If the stars in distant galaxies have the same composition as those in our own galaxy, the light we receive from them should exhibit the characteristic signatures of hydrogen and helium. This is indeed observed. However, there is one crucial difference between galactic and stellar light, and this has profound consequences for our understanding of the universe. Although it is true that the relative separation of the dark lines observed in the galactic light corresponds to hydrogen and helium atoms, the lines do not appear quite in their expected positions. When we examine the light from galaxies, we find that the dark lines have all been shifted slightly along the photograph.

Figure 3.1 shows schematic pictures of the light emitted by a typical star in our galaxy and the light received from an average distant galaxy. Light of a longer wavelength is positioned towards the right of the diagram. If this were a colour photograph, the picture would appear redder on the right-hand side and bluer on the left. Notice how the absorption lines in the galactic light are all positioned at slightly longer wavelengths than in the stellar case.

The key to understanding what causes the dark lines to be moved towards longer wavelengths lies in the fact that light travels through space as a wave. Consider what happens when a wave is emitted by a source and is then picked up by a receiver positioned some distance away.

Consider what happens when you observe a particular object, such as a page of this book. Light, either from the sun or a lamp, is continually being reflected off the page, and some of this light will enter your eyes. This will stimulate the retina, thereby causing a signal to be sent via the optic nerve to your brain. The brain then deciphers this signal and ‘reads’ the words that are printed. The key point is that light has to be reflected if you are to see the words. In effect, it must escape from the surface of the page.

Suppose that we are located on the surface of a star and, using a cannon, fire a tennis ball upwards. The distance travelled by the ball is determined by the speed it has when it is released. If the ball is moving sufficiently fast, it can escape completely from the star's influence. In that case, it need never fall back to the surface.

What would happen if the star began to collapse? Its density would gradually increase, and the force of gravity near its surface would become stronger. This means that the ball would have to be released with a greater speed than before in order to escape. If the collapse were to proceed unhindered, the density and gravitational pull of the star would soon become extremely high.

During the past few decades of research a plausible picture of the universe's most distant past has begun to emerge. The current view is that the universe came into existence some ten billion years ago in the form of a huge, exploding ‘fireball’. This was the big bang.

We are going to discuss some key features of the big bang in this book. In particular, we will look at the central question of just how ‘big’ it really was. But before we can begin our journey back towards the origin of the universe, we must work out our present location within it. Let us therefore embark on a brief sight-seeing tour of the universe.

The Earth belongs to a collection of objects known as the solar system. The central and largest object in this system is the sun. Nine planets, including the Earth, orbit the sun. Pluto is the planet most distant from the sun, and Pluto's orbit may be viewed as the edge of the solar system.

The nearest significant object to the Earth, its moon, is some four hundred thousand kilometres away. For comparison, the distance between the Earth and the sun is roughly one hundred and fifty million kilometres, whereas the average distance between the sun and Pluto is approximately six billion kilometres.

What lies beyond the solar system? As we travel past Pluto, the vastness of empty space soon becomes apparent.

The Planck time is the furthest back in time that we can go before quantum gravitational effects become significant. It has been suggested that inflation may have occurred when the universe was just 10−43 seconds old. This is precisely the era when superstring theory is supposed to be important.

More significantly, at least for our discussion, is that all the spatial dimensions of the universe would have had similar sizes. We discussed some of the theoretical arguments as to why there may be higher dimensions in Chapter 6. The existence of these extra dimensions has always been viewed as a problem. We know that the universe today contains only three large space dimensions, so why have the others remained so small that they cannot be seen? In other words, why is it that only three of the dimensions have grown to cosmological sizes? What is it that prevents the universe from having a different number of large dimensions?

Inflation, by its very nature, increases the size of the universe by a huge factor in a very small amount of time. We might therefore expect it to shed some light on the question of why three of the dimensions in the universe are so much larger than the others. The problem of small dimensions would be solved if only three of them were able to inflate to large sizes.

The theory of gravity that describes the large-scale dynamics of the universe was developed by Albert Einstein during the first two decades of the twentieth century. It is referred to as the general theory of relativity. It will be helpful if we now consider some of the ideas behind this theory.

We should begin by considering the speed of light. In 1865 the English physicist James Clerk Maxwell derived equations proving that electromagnetic radiation travels in a vacuum at a constant and finite speed. One of the key assumptions that Einstein later made was to suppose that two observers who are moving at a constant speed relative to one another would measure the same value for the speed of light.

Einstein's assumption goes against our intuition, to say the least. What might we expect? Speed is a relative quantity; we can measure the speed of an object only in terms of its relationship to something else. For example, when we say that a train travels through a station at a constant speed of one hundred kilometres an hour, what we really mean is that the distance between the train and the platform changes at this rate.

Let us consider two trains, A and B, that simultaneously travel through the station at this speed. If the two trains are moving in the same direction, they will appear to be at rest relative to each other.

Our journey has been remarkably successful. We have followed the development of the universe from its earliest moments through to the present. Our picture is that the universe has been expanding ever since it came into existence some ten billion years ago. However, Einstein's theory of general relativity breaks down at extremely early times when the universe was very small. The critical time scale over which the theory does not apply is the Planck time, corresponding to 10−43 seconds. We cannot employ Einstein's theory to determine the nature of the universe before this time.

The question we will address in this concluding chapter is whether we can travel back beyond the Planck time. Are we able to complete the story and cross this final barrier, or does the breakdown of Einstein's theory at this point represent a fundamental limit to our understanding?

If we are going to discuss the origin of the universe, we must first extend Einstein's theory in some suitable way. How might we proceed to modify the theory? We may answer this question by appealing to quantum effects. We have seen in the preceding chapters how these effects become significant on very small scales. The reason why Einstein's theory breaks down before the Planck time is that it fails to take into account the quantum fluctuations that are inherently present in any physical process involving gravity.

These uncertainties are not important on large scales and can be ignored.

Exploring the subject of elementary particles is rather like trying to find our way around an enormous zoo without the help of a guidebook to identify the different species of animal. How are we to make sense of it all? We will begin by summarizing some of the properties exhibited by the elementary particles. It is helpful to picture each particle as a tiny sphere that has three fundamental characteristics: electric charge, mass and spin. The different particles can be described in terms of these three basic quantities.

Electric charge is a familiar concept. Some particles carry it, but others do not. Those that do not are said to be electrically neutral. Likewise some particles have a mass, but others are massless. The mass of a particle contributes to its total energy, because mass is just another form of energy. Mass may be converted into energy and vice versa, and, indeed, a huge amount of energy may be produced from a relatively small mass.

The amount of spin that a particle carries determines its rate of rotation. We can view spinning particles as rotating about an axis. The electron is an example of a spinning particle. The spin of all elementary particles is severely restricted. Those particles that do not rotate have zero spin. Particles that do rotate have a spin that is directly related to that of the electron.

Elementary particles are divided into two main groups depending on the amount of spin that they carry.

The planets had finally finished growing. Now they would begin their long process of evolution towards the way we see them today. By now, about 100 million years had passed and the Solar Nebula was relatively sparse. Yet its activity did not stop completely. For the Solar System was still littered with fragments of debris that had not yet been ejected from the system by the giants or been swept up by the terrestrials. It was at this point that the Solar System entered what astronomers call, quite justifiably, the heavy bombardment phase.

For hundreds of millions of years, leftover scraps continued to rain down on the planets and their satellites. This is the battering that shaped the planets' and moons' crusts, and the majority of it occurred in the first 600 million years or so of their creation. A glance at the surface of the Moon gives ample reminder of this violent phase in the Solar System's history. Many of the craters there are well over 100 kilometres across. One of them is about 12 kilometres deep and 2500 kilometres across – greater than half the Moon's diameter. Called the Aitken basin, it is the largest known impact structure in the entire Solar System, carved out when the Moon was struck a glancing blow from a piece of rock and metal some 200 kilometres across. This constant barrage meant that the crusts of the terrestrial planets and moons oscillated between molten and solid states for many hundreds of millions of years. The heaviest elements sank to their centres, while the lighter substances, buoyed up, stayed near the surfaces.

The Sun is already dying, in a sense. All the while our star burns hydrogen on the main sequence, its core becomes gradually more depleted in that element, and a helium ‘ash’ is left in its place. As the core adjusts itself to this steadily changing composition, the star's diameter and brightness both slowly increase. When the Sun took its first steps onto the main sequence it was only 90 per cent of its current radius and 60 or 70 per cent of its present luminosity. It is quite a bit warmer and larger now than it used to be. And that trend is not going to change.

The next billion years will see a hike in the Sun's luminosity by about 10 per cent. That may not sound like a cause for concern, but for the innermost planets the change will be overwhelming. And, for the Earth in particular, this slight escalation in luminosity will signal the beginning of the end of billions of years of evolution. With that much extra energy flowing away from the Sun, our planet's polar caps will start to melt and its oceans will begin to warm up. Slowly, they'll evaporate into the atmosphere. Too much water vapour, like carbon dioxide, has a serious effect on our planet's climate. The Sun's energy heats the surface, but the heat is partially trapped. Infrared radiation cannot travel through water vapour or carbon dioxide, because they absorb it. And so the planet steadily warms up. Today, Earth is about 32 Celsius warmer than it would be without its atmospheric blanket.

Once the collapse of the giant molecular cloud had started, it continued under its own momentum. By the time two million years had passed, a multitude of nuclei had developed in the cloud, regions where the density was higher than average. These concentrations began to pull in more gas from their surroundings by virtue of their stronger gravity, and the original cloud fragmented into hundreds or even thousands of small, dense cores. Most of them would later form stars. One of them was destined to become the Sun.

By now, the cloud core from which the Sun would form was perhaps a tenth of a light-year across, more than a hundred times the present size of the Solar System out to Pluto. Gradually, this tight clump of gas continued to fall in on itself like a slow-motion demolished chimney stack, a process known as gravitational freefall. The innermost regions fell the fastest; they were closest to the central condensation where the gravitational pull was greatest. The outermost edges of the cloud core took longer to succumb to their inevitable fall. Thus, because of these differences in infall rates, the cloud's contraction essentially amounted to an implosion, an explosion in reverse. In time, as the gas closest to the centre plunged inward and accelerated, the material there grew steadily hotter, the atoms and molecules within it rubbing against each other frantically. After perhaps millions of years in a deep freeze, the molecular cloud was finally warming up. The eventual result was a gas and dust cocoon: a shell of dark material surrounding a denser, warmer core. Such an object is known as a globule. It was the Sun's incubator.

The planets, their moons, the asteroids and the comets – all are part of the Sun's family. And they are just as ancient as their parent. Evidence suggests that the Solar System's contents started to form even while the Sun itself was still only a protostar, almost as soon as the Solar Nebula was in place.

We have seen that, in some ways, the Sun formed in much the same manner in which a sculpture is made. What began as a single, large block of material – the giant molecular cloud – was gradually whittled away to reveal a smaller end product. But the planets' origins are more like those of buildings. They grew bit by bit, from the bottom up, by accumulating steadily larger building blocks. The very first process in the planet-building production line is a familiar concept known as condensation. You can see it in action when somebody wearing spectacles enters a warm room after being outside in the cold. As soon as air-borne water molecules hit the cold lens surfaces, the molecules cool down and stick to the lenses one at a time to produce a thin – and very annoying – film of tiny water droplets. Exactly the same phenomenon was big business in the very earliest stages of the Solar Nebula. As more and more material spiralled from the Solar Nebula into the newly forming Sun, the disc grew less dense.

It is now more than four and a half billion years since the Solar System came into being. Generally there has been little evolution in the grand scheme of things. Along with some of the planets' moons, the asteroids have surfaces that have not been modified extensively since the heavy bombardment stopped, 3300 million years ago. They still populate the belt between Mars and Jupiter. The comets still surround the Sun in the Oort cloud, and the planets' orbits are essentially unchanged.

But on local scales it is a very different story. Since the initial fires of their births so long ago, the planets, their moons and even the Sun have seen significant changes. We will look at these in this, the third part of the book. Starting with the Sun and working our way outwards, we will investigate each of the elements of the Solar System in turn to see what they are like now – and how they have come to be that way. We shall see that the Sun is slightly larger and a bit more luminous today than it was when it was born. It will become evident why the Earth became the only place capable of supporting life, and why it no longer shows signs of its formation. We will discover a different Mars, a waterworld like Earth, and learn how it evolved into the cold and barren desert it is today. And there are other sights too: a larger Mercury than exists today; volcanoes that have changed planets' surfaces beyond recognition; and moons that have been shattered in cosmic impacts and later reformed into new and unusual forms. Having seen where the Solar System came from, it is time now to see how it has evolved.