindrop on the surface of a pond. Inflation, the super-fast expansion of space, would have involved just about the most violent movement of matter imaginable. So, if it did really happen during the first split-second of the Universe’s existence, it should have generated copious gravitational waves. They should survive in today’s Universe as a chaotic ‘background’ of space-time ripples. Crucially, however, no such gravitational waves are generated in the cyclic universe.
Here then is a critical test. If there is no background of gravitational waves, the cyclic universe is correct. If there is, it is wrong. Simple. Except that gravitational waves are extremely weak and nobody has yet succeeded in detecting them on Earth.
Nevertheless, the hypothetical gravitational waves of inflation should affect the way the temperature of the cosmic background radiation varies from place to place in the sky. It is a very subtle effect but it might just be detectable by the European Space Agency’s ‘Planck’ space probe, due for launch in 2007. If so, we should soon know whether our universe did indeed begin in the ultimate train crash – a collision between island universes.
* Nature’s four fundamental forces are the electromagnetic force, which glues together the atoms in our bodies; the ‘strong’ nuclear force and the ‘weak’ nuclear force, which orchestrate what goes on in the ‘atomic nucleus’, the tight knot of matter at the heart of an atom; and the gravitational force, which governs the behaviour of planets, stars and the entire Universe.
* See Chapter 7, ‘Patterns in the Void’.
* String theory is not the only hope for uniting quantum theory and Einstein’s theory of gravity. There are at least two other promising routes currently being taken by physicists.
* See Chapter 6, ‘God’s Number’, for a discussion of the concrete evidence that this is indeed so. Also, see Chapter 7, ‘Patterns in the Void’, for a claim that our Universe is simple because it actually contains nothing!
* See my book, Afterglow of Creation (University Science Books, Sausalito, California, 1996).
* To be precise, this requires the material’s pressure to be less than -1⁄3 its energy density.
* From the point of view of Einstein’s theory of gravity, a black hole is a region of space so grossly warped that it can be imagined as a bottomless well. Not surprisingly, nothing, not even light, can clamber out.
* Or maybe not. See Chapter 7,‘Patterns in the Void’.
† A central feature of the Big Bang, which is often difficult to comprehend, is that it happened everywhere at once. It was not an explosion located at a single place like the explosion of a stick of dynamite. Rather, all of space was filled with light and matter and began expanding everywhere simultaneously. Astronomers often use the crude analogy of a rising raisin cake. Every raisin recedes from every other raisin. None is at the centre of the expansion. So it is with galaxies in our Universe.