In the beginning was nothing. Then the Universe was born in a searing hot fireball called the big bang. But what was the big bang? Where did it happen? And how have astronomers come to believe such a ridiculous thing?
Fifteen thousand million years ago, the Universe that we inhabit erupted, literally, out of nothing. It exploded in a titanic fireball called the big bang. Everything – all matter, energy, even space and time – came into being at that precise instant.
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In the earliest moments of the big bang, the stuff of the Universe occupied an extraordinary small volume and was unimaginably hot. It was a seething cauldron of electromagnetic radiation mixed with microscopic particles of matter unlike any found in today’s Universe. As the fireball expanded, it cooled, and more and more structure began to “freeze out”.
Step by step, the fundamental particles we know today, building blocks of all ordinary matter, acquired their present identities. The particles condensed into atoms, galaxies began to grow, then fragment into stars such as our Sun. Just over four thousand million years ago, the Earth formed. The rest, as they say, is history.
It is an extraordinarily grand picture of creation. Yet astronomers and physicists, armed with a growing mass of evidence to back their theories, are so confident of the scenario that they believe they can predict the detailed conditions in the early Universe as it evolved, instant by instant.
Unfortunately, when it comes to answering the ultimate question – just how could space and time, matter and energy have come out of absolutely nothing – the theories are not yet good enough. The best that physics can do is to attempt to describe what was happening when the Universe was already about 10-35 seconds old – a time that can also be written as a decimal point followed by 34 zeroes and a 1.
This is an exceedingly small interval of time, but you would be wrong if you thought that it was so close to the moment of creation as to make no difference. Although the structure of the Universe no longer changes much in even a million years, when the Universe was young, things changed much more rapidly.
For example, physicists think that as many important events happened between the end of the first tenth of a second and the end of the first second as in the interval from the first hundredth of a second to the first tenth of a second, and so on, logarithmically, back to the very beginning. As they run the history of the Universe back, like a movie in reverse, space is filled with ever more frenzied activity.
This is because the early Universe was dominated by electromagnetic radiation – in the form of little packets of energy called photons. And the higher the temperature, the more energetic the photons. Now, high-energy photons can change into particles of matter because, as Einstein revealed, energy and matter are simply different faces of the same coin. They are connected by the famous equation E=mc2, where c is the speed of light.
What Einstein’s equation says is that particles of a particular mass, m, can be created if the packets of radiation, the photons, have an energy of at least mc2. There is, therefore, a temperature above which the photons are energetic enough to produce a particle of mass, m, and below which they cannot create that particle.
If we look far enough back, we come to a time when the temperature was so high, and the photons so energetic, that photons could collide and produce particles out of pure energy. What those particles were before the Universe was 10-35 seconds old, we do not know. All we can say is that they were very much more massive than the particles we are familiar with today, such as the proton and electron.
As time progressed, and the temperature fell, so the mix of particles in the Universe changed, to a soup of less and less massive particles. Each particle was “king for a day”, or at least for a split second. For the reverse process was also going on – matter was being converted back to energy as particles collided to produce photons.
What do physicists think the Universe was like a mere 10-35seconds after the big bang?
Well, the volume of space that was destined to become the entire visible Universe, thousands of millions of light years across, was contained in a volume roughly the size of a pea. And the temperature of this superdense material was an unimaginable 1028 degrees Celsius.
At this temperature, physicists predict, colliding photons had just the right amount of energy to produce a particle called the X-boson.
This particle was a thousand million million times more massive than the proton. No one has yet observed an X-boson, because to do so we would have to recreate, in an Earth-bound laboratory, the extreme conditions that existed just 10-35 seconds after the big bang.
How far back can physicists probe in their laboratories?
The answer is to a time when the Universe was about one-hundredth of a second old. At that time, the Universe had grown to fill a volume roughly the size of the Sun. By then, it had cooled down to 1014 or 100 million million degrees – still many millions of times hotter than the centre of the Sun. But this temperature is not beyond the reach of experiments.
In 1983, physicists at CERN in Geneva managed to recreate these conditions in a giant particle accelerator. They created the W and Z bosons, particles which vanished from the Universe one-hundredth of a second after the big bang.
The gulf between 10-35 seconds and one-hundredth of a second is gigantic. We know that, for most of this period, matter was squeezed together more tightly than the most compressed matter we know of – that inside the nuclei of atoms. And, as the temperature fell, so the energy level of photons declined, creating smaller and smaller particles. Most were unstable, and collided with each other to produce photons.
At some point, the hypothetical building blocks of the neutron and proton – known as quarks – came into being. Unfortunately, no one has developed a satisfactory theory which explains how a quark soup behaves, so we know little about this period.
By about one-hundredth of a second, however, the Universe had cooled sufficiently to be dominated by particles that are familiar to us today: photons, electrons, positrons and neutrinos. Neutrons and protons were around, but there weren’t many of them. In fact, they were a very small contaminant in the Universe.
About one second into the life of the Universe, the temperature had fallen to about 10 thousand million degrees, and photons had too little energy to produce particles easily.
The next important stage in the history of the Universe was at about 100 seconds. The temperature had dropped to a mere thousand million degrees – the temperature in the hearts of the hottest stars. Now, the particles were moving more slowly. In the case of the protons and neutrons, it meant that they stayed close to each other long enough for the strong nuclear forces, which bind them together in the nuclei of atoms, to have a chance to take a hold. In particular, two protons and two neutrons could combine to form nuclei of helium.
Solitary neutrons decay into protons in about 10 minutes, so any neutrons that were left over after the helium formed became protons. According to physicists’ calculations, roughly 10 protons were left over for every helium nucleus that formed. And these became the nuclei of hydrogen atoms, which consist of a single proton.
This is one of the strongest pieces of evidence that the big bang really did happen. For much, much later, when the temperature had cooled considerably, the hydrogen and helium nuclei picked up electrons to become stable atoms. Today, when astronomers measure the abundance of elements in the Universe – in stars and galaxies and interstellar space – they still find roughly one helium atom for every 10 hydrogen atoms.
At one time, almost all the electrons and their positively charged opposites, the positrons, were colliding and cancelling each other out by forming photons. There were roughly a thousand million photons for every proton and neutron in the Universe, a ratio which persists to this day. But a slight lopsidedness in the laws of physics meant that at about half an hour after the big bang, at the end of all the collisions, there was a tiny number of electrons remaining.
The point at which it was cool enough for these electrons to combine with protons to make the first atoms was a long way down the stream of time – 300 000 years after the big bang. The Universe was now cooling very much more slowly than in its early moments, and the temperature had reached a modest 3000 degrees. This also marked another significant event in the early history of the Universe.
Until the electrons had combined with the hydrogen and helium nuclei, photons could not travel far in a straight line without running into an electron. Free electrons are very good at scattering, or redirecting, photons. As a consequence, every photon had to zigzag its way across the Universe.
This had the effect of making the Universe opaque. If, for instance, the light from the stars zigzagged its way across space to your eyes rather than flew in straight lines, on a clear night you would see only a dim milky glow across the whole sky rather than a myriad of stars.
We can still detect photons from this period. No longer creating matter, they have been flying freely through the Universe for about 15 thousand million years, and astronomers observe them as the so-called cosmic microwave background. Whereas these photons started their journey when the temperature was 3000 degrees, the Universe has expanded a thousand times while they have been in flight. This has decreased their energy by this factor, so that we now record the signals as just 3 degrees above absolute zero.
The temperature dropping to about 3000 degrees also signalled another event – the point at which the energy levels of the radiation, or photons, in the Universe fell below that of the matter. From then on, the Universe was dominated by matter and by the forces of gravity acting on that matter.
The building of elements, however, had stopped abruptly after the Universe had reached an age of 100 seconds and the protons and neutrons had formed the nuclei of hydrogen and helium. For elements such as carbon and oxygen to form, higher temperatures were needed, but the Universe was getting colder all the while. The heavy elements in the planets and in your body were created, billions of years later, in the nuclear furnaces of stars.
Instead, as the Universe continued to expand, gravity caused clumps of matter to accumulate in large islands. Those islands were to become the galaxies. The galaxies continued their headlong rush into the void, fragmenting into smaller clumps which became individual stars, producing heat and light by nuclear reactions deep in their cores. At one point, about 10 thousand million years after the big bang, a yellow star was born towards the outer edge of a great spiral whirlpool of stars called the Milky Way.
The star was our Sun.
Getting back to the moment of creation
Physicists can run the expansion of the Universe backwards. In this way, they can watch it get hotter as it gets smaller, just as the air in a bicycle pump heats up as it is compressed. But theory predicts that, at the big bang itself, the temperature was infinite. Infinities warn physicists that their theory is flawed.
At the moment, the theories which take us furthest back in time are the Grand Unified Theories. These GUTs are an attempt to show that three of the basic forces that govern the behaviour of all matter are no more than facets of a single superforce.
Each force of nature arises from the exchange of a different “messenger” particle. The messenger transmits a force between two particles, just as a tennis ball transmits to a player the force of his opponent’s shot. At high enough temperatures – such as those that occurred when the Universe was 10-35 seconds old – physicists believe the electromagnetic, and strong and weak nuclear forces were identical, and mediated by a messenger dubbed the X-boson.
Physicists want to show that gravity, too, is a facet of the superforce. They suspect that gravity split apart from the other three forces at about 10-43seconds after the big bang. But before they can “unify” the four forces, they must describe gravity by “quantum” theory, a type of theory hugely successful in describing the other forces. Physicists are currently finding this difficult.
When they have their unified theory, physicists believe that they will be able to probe right back to the moment of creation and explain how the Universe popped suddenly into existence 15 thousand million years ago.
How do we know there was a big bang?
Our modern picture of the Universe is due in large part to an American astronomer, Edwin Hubble. In 1923, he proved that the Milky Way, the great island of stars to which our Sun belongs, was just one galaxy among thousands of millions of others scattered throughout space.
Hubble also found that the wavelength of the light from most of the galaxies was “red shifted”.
Astronomers interpreted this as a doppler effect, familiar to anyone who has noticed how the pitch of a police siren changes as it passes by. The siren becomes deeper because the wavelength of the sound is stretched out. Similarly with light, the wavelength of light from a galaxy which is moving away from us is stretched out to the longest, or reddest, wavelength.
Hubble had discovered that most galaxies are receding from the Milky Way. In other words, the Universe is expanding. And the farther away the galaxy, the faster it is receding.
One conclusion, therefore, was inescapable: the Universe must have been smaller in the past. There must have been a moment when the Universe started expanding: the moment of its birth. By imagining the expansion running backwards, astronomers deduce that the Universe came into existence about 15 thousand million years ago.
This idea of a big bang means that the red shifts of galaxies are not really doppler shifts. They arise because in the time that light from distant galaxies has been travelling across space to us, the Universe has grown, stretching the wavelength of light.
The picture of a Universe which is expanding need not have been a surprise to anyone. If Albert Einstein had only had faith in his equations, he could have predicted it in 1915 with his theory of gravity – general relativity. But Einstein hung on desperately to the idea that the Universe was static – unchanging, without beginning or end.
The vision of a static Universe appealed strongly to astronomers also. In 1948, Hermann Bondi, Thomas Gold and Fred Hoyle proposed the steady-state theory of the Universe. The Universe was expanding, they said, but perhaps it was unchanging in time.
Their theory said that space is expanding at a constant rate but, at the same time, matter is created continuously throughout the Universe. This matter is just enough to compensate for the expansion and keep the density of the Universe constant. Where this matter would come from, nobody could say. But neither could the proponents of the big bang.
The steady-state theory held its own as the principal challenger to the big- bang theory for two decades. Then, in the 1960s, two astronomical discoveries dealt it a fatal blow.
The first discovery came from Martin Ryle and his colleagues at Cambridge. They were studying radio galaxies – enormously powerful sources of radio waves, a type of light invisible to the naked eye. In the early 1960s, the Cambridge astronomers found that there were many more radio galaxies at large distances than nearby.
The radio waves from these distant objects have taken billions of years to reach us. Ryle and his colleagues, therefore, were observing our Universe as it was in an earlier time. The excess of radio galaxies at great distances had to mean that conditions in the remote past were different from those today. A Universe which changes with time ran counter to the steady-state theory.
Then in 1965, Arno Penzias and Robert Wilson, two scientists at the Bell Telephone Labs in New Jersey, detected an odd signal with a radio horn they were using for satellite communications.
The signal did not come from the Earth nor the Sun. It seemed to come from all over the sky, and it was equivalent to the energy emitted by a body at 3 degrees above absolute zero.
There could be no doubt. Penzias and Wilson had discovered the remnant of the radiation from the big bang – the cosmic microwave background. They shared the Nobel prize. The steady-state theory was dead.
The fate of the Universe
The big bang was not at all like the explosion of a lump of material in which fragments are blown away into an existing void. There was no void. Space itself popped suddenly into existence 15 billion years ago and began expanding.
So, when astronomers look at a distant galaxy and find that it appears to be rushing away from us, it is because the space between us and the galaxy is swelling in the aftermath of the big bang. Imagine a balloon with dots drawn on its surface to represent galaxies.
Now imagine inflating the balloon. The dots, or galaxies, move apart. They do not move within the surface, they move because the surface expands. The real Universe is a three-dimensional version of the surface of the balloon, and so difficult to imagine.
The flaw in the analogy is that the dots on the balloon grow as the balloon inflates. Galaxies do not grow as the Universe expands – the gravity of each galaxy is strong enough to bind its stars together.
Will the expansion of the Universe continue for ever? This depends on how much matter there is in the Universe. The gravity of each galaxy tries to pull every other galaxy towards it. Ever since the big bang, the gravity of the galaxies has been acting as a brake on the expansion of the Universe.
If there is enough matter in the Universe, gravity will eventually slow, then reverse, the expansion until all the matter is recompressed into a tiny volume: a “big crunch”. The Universe might then rebound in another cycle of big bang and big crunch. If, however, there is not enough matter in the Universe, the expansion will continue.
Further reading
The First Three Minutes by Steven Weinberg (Fontana) is the best popular account of the standard model of the big bang. Because it was written 10 years ago, however, it does not describe the first hundredth of a second. Superforce by Paul Davies (Counterpoint) describes the quest to unify the forces of nature. It may be heavy going, though, for anyone without a science background.
In Search of the Big Bang by John Gribbin (Corgi) is a popular account of the big bang which speculates on the precise nature of the big bang and what happened before.