If you wish to make an apple pie from scratch you must first invent the universe.
CARL SAGAN
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
DOUGLAS ADAMS, The Restaurant at the End of the Universe
If you spent your whole life sitting on a chair in the middle of a field, you would find it hard – if not impossible – to create a mental picture of the Earth. Astronomers are similarly handicapped. They spend their whole lives pinned to the surface of a tiny ball of rock in an anonymous cosmic backwater. But, despite their overwhelming handicap, they have had remarkable success in concocting a picture of the cosmos. Not only do they know the content and extent of the Universe but they also have a pretty good idea of how it all came into being in the first place.
Nature has been kind to us. We do not live on a planet such as Venus shrouded in impenetrable clouds. We do not live in a star-choked region of the Galaxy such as the heart of the Milky Way where night is unknown. We have not appeared on the cosmic scene so late in the day that most of the stars have exhausted their fuel and sputtered out. Instead, using our Earthbound telescopes, we can see all the way to the Universe ’s distant horizon.
Previous generations would have killed for the kind of picture we have of our Universe. The Earth, along with a handful of other planets and moons and assorted leftover rubble from the formation of the Solar System, orbits the Sun. The Sun, in turn, orbits the centre of the Milky Way, a great pinwheel of about 100 billion stars turning ponderously in the night. At the Sun’s location, about two-thirds of the way out towards the outer rim, it takes about 220 million years to complete a circuit, which means the last time the Earth was where it is at this very moment the dinosaurs were just beginning their 150-million-year reign.
The Milky Way, however, is but one island of stars, or galaxy, among 100 billion others. To get some idea of the scale of things, imagine the Universe is a sphere a kilometre across. It would be filled with 100 billion galaxies, each roughly the size of an aspirin, with the nearest galaxy, Andromeda, just over 10 centimetres away from us. Andromeda and a handful of the closest galaxies are bound to us by gravity. But all the other galaxies are fleeing from each other like pieces of cosmic shrapnel in the aftermath of an explosion.
The recession of the galaxies was discovered by American astronomer Edwin Hubble in 1929. An unavoidable consequence of this recession is that the Universe must have been smaller in the past. In fact, if the expansion of the Universe is imagined running backwards like a movie in reverse, a time is reached – about 13.8 billion years ago – when everything in creation was crowded into the tiniest of tiny volumes. This was the moment of the Universe’s birth: the big bang.
One of the most profound discoveries in the history of science is undoubtedly that the Universe has not existed for ever, that it was born, that, in the words of the Belgian priest and mathematician George Lemaître, there was ‘a day without a yesterday’.
When the Universe was squeezed into a small volume, it must have been hot, for the same reason that air squeezed in a bicycle pump gets hot. The big bang was a hot big bang. The Universe was therefore born in a hot, dense state – a fireball. It has been expanding and cooling ever since and, out of the cooling debris, have congealed the galaxies we see about us, including the Milky Way.1
Out of the cold and fleeing dust
that is never and always,
the silence and waste to come …
The fireball of the big bang was like the fireball of a nuclear bomb. But whereas the heat of a nuclear fireball dissipates into the surrounding air in an hour, a day, a week, the heat of the big-bang fireball had nowhere to go. It was bottled up in the Universe, which, by definition, is all there is. Consequently, the heat of the big bang is still all around us today.
Although this cosmic background radiation was once blindingly bright, it has been greatly cooled by the expansion of the Universe since the big bang and no longer appears as visible light. Instead, it appears as microwaves, a type of light invisible to the naked eye but that can be picked up by your TV.3 Tune your television between the stations. One per cent of the static, or snow, on the screen is the leftover heat from the big bang. Before it was intercepted by your TV aerial, it had been travelling for 13.8 billion years across space – and the very last thing it touched was the fireball of the big bang.
The cosmic background radiation is the most striking feature of our Universe. A remarkable 99.9 per cent of the particles of light, or photons, in the Universe are tied up in this afterglow of the big bang and a mere 0.1 per cent in the light of stars and galaxies. If we had eyes sensitive to microwaves rather than to visible light, we would see the whole of space glowing white like the inside of a giant light bulb.4
The afterglow of the big-bang fireball together with the expansion of the Universe are two powerful pieces of evidence that the Universe started out in a hot and dense state and has been expanding and cooling ever since.5 One other major piece of evidence is that about 25 per cent of the mass of the Universe is in the form of helium, the second heaviest element. Starlight is a by-product of the fusion of the lightest element, hydrogen, into helium.6 By estimating how much starlight there is in the Universe, astronomers can deduce that stars have converted only 1 or 2 per cent of the Universe ’s initial hydrogen into helium. So the helium had to be forged somewhere else.
The cores, or nuclei, of hydrogen atoms, being charged, repel each other ferociously. They can overcome their mutual aversion and stick together to make helium nuclei only if they slam into each other at high speed, which is synonymous with high temperature, and if they run into each other frequently, which is synonymous with high density. These twin conditions are believed to have been satisfied in the fireball of the big bang between about 1 and 10 minutes after the birth of the Universe. Calculations show that 25 per cent of the Universe ’s hydrogen should have been transformed into helium. And this is exactly the percentage of helium astronomers observe throughout the Universe.
The big-bang picture of the birth of the Universe provokes a host of questions. One of the most common is: where did the big bang happen? Here, the very term ‘big bang’, coined by English astronomer Sir Fred Hoyle on a BBC radio programme in 1949, sows seeds of confusion. After all, a bang, or explosion, happens at a particular location and the shrapnel flies outwards into pre-existing space. But the big bang did not happen at one location. It happened everywhere. And there was no pre-existing space. Space, along with matter, energy – and even time – were all created together in the big bang.
When we look out at the Universe and see all the galaxies fleeing from us, it does not mean that the big bang happened here on Earth. When astronomers say the Universe is expanding, all they mean is that every galaxy is receding from every other galaxy. In other words, if we could magically transport ourselves to another galaxy far across the Universe, we would also see all the galaxies fleeing from us. Everyone is at the centre and no one is at the centre because there is no centre.
An image often used to convey the idea is that of a cake with raisins baking in an oven. As the cake rises, every raisin recedes from every other raisin. No raisin is at the centre of the expansion. Of course, it is necessary to overlook the fact that a real cake has an edge – and imagine an infinite cake. But, then, all visual analogies of the big-bang expansion of the Universe provide at best a partial picture because it is fundamentally unvisualisable. The big bang, after all, happened in four dimensions of space–time – one dimension beyond what we, as lowly three-dimensional beings, can comprehend directly.
Many other questions provoked are by the big-bang picture of the birth of the Universe. What was the big bang? What drove the big bang? And, of course, what happened before the big bang? It is possible to answer all these questions. But only within the context of a major – and it must be stressed, speculative – extension of the basic big-bang model.
The basic big-bang idea, for all its successes, contradicts our observations of the Universe in several serious ways. For one thing, the cosmic background radiation comes to us more or less equally from all directions in the sky. Or, to put it another way, everywhere in the sky has almost exactly the same temperature – 2.725° above absolute zero.7 This a problem because, if we imagine the expansion of the Universe running backwards like a movie in reverse to the time of the origin of the big-bang radiation, we find that regions of the Universe that are today more than 1° apart on the sky – twice the apparent width of the Moon – were not in contact with each other.8 Or, to be precise, there had been insufficient time since the beginning of the Universe for any influence – travelling even at the cosmic speed limit set by light – to pass between them. Consequently, if one bit of the fireball cooled down a bit faster than another, heat could not have travelled to it from its surroundings to equalise the temperature. The cosmic background radiation should therefore have an uneven temperature across the sky. It should not, as is the case, have the same temperature everywhere in the sky.
The bizarre explanation, which has been embraced by many physicists, is that, in the Universe’s first split second of existence, it expanded far faster than at any time since – faster even than the speed of light.9 This period of inflation was so incredibly, mind-bogglingly fast that the Universe doubled in size, and doubled again, more than 60 times over. Inflation has been likened to the explosion of an H-bomb compared to the puny stick of dynamite of the big-bang expansion that followed in its wake.
Inflation neatly explains why the temperature of the cosmic background radiation is the same everywhere we look in the sky. After all, if the Universe expanded far faster than we thought, it could have been smaller than we thought early on and yet still have reached its current size in 13.8 billion years. And, if it was smaller than we thought, then all bits could have been close enough to have exchanged heat, keeping the temperature of the Universe the same as it expanded.
Inflation, an idea from particle physics, was proposed in 1979 by the Russian physicist Alexei Starobinsky and independently in 1981 by the American physicist Alan Guth. Although the detailed physical mechanism underpinning the theory remains, frustratingly, obscure, inflation provides a majestic picture of the birth of our Universe and, most importantly, an explanation of what the big bang was.
This is the bizarre story now accepted by the majority of cosmologists. In the beginning was the inflationary, or false, vacuum. This was a weird, high-energy version of the true vacuum around us today.10 For a start, it had repulsive gravity.11 This caused the vacuum to expand, creating more vacuum, with more repulsive gravity, which caused the vacuum to expand even faster. Imagine you are holding a stack of banknotes between your hands and you pull your hands apart and more and more banknotes pop into existence. This is the way it was for the inflationary vacuum. Not surprisingly, physicists have dubbed inflation the ‘ultimate free lunch’.
But the inflationary vacuum was intrinsically unstable.12 Here and there, and totally at random, small patches disintegrated, or decayed, into normal, lower-energy vacuum. And, when this happened, the tremendous energy of the inflationary vacuum had to go somewhere. It went into creating matter and simultaneously heating it to a tremendously high temperature. It made hot big bangs.
Imagine a never-ending sea in which bubbles are appearing at random times and at random locations. Inside each bubble is a big-bang universe. One of those big-bang universes was our Universe.
Now it is possible to answer some of those nagging questions about the big bang. The big bang was not a one-off. It was merely a local event in an ever-expanding ocean of inflationary vacuum. It was driven by the energy of that decaying vacuum. And the big bang was not the beginning. Other big bangs have been going off like stuttering firecrackers across the length and breadth of the inflationary vacuum ever since the inflationary vacuum began, well, inflating.
As fast as bubble universes are created, they are driven apart. In fact, new vacuum is created far faster than it is eaten away, so inflation, once started, is unstoppable. It is eternal. But, even though inflation will continue into the infinite future, surprisingly this does not mean that inflation started in the infinite past. It must have had a beginning. The question of ‘What happened before?’ is therefore simply pushed back from the big bang to an earlier time. Quantum theory might come to the rescue, however, since quantum theory allows stuff literally to pop out of nothing. All that would have been necessary was for a tiny patch of false vacuum to pop into existence and begin inflating. Since a prerequisite of this happening is the existence of quantum theory, the question now becomes: ‘Where did the laws of physics come from?’
The picture of the big bang as one among perhaps an infinite number of other bangs going off in an ever-expanding sea of vacuum is a remarkable one. But it is far from being bedded in firm theoretical ground. Inflation is an add-on, bolted onto the basic big-bang picture. It is not part of a single seamless theory of the Universe. And, worse, it is not the only thing bolted on.
The basic big-bang model not only predicts that the temperature of the cosmic background radiation should vary over the sky when it does not – something fixed by inflation – it predicts two other things that conflict with observations. For instance, it predicts that the expansion of the Universe should be slowing down. The galaxies, after all, are pulling on each other with their mutual gravity. It is as if they are connected by a vast web of elastic, dragging on them and hindering their headlong flight from each other. However, in 1998, physicists discovered that the expansion of the Universe, contrary to all expectations, is not slowing down. It is speeding up.
On the largest scales, another force must be operating in the Universe, overwhelming the force of gravity and driving apart the galaxies. This mysterious force appears to have switched on about 10 billion years ago and has been calling the cosmic shots ever since. The gaze of physicists has settled on the vacuum between the galaxies. They claim it is filled with dark energy. It is invisible. It fills all of space. And it has repulsive gravity. It is this repulsive gravity that is speeding up the expansion of the Universe.
Dark energy accounts for a 68.3 per cent of the mass energy of the Universe. Imagine how embarrassing it is to have overlooked the single biggest mass component of the Universe until 1998.
Dark energy could be an intrinsic energy of space predicted by Einstein’s theory of gravity or it could have some other origin. Nobody knows. Physicists are pretty much at sea in explaining it. When quantum theory is used to predict the energy density of the cosmic vacuum – the dark energy – it comes up with an energy density of 1 followed by 120 zeros bigger than what is observed.13 This is the biggest discrepancy between a prediction and an observation in the history of science. It does not take a genius to realise that some big idea is missing.
The dark energy, with its repulsive gravity, is reminiscent of the inflationary vacuum that speeded up the expansion of the Universe in its first split second of existence. The difference is that it was hugely more puny and nowhere near as short-lived. Nobody knows whether there is a connection between the dark energy and the inflationary vacuum.
Dark energy and inflation, however, are not the only two things that must be bolted on to the basic big-bang model to make it agree with what we observe. There is a third thing predicted by the big-bang model that is at odds with reality. In fact, it is quite a serious thing. The big bang predicts that we should not exist.
Recall that the galaxies, such as our own Milky Way, congealed out of the cooling debris of the big-bang fireball.14 This was possible because the fireball was not completely uniform. The temperature of the cosmic background radiation is remarkably even all over the sky but it is not totally even. There are places where the temperature departs by a few parts in 100,000 from the average. The temperature undulations are believed to reflect the fact that some parts of the big-bang fireball were ever so slightly denser than their surroundings.
The small unevenness in the matter of the big-bang fireball is believed to have been caused by microscopic convulsions, or quantum fluctuations, of the inflationary vacuum in the first spilt second of the Universe. These were then magnified by the tremendous expansion of inflation. By about 379,000 years after the birth of the Universe, they had created slight bumps in the distribution of matter in the big-bang fireball. With slightly stronger gravity, these gathered matter about them faster, which boosted their gravity yet more. In a process akin to the rich getting ever richer, they grew remorselessly, eventually becoming the galaxies we see around us today.
It is a detailed and compelling picture. Only there is a problem. The 13.8 billion years since the big bang has not been enough time to assemble galaxies as big as the Milky Way. Not nearly enough. In short, we should not be here.
Undeterred, astronomers fix this problem by postulating that the Universe contains a vast quantity of invisible, or dark, matter, whose extra gravity speeded up the process of galaxy formation so it was completed within 13.8 billion years.15 In fact, the dark matter in the Universe amounts to about 26.8 per cent of the mass of the Universe.16 It outweighs the visible stars by a factor of more than five.
Nobody knows the identity of the dark matter. It could be in the form of fridge-sized black holes formed in the first split second of the Universe ’s existence.17 Or it could be in the form of hitherto undetected subatomic particles. Certainly, theories of particle physics are not short of possible candidates. But the bottom line is that nobody knows.
To summarise, then, the basic big-bang picture must be supplemented by three bolt-ons: inflation, dark energy and dark matter. A figure of 68.3 per cent of the mass of the Universe is mysterious dark energy. Another 26.8 per cent is mysterious dark matter. That leaves a mere 4.9 per cent of the Universe made of ordinary matter – the stuff that you and I and the stars and galaxies are made of.18 And, actually, we have only ever seen about half of that with our telescopes. The rest is ultra-hot hot gas floating around the galaxies that gives out little visible light.
To say that this is an embarrassing situation is an understatement. We have based the great edifice of our cosmological model on a mere 4.9 per cent of the Universe we have seen directly, whereas 95.1 per cent is made of invisible stuff whose identity eludes us. Imagine if Charles Darwin had tried to concoct a theory of biology knowing only of frogs but nothing of fish or birds or elephants.
Actually, it is not quite as bad as this. Dark matter and dark energy, to steal a phrase from Donald Rumsfeld, are ‘known unknowns’. Astronomers, though hazy on the details, are confident that their overall picture is correct. Nevertheless, few would deny that there must be a deeper theory of the Universe out there, which unites inflation, dark matter and dark energy into a seamless whole.
Such a deeper theory might have to acknowledge one rather basic thing. Our Universe is not all there is. There might be other universes.
The key thing to remember is that the Universe was born 13.8 billion years ago. This means that we can see only those galaxies whose light has taken less than 13.8 billion years to reach us. Those whose light would take more than 13.8 billion years, well, their light is still on its way. Consequently, the Universe is bounded by a horizon – the light horizon. Think of it as the surface of a bubble. The bubble, centred on the Earth, contains about 100 billion galaxies, and is commonly known as the observable Universe.
But, just as we know there is more of the ocean over the horizon at sea, we know there is more of the Universe over the cosmic horizon. In fact, according to the theory of inflation, there is effectively an infinite amount. In other words, beyond the soap bubble of our observable Universe, are an infinite number of other soap bubbles. What is it like in them? Well, each had its own big bang – or, to look at it another way, a portion of our big bang. And, out of the cooling debris, congealed galaxies and stars – different galaxies and stars.19 In other words, each bubble had a different history. ‘Many and strange are the universes that drift like bubbles in the foam of the river of time,’ said English science-fiction writer Arthur C. Clarke.20
There is a twist. Because the Universe is quantum, or grainy, there are only a finite number of possible histories for each bubble. Here is the reasoning …
According to quantum theory, the world at a microsocopic level is grainy, like a newspaper photograph. Ultimately, everything comes in indivisible chunks, or quanta. Energy comes in chunks. Matter does. Time does. And so does space. So, if we were able to look at space closely with some kind of super-microscope, we would see space resolve itself into indivisible grains. Think of it as a chessboard with squares of space. Now, if we run the expansion of the Universe all the way back to the beginning of inflation, we find that there were a mere 1,000 squares of space. The number is not relevant – although it is amazingly small. It is the fact that there is only a finite number of squares.
The seeds of galaxies turn out to be stuff on those squares. If a square contains energy that energy is the seed for a galaxy. But, just as there are only a finite number of ways to arrange the chess pieces on a chessboard, there is only a finite number of ways to fill the squares, some with energy, some empty of energy. In other words, the inflationary chessboard can create only a finite number of possible arrangements of galaxies, only a finite number of possible cosmic histories.
So we have a finite number of possible histories and an infinite number of locations for them to be played out. Consequently, every history occurs an infinite number of times. So there is an infinite number of copies of you whose lives until this moment have been exactly like yours. In fact, it is possible to calculate how far you would have to go to meet your nearest doppelgänger. The answer is roughly 10^10^28 metres.
In scientific notation, the number 10^28 is 1 followed by 28 zeros, which is 10 billion billion billion. Consequently, 10^10^28 is 1 followed by 10 billion billion billion zeros. It is a tremendously big number. It corresponds to a distance enormously further than furthest limits probed by the world’s biggest, most powerful telescopes. But do not get hung up on the size of this number. The point is not that your nearest double is at a mind-bogglingly great distance from the Earth. The point is that you have a double at all.
Don’t believe this? Unfortunately, it is an unavoidable consequence of two things: a fundamental theory of the Universe and our fundamental theory of physics – quantum theory. If it is wrong, one or both of these must be wrong. This would not be an unusual state of affairs. ‘Cosmologists are often wrong,’ said the great Russian physicist Lev Landau, ‘but they are never in doubt.’
1 Philip Anderson, ‘More Is Different’, Science, vol. 177 no. 4047 (4 August 1972).
2 ‘Laws of Physics for Cats’, http://www.funny2.com/catlaws.htm.
3 Actually, Newton’s law of gravity turns out to be true only when gravity is relatively weak, which is in most normal circumstances. The theory that describes the behaviour of gravity, both weak and strong, is Einstein’s general theory of relativity. See Chapter 17, ‘The sound of gravity: General relativity’.
4 One of the central characteristics of a scientific theory, as encapsulated in a scientific law, is that you get more out than you put in. Pseudo scientific explanations all fall at this hurdle. To get out a lot, it is generally necessary to put in a lot. In fact, in the case of Creationism, in order to explain the Universe, it is necessary to postulate something even more complicated than the Universe – namely, God. This amounts to putting in more than you get out, the very opposite of science.
5 One controversial explanation for why mathematics is such a perfect metaphor for physics is that mathematics is physics. The Swedish-American physicist Max Tegmark has taken the increasingly popular idea that we live in one universe within a vast ensemble of other universes, or a ‘multiverse’, and run with it. He says every discrete piece of mathematics is implemented in a universe. In other words there is a universe with only flat-paper geometry, another with Boolean logic, and so on. But most of these universes are dead. Only in universes with mathematics/physics complicated enough to generate intelligence will intelligence arise. We live in such a universe, says Tegmark. After all, how could we not?! (Max Tegmark, ‘Is the “Theory of Everything” Merely the Ultimate Ensemble Theory?’, Annals of Physics, vol. 270, issue 1 (20 November 1998), pp. 1–51.)
6 Neil deGrasse Tyson, Death by Black Hole: And Other Cosmic Quandaries.
7 Alan Sokal, ‘A Physicist Experiments with Cultural Studies’, Lingua Franca, May/June 1996; http://tinyurl.com/mv0w.
8 Fundamental physics is the search for laws that are not dependent on our viewpoint – for instance, on how fast we are moving or how strong is the gravity we are experiencing – laws, that is, that we can all agree on. In relativity, such observer-independent laws are called ‘covariant’.
9 Actually, in 2012, I saw a four-armed starfish near Broome in Western Australia.