Edwin Hubble’s discovery that the Universe we live in is expanding in the aftermath of a gigantic explosion should have surprised no one. Not only had several scientists predicted it more than a decade earlier, but their predictions had also been published in the scientific literature for everyone to see. No one had taken a blind bit of notice – least of all Hubble.
The man who had made it possible to think seriously about what kind of Universe we live in was Albert Einstein. In 1915, he had published his theory of gravity, which described the way in which every chunk of matter pulls on every other chunk.1 Never one to shy away from the really big problems in science, two years later Einstein applied his theory of gravity to the biggest collection of matter he could think of – the entire Universe. In doing so, he created cosmology, the science which concerns itself with the nature of the Universe we live in – where it has come from and where it is going.
According to Einstein’s theory, matter does not influence other matter directly but only through the intermediary of space. This is the crucial difference between Einstein’s view of the Universe and the view of his famous predecessor, Isaac Newton. To Newton space was simply the backdrop against which the cosmic drama was played, but in Einstein’s theory it has a far more active role.
The essential idea is that space is malleable – it can be warped or curved by the presence of matter. Warped space is a hard thing to imagine, but though we cannot visualise it, we can gain some insight into its most important properties by thinking of it as a pliable rubber sheet. If a heavy ball bearing is placed on such a sheet, it creates a depression or valley around it.
In the same way, a massive body like the Earth creates a valley in the space around it.
Now imagine placing a second ball bearing on the rubber sheet. Since the first one rests at the bottom of the valley it has created in the rubber sheet, the second ball bearing will naturally roll down towards it.
In the same way, small bodies in space fall into the ‘warped space’ around the Earth.
We say that the Earth attracts other bodies with its gravitational force. But in reality the Earth warps space, and it is this warped space that affects other bodies. This is what gravity is: warped or curved space.
The whole idea can be neatly summarised in one sentence: ‘Matter tells space how to warp, and warped space tells matter how to move.’ It’s all rather chicken-and-egg-like, but many observations since Einstein proposed his theory of gravity in 1915 have confirmed this is indeed the way things work.
In 1917, when Einstein applied his theory of gravity to the Universe as a whole, he should by rights have discovered that it was expanding there and then. It was crying out at him from his equations. But the greatest physicist of the twentieth century did not see it. Or rather he did see it, but ignored it.
What obscured the truth for Einstein was simple prejudice. He had already decided how the Universe should be, so he was primed to ignore all competing possibilities.
Einstein had a deep-seated belief that the Universe we lived in was ‘static’: that all the galaxies were essentially suspended motionless in space. It was possible for individual galaxies to wander about a little within the Universe, but not so that it changed the overall density. That had to stay the same for ever.
A static universe appealed to Einstein because it made things simple. A static universe could never surprise you. It would remain exactly the same throughout time. There was no need to worry about answering sticky questions, such as where had the Universe come from or where was it going. There was no beginning. There was no end. The reason the Universe was the way it was was because that was the way it had always been.
But when Einstein applied his theory of gravity to the Universe he found that the galaxies seemed to have a restless need to be on the move. The reason is clear: every galaxy is pulling on every other galaxy with the force of gravity, so the net effect should be to pull all the galaxies together.
This was all a worry to Einstein, but his belief that the Universe must be static was so great that he was not going to let go easily.
It was very difficult indeed to make the Universe static. To salvage the idea, Einstein had to resort to mutilating his elegant equations. He inserted a mysterious force of cosmic repulsion. The force could be felt only over enormous distances, which is why we had not noticed it before. It counteracted the gravitational force which was remorselessly pulling all the galaxies together.
There was no evidence that such a peculiar force existed, but if it did, Einstein reasoned, it would stop all of creation from collapsing in on itself. The static Universe would be rescued from a premature grave.
If this sounds contrived, that’s because it was. In fact, there were much more natural solutions of Einstein’s equations, though ironically it was left to others to see the truth in them.
One of the first people to accept Einstein’s theory of gravity was a friend of his, the Dutch astronomer Willem de Sitter. In 1917, he, too, had applied the theory to the entire Universe. But, unlike Einstein, he did not insist that the density of the Universe remain constant for all time. Instead, he looked at the equations with a slightly more open mind.
De Sitter discovered an entirely different design of the Universe, which also obeyed Einstein’s equations. In one way it was greatly at odds with the Universe we live in because it was completely devoid of matter. But it had another property that was remarkably like the Universe we live in: its space was expanding.2
If two particles were placed somewhere in this empty universe, they would move steadily apart as the space between them expanded. If a large number of particles were scattered throughout such a universe, the general expansion of space would steadily increase the distance from one of them to any other. In fact, every particle would recede from every other particle at a speed proportional to the distance between them. In de Sitter’s universe, Hubble’s expansion law naturally applied.
The red shifts in the light of distant galaxies have a rather simple explanation in such an expanding universe. Rather than being Doppler shifts, they arise because in the time that light from a distant galaxy has been travelling across space to us the Universe has grown in size, stretching the wavelength of light along with it. Imagine drawing a wiggly wave on the surface of a balloon and then inflating it. This illustrates how light is stretched in wavelength, or red-shifted.
Apart from having a rather interesting expansion law, de Sitter’s universe did not have much going for it. After all, it was empty of matter. But, in 1922, this was rectified by the Russian astronomer Aleksandr Friedmann at the University of Petrograd. He discovered a whole class of universes which obeyed Einstein’s equations and which, like the real Universe, contained particles of matter.
Friedmann found that his universes would almost certainly not be motionless; they would change their appearance with time, either by expanding or contracting. In the expanding universes, the particles of matter naturally obeyed Hubble’s law.
Astronomers call universes which change with time ‘evolving’ to distinguish them from static universes, which stay the same. The evolving universes of Friedmann were discovered independently five years later by Georges Lemaître, a Belgian Catholic priest turned astronomer.
A characteristic feature of the universes of Friedmann and Lemaître was that they began with a violent expansion from a small and highly compressed state – a Big Bang. Particles of matter were born on the move and have been flying apart ever since.
Lemaître went on to speculate about what had actually caused the explosion at the beginning of the Universe. He knew about the phenomenon of radioactivity, in which an unstable atomic nucleus disintegrates, releasing a lot of energy. It was, therefore, natural for him to suppose that the Universe had begun when a giant ‘primeval atom’ exploded, sending all of creation flying apart. There was little evidence for this, but then again no one had a better idea.
When Hubble discovered that the Universe was expanding, it was a vindication of what Friedmann and Lemaître had been saying for years. Our Universe is evolving. It began in a Big Bang and has been expanding ever since. Questions like what was the Big Bang and what happened before might be difficult but they would simply have to be faced. The plus was that a universe that was forever changing was bound to be richer in possibilities than a static cosmos, frozen into eternal immobility.
When Einstein learnt of Hubble’s discovery, he realised his error in inventing his cosmological repulsion. Immediately, he renounced it, calling it ‘the biggest blunder of my life’.
Actually, Einstein’s static universe could never have worked, and this was shown by the British astronomer, Arthur Eddington, in 1930. A static universe was inherently unstable, balancing precariously on the knife edge between expansion and contraction. The slightest of nudges would have sent it careering either way.
In Einstein’s defence, it should be said that in 1917, when he applied his theory of gravity to the Universe, nobody even knew that the major constituents of the Universe were galaxies. He can be forgiven this uncharacteristic lapse.
Naively, we thought of the Big Bang as a titanic explosion centred on the Earth in which the galaxies were blasted apart like cosmic shrapnel. But the equations of Friedmann and Lemaître describe something quite different. If the Big Bang was an explosion, it was an explosion unlike any other.
For one thing, when a bomb goes off, shrapnel is blown outwards into a void that already exists – the surrounding air. But no such void existed before the Big Bang. There was literally nothing. The Big Bang created everything, and that included empty space, matter, energy and even time. As soon as it was created, the Universe began expanding.
If you are having trouble visualising this, do not worry. The Big Bang was unique. A one-off event. There is nothing in our everyday experience to compare it to. Words are inadequate.
Another major difference between a familiar explosion and the Big Bang is that the Big Bang happened everywhere at once. It would have been impossible to point to a place and say that was the centre of the explosion, in the way that you can point to the place where a bomb went off. About 13.7 billion years ago, every particle of matter was simply set in motion, rushing away from every other particle of matter.3
An explosion which occurs everywhere in space has an important consequence. It gives every observer in the Universe the illusion that they are at the centre. So, although we see every other galaxy rushing away from us, it does not mean that we are in a privileged position at the centre of the Universe.
The best way to see why this is so is to imagine the Universe as a rising cake, with raisins representing the galaxies. There are flaws in this picture – for instance, a cake has an edge, whereas the Universe goes on for ever – but, by and large, the picture works.
As the cake rises, the cake mixture expands in all directions, driving the raisins further and further apart. Now, if you were to look at the view from any raisin – it doesn’t matter which one – you would always see every other raisin moving away. In the same way, it would not matter if we lived in the Andromeda galaxy or a galaxy at the limit probed by our most powerful telescopes; the galaxies would always appear to be rushing away from us just as they do from the Milky Way. In our expanding Universe everyone sees the same view, and everyone thinks they are at the centre of creation.
Astronomers have a name for this feature of the Universe, namely that no place is more special than any other. They call it the Cosmological Principle. It is a natural extension of a principle formulated by the great Polish astronomer Nicolaus Copernicus in the sixteenth century. He lived at a time when ancient Greek ideas of an Earth-centred cosmos still flourished. But his observations showed that the Earth revolved around the Sun, not the other way around. The Copernican Principle can be simply stated: our place in the Universe is in no way special. The Cosmological Principle is a natural extension of this idea from the sixteenth-century Universe consisting of the Sun and planets to the twenty-first-century cosmos crowded with galaxies.
It turns out that Hubble’s law is a natural consequence of living in an expanding universe where the Cosmological Principle applies. The speed of a receding galaxy has no option but to be proportional to its distance.
To see why, think of three galaxies A, B and C which happen to lie in a straight line. Let’s say the distance between A and B is the same as the distance between B and C.
Now, imagine that B is receding from A at 100 kilometres per second. This means that C must be receding from B at 100 kilometres per second as well, because we know the Universe looks the same from every point. This is the Cosmological Principle.
How fast is C receding from A? Well, it must be 100 kilometres per second plus 100 kilometres a second – 200 kilometres a second. So C, which is twice as far away from A as B, is receding at twice the speed.
If we extended this reasoning to all galaxies in the Universe, we would find that a galaxy three times as far away as another will be moving three times as fast, and so on. This is precisely the expansion law which Hubble discovered in 1929. It turns out, then, that if the Universe is expanding and also looks the same from every point, this expansion law has to be true.
Although Einstein wanted the Universe to be static and infinite, the evidence that this is not so has always been around for people to see. In fact, evidence can be found in the simple observation that the sky is dark at night.
If the Universe stretched for ever in all directions with stars marching on, rank after rank, out to infinity, then in every direction you looked out from Earth you would see a star. Between the bright stars in the sky there would be fainter stars, and between them fainter stars still, on and on for ever, so that there would be no gaps at all between the stars. Since every line of sight from the Earth would sooner or later strike the surface of a star, the entire night sky would appear as bright as the surface of a typical star, a result in spectacular disagreement with what we actually observe.
It was the German astronomer Johannes Kepler, renowned for discovering the laws which govern the motion of the planets around the Sun, who first pointed out this apparent paradox in 1610. Other astronomers, including Edmund Halley, the man the famous comet was named after, also recognised the contradiction between theory and observation, but it was the German astronomer Heinrich Olbers who popularised it in the early nineteenth century. Today, it is generally known as Olbers’ paradox.
Another way to see the argument is to think of the Universe as made of concentric shells of space rather like the layers of an onion. Sheer distance will make the stars in a remote shell appear much fainter than the stars in a shell close to the Earth. But although these distant stars may be individually fainter, there will be more of them, because the distant shell will be larger. In fact, it turns out that no matter how far away a shell is, the number of stars will always compensate for their faintness, so that each successive shell will contribute the same amount of light. Since there are an infinite number of such shells in a never-ending universe, the brightness of the sky should therefore be infinite!
Actually, this is not quite right. The stars may seem no more than pinpricks, but in fact they are tiny discs – although no telescope is powerful enough to discern them. Because of this, nearby stars will block out the light from more distant ones behind them. When this effect is taken into account, a slightly less ridiculous answer is obtained: the night sky should not be infinitely bright but as bright as the average star.
Most of the stars in the Universe – about 70 per cent – are of a type known as red dwarfs, quite a bit cooler than our own Sun. So the night sky should appear completely red, as if we lived on the surface of a red dwarf! In fact, the night sky is about a thousand million million million times fainter at visible wavelengths than the surface brightness of such a star.
The fact that the sky is dark at night, an apparently trivial observation, is therefore telling us that the Universe cannot be static and filled with stars marching on and on for ever.
In a universe like ours which has undergone a Big Bang two obvious things stop the night sky from being bright. The first is the expansion of the Universe. Because the Universe is expanding, the light coming from ever more distant galaxies is progressively more red-shifted. Since red light carries less energy than blue light, the effect of this is to reduce the energy of light from distant galaxies. As a result, galaxies at great distances contribute less to the brightness of the night sky than they would if the Universe were static.
But there is another, much more important, effect in a Big Bang universe which helps to keep the night sky dark: the fact that the Universe had a beginning and so has not existed for ever. This means that not every line of sight ends in a star as Kepler, Olbers and the rest assumed.
To understand why this is so, you have to realise that we only see a distant star or galaxy if there has been enough time since the Big Bang for the light to have reached us. If there has not, we simply do not see it.
It all comes down to the speed of light, which though exceedingly fast by everyday standards is not infinite. Light travels at a speed of about 300,000 kilometres a second, or about a billion kilometres an hour. Click your fingers. In the time it took to do that, a ray of light could have made the round trip between Europe and America about 30 times.
But although light is swift, the Universe is a very big place. Light takes about eight minutes to reach the Earth from the Sun, more than four years to come from Alpha Centauri, the nearest star, but billions of years to reach us from the most distant galaxies. If the Sun were to wink out at this moment, we would not know about it for eight minutes. Almost certainly, the most distant galaxies have changed in the time their light has taken to reach us (they may all be long dead for all we know). The finite speed of light means that as we look further and further into space, we see objects as they were further and further in the past.
But the finite speed of light has another consequence in a universe with a beginning. Although stars may march on for a very long way indeed, there is a limit or ‘horizon’ beyond which we cannot see them. There has simply not been enough time since the beginning of the Universe for their light to reach us. An analogous horizon exists around a ship at sea. It is not the end of the Universe as far as the ship’s captain is concerned. It is simply as far as he can see.
This effect of seeing only stars or galaxies within a certain horizon is the most important reason why the sky is dark at night. Today, the distance an arbitrary line of sight must hypothetically extend before intercepting the surface of a star greatly exceeds the distance to the horizon.
So, in a Big Bang universe, the sky is dark at night because of the Universe’s finite age and, to a lesser extent, its expansion. In fact, the neat thing about a Big Bang universe is that these two effects go hand in hand. The expansion was caused by the Universe exploding into being relatively recently in a Big Bang.
There is a footnote to all this. It turns out that although Kepler, Olbers and the rest were right to point out that it was a great mystery why the night sky is dark – the mystery being explained in a Big Bang universe – they were wrong to go on to say the night sky should be as bright as the average star. What they had forgotten was that stars do not live for ever. They run out of fuel and wink out, usually within 10 billion years or so. But as the astrophysicist Ed Harrison pointed out in 1964, it would take the stars in the Universe something like 100,000,000,000,000,000,000,000 years to fill space with enough radiation to make the night sky appear as bright as the surface of the average star. So Olbers’ paradox never really was a paradox. It was a red herring.
Nevertheless, it generated a lot of hard thinking about an infinite static universe – thinking that eventually showed that such a universe could never exist. Einstein had missed the message in his own equations of gravity: the Universe would not have been possible had there not been a beginning to time, followed by an expansion.
But the idea of a universe that always stays the same was not dead yet. So great was its aesthetic appeal that Einstein had resorted to inventing a cosmological repulsive force to keep the Universe unchanging in space and time. Others were prepared to bend the laws of physics in other ways to keep the static universe alive.
In 1948, the British cosmologists Fred Hoyle, Hermann Bondi and Thomas Gold proposed the steady-state theory of the Universe. It was based on something known as the Perfect Cosmological Principle. The Perfect Cosmological Principle went one step further than the standard version. It maintained that the Universe looks the same wherever you are for all time.
Hoyle and his colleagues maintained that the Universe expands at a constant rate, and that matter is created continuously throughout the Universe to fill the voids left behind. This matter, popping into existence out of empty space, is just enough to compensate for the expansion and keep the density of the Universe constant.
Where this matter would come from neither Hoyle, Bondi nor Gold could say. But then nobody could say where the matter in the Big Bang came from either. For the next decade and a half, it was a two-horse race between the Big Bang theory and the steady-state theory.4 But, by the early 1960s, the Big Bang was nosing ahead.
At the University of Cambridge in England, the astronomer Martin Ryle had been carrying out a survey of radio galaxies, objects which generated intense radio waves, radiation physically identical to light waves but with a wavelength a million times longer. He was finding that there were many more radio galaxies far away than in the neighbourhood of the Milky Way. Since the radio waves from the distant galaxies had taken billions of years to reach the Earth, Ryle concluded that these objects were far more common in the remote past than they are today. In other words, the Universe had changed with time, in clear conflict with the steady-state theory.
1. Also known as the general theory of relativity.
2. The irony was that de Sitter had been looking for a static universe which obeyed Einstein’s equations and which was less contrived than Einstein’s.
3. This is the modern estimate for the age of the Universe.
4. Ironically, it was Hoyle who coined the phrase ‘big bang’ to describe the alternative to the steady-state theory during a BBC radio programme in 1949.