The big question
How did it all begin? That question has been asked and answered, in an infinite variety of creation stories, ever since human beings discovered speech. People have always felt certain of the answer, probably rather more certain than now. But then the answer in the past was provided by priests and by holy texts, easily trusted by believers.
Now it is delivered by scientists, whom we trust but find hard to believe.
Yet everyone in the past has been wrong, and we appear to be the first generation to stand a reasonable chance of being right. The turning point comes when the American astronomer Edwin Hubble is able to prove, in 1929, that the entire universe is expanding.
One possible explanation is that it began with a single mighty explosion and has been expanding ever since. It is predicted on a theoretical basis that if there was a Big Bang, echoes of it must still survive.
In 1965 faint background radiation of precisely this kind is discovered. That is the scientific evidence underpinning the statement that for the first time in history a creation theory stands a good chance of being correct.
Even so, the story which the scientists ask us to believe taxes the imagination more profoundly than any myth put about by the priests.
The first moment
The very first moment of the universe has seemed, in recent years, extraordinarily hard to pinpoint. In the late 1980s the conventional wisdom was that Big Bang occurred about 15 billion years ago. But evidence collected during the early 1990s by telescopes in orbit (Hubble and Hipparcos) seemed to reduce the universe's probable age to less than 10 billion years. Meanwhile other methods of analysis suggested that some stars are much older than this. The universe seemed in danger of being younger than its contents.
By the end of the decade more refined techniques and improved images from Hubble resolve the issue. Astronomers agree in placing Big Bang in the region of 13.7 billion years ago.
From the start things move fast. At that first moment all matter is in one speck, unimaginably hot and unimaginably tiny. The word million is more evocative than any string of noughts or powers of ten: the suggested temperature for that speck is a million million million million million million degrees Centigrade. It has a diameter of only a hundredth of a millimetre.
And there are more millions to grapple with. By the time a millionth of a second has passed, the primeval pinpoint has expanded into a seething mass of elementary particles occupying a space almost as large as our solar system. The priests never asked us to believe anything like this. But the evidence suggests that somehow or other we must.
From elements to galaxies
Tiny particles of matter (protons and neutrons) collide ceaselessly in the expanding cloud of chaos, and some of them merge to form the nuclei of hydrogen and helium. These exist by the end of the first three minutes, according to modern calculations, but it will be another 300,000 years before electrons combine with these nuclei to form hydrogen and helium atoms.
These atoms of hydrogen and helium are the first building blocks of the universe; with them, matter acquires enough density to become subject to the laws of gravity. And so the dense scalding fog of the expanding fireball becomes increasingly transparent, separating into cooler areas of space and hotter clusters of gaseous elements.
In these more localized infernos, matter begins to recover a fraction of its original density. The protons, neutrons and electrons, which in relatively simple combinations have formed hydrogen and helium atoms, now begin to merge in the more complex patterns of the other elements. From this developing gas, the galaxies coalesce.
The first galaxies are believed to have been formed between 1 and 3 billion years after Big Bang. They become self-contained gravitational systems, with the gas steadily pulling into tighter clumps to form individual stars. Even in our time many galaxies still contain much free-floating material which has yet to coalesce into stars.
Our solar system: about 4.6 billion years ago
In our own galaxy, the Milky Way, a star is formed about 4.6 billion years ago - about two thirds of the way through the story so far of the universe. It is the star which we know as the sun.
As its material contracts, many particles are left spinning freely round the central mass of the new star. It is these which coalesce to form the planets, including earth.
In our emerging solar system many smaller lumps of matter are also within the sun's gravitational field. Some of them, the asteroids (varying from a few millimetres to a kilometre and more in diameter), settle into orbit round the sun.
Larger bodies, such as our moon or the satellites of Jupiter, begin orbiting individual planets - as do the particles, varying in size from pebbles to rocks, which form the rings of Saturn.
Mercury, the nearest planet to the sun, has a daytime surface temperature of around 350° Centigrade - far too hot to support life. Pluto, the outermost planet, is believed to be covered in a blanket of ice some 150 miles thick.
By contrast the earth, third in distance from the sun, has the moderate temperature range with which we are all familiar. It is one, but only one, of the factors which make life on earth possible.
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