Cosmology is the study of the Universe, and how it came into being. It's an interesting example of the way that science has progressed in the last century; up until a hundred years ago, cosmology was considered to be the realm of theologians, not scientists. How the Universe came into being had to be the one question that science couldn't tackle. And yet today, cosmology is a hard science with the world's most brilliant physicists working on the most important question of all: why are we here?

Many people who do not have a background in science misunderstand science. They assume that scientists come up with a theory, that this remains "fashionable" for a while, and then someone else comes along and they all change their mind. "How can you trust science?" they say - "Everyone thought Newton was right, then Einstein comes along and turns it all on its head." This attitude is particularly prevalent when it comes to cosmology, because the science is still young and somewhat speculative. In the fifties, the distinguished scientist and astronomer Fred Hoyle was a fervent supporter of the "Steady State Universe" - we've always been here and always will be. Nowadays, the vast majority of scientists accept the "Big Bang Theory", which implies the Universe was created from a single point, billions of years ago. Since these two viewpoints are diametrically opposed, surely cosmology is no different from religion, where it's a matter of faith which belief you subscribe to?

Not so. Science does get things wrong sometimes, to be sure, but scientific "belief" is based on observation, rather than faith. And by and large, science does not turn everything on its head by accepting theories that completely refute earlier ideas. Science does, however, make better and more accurate observations that subsequently modify existing theories. Let's look briefly at three examples: orbits of planets, relativity and the creation of the Universe.

From earliest times, people have noticed that the sun rises, moves through the sky and sets. The ancient Greeks also noticed that certain "stars" appeared to move relative to the rest of the sky, and called these wandering stars "planets" (from the Greek word for wander). They realised that there were two possibilities for this phenomenon: either the Sun and the planets are revolving around the Earth, or the Earth and the planets are revolving around the Sun. Either viewpoint was equally valid. However, the Moon presented a bit of a problem. If the Earth was the centre of the solar system, there was no problem - the Sun, planets and Moon all revolved around the Earth. If the Sun was the centre of the solar system, though, the Moon didn't seem to fit in. It clearly did not revolve around the Sun. So the Greeks were left with two models: either everything revolved around the Earth; or everything revolved around the Sun except the Moon, which revolved around the Earth.

Of course, we now know that the latter theory is the correct one; but to the Greeks it seemed unnecessarily complicated, so they opted for the Earth-centred solar system. This idea carried through into Western belief and fitted well with the Christian belief that since humanity was created in God's image, then it was reasonable that the Earth should occupy a special place too. So it became an article of faith that the Sun and the planets revolved around the Earth.

When Galileo dared to suggest otherwise, he was treated as a heretic. Other renaissance scientists who realised that the Earth-centred belief was not tenable were a little more circumspect - they "pretended" that the Earth revolved around the Sun "to make the mathematics easier". Provided this was done as a mathematical trick rather than as a statement of what was actually happening, the Church was happy.

Thus modern science was born. Rather than using philosophical argument to establish the truth or falsehood of something, renaissance scientists started to use observation. It didn't matter how convincing a philosophical argument was - if it didn't fit the facts, the observations, then it was wrong. This is the fundamental yardstick by which science still operates.

So science now accepted that the Earth and other planets revolved around the sun. The Moon revolved around the Earth. Galileo had spotted - using his new invention, the telescope - that the planet Jupiter also had moons of its own, which revolved around Jupiter. It was believed that these orbits were circular.

However, even with this new theory, the observed motions still did not completely fit the theory. Johannes Kepler (1571-1630) proposed a new theory - instead of circular orbits, he proposed that orbits were elliptical. This then meant that the observed motions of planets did fit in with his theory.

This stroke of genius did not turn scientific orthodoxy on its head. The original theory - that everything revolved around the Sun in a circular orbit - was wrong, but it was only a little bit wrong. The planetary orbits are, strictly speaking, elliptical - but they are ellipses that approximate a circle. The original theory was more or less correct, and Kepler's theory modified the original theory to make it even better.

Relativity is often quoted as the theory that "proved Newton wrong." (Actually, one of the anomalies that led Einstein to develop his theory in the first place was that Kepler's theory of planetary orbits didn't quite work for Mercury - the motion of this tiny planet didn't quite fit in with its theoretical orbit).

For over two hundred years, Sir Isaac Newton's theories of physics had held sway, and had advanced science enormously. Albert Einstein did not come along and "overturn" Newton's laws at all - he modified them. He discovered that Newton's laws did break down at very high speeds, close to the speed of light. At everyday speeds, Einstein's equations give results that are virtually identical to Newton's.

This whole process of scientific modification is a little bit like looking at a map - when I look at my road atlas of Britain, I may see a straight road between two towns. When I look at a larger-scale walker's map of the area, I can see that this straight road actually has a couple of small kinks in it. Looking at a street atlas of the same area, I see lay-bys, groups of houses offset from the road and so on. This does not mean that my road atlas is wrong - simply that it is only correct to within a certain limit. I can narrow down this limit by purchasing a larger-scale map. Similarly, new scientific theories rarely "overthrow" old ones - they usually build upon them.

This of course does not hold where a science is very new. In the pioneering days of looking at a subject, science does not have much to go on. Scientists can propose a theory, and then wait to see if the facts fit. This is why Fred Hoyle's Steady State Universe appears so opposed to the Big Bang Theory. One of the theories had to go.

This does not mean that scientists tossed a coin to see which one they should "believe in." The current consensus is that the Big Bang Theory is correct - it may well be modified in the future, but the consensus is that the theory is basically correct - because not only do the observations fit, but it was capable of making predictions that have subsequently turned out to be correct. This is the real power of science - anyone can look at the way things are today, and make up a plausible story as to how it happened; but the ability to say "if this theory is correct, then we should be able to observe the following..." and then subsequently to find that the predicted events do occur - that's powerful stuff.

The Big Bang theory is not just a wild guess. When a train goes whizzing past you, you hear a drop in its tone - this is called the "Doppler effect" and is caused by the fact that the sound waves that were coming towards you were reaching you quicker than they are now they are receding from you. By measuring the difference in tone, you can calculate how fast the train is going. Similarly, stars a long way away are emitting light that we can measure. Since most stars consist largely of hydrogen, we can measure a kind of "hydrogen fingerprint" in a star using a technique called "spectroscopy". This fingerprint is shifted in one direction for stars receding from us, and in the other direction for stars coming towards us. This difference, for receding stars, is known as the "red-shift". Astronomers noted that although some close stars were moving towards us, distant galaxies were all receding from us. And furthermore, the more distant galaxies were receding faster than closer galaxies. This implied that the entire Universe was expanding.

The implication of an expanding Universe was extraordinary - play the expansion in reverse, and you end up with a point, billions of years ago, when the entire matter of the Universe must have been gathered together in a single point. The Universe must have had a beginning. So an observed fact - that distant galaxies are receding from us - led to a theory that the Universe must have had a beginning. Then scientists realised that if the Universe started as a tiny point with a vast amount of energy - the "big bang" - which then expanded, the Universe would have been very hot in its early stages, and would be in the process of cooling down. This means that the Universe should still have a background temperature, just like the embers of a fire cooling down. This particular fire would be very cold by now, but should still have an observable temperature - scientists estimated between 2 to 3 degrees above absolute zero. This was a prediction made by science. Then, in the sixties, observations were made - and the Universe did indeed have this background temperature.

This is what makes science so powerful - not its ability to come up with a plausible explanation for something, but its ability to predict something that hasn't yet been observed.

Further refinements to the Big Bang model meant that further predictions came about - namely, that there should be tiny fluctuations in the background temperature. In the 1980s, these fluctuations were found. So the Big Bang theory is not just wild speculation - it is very plausible indeed.

For me, cosmology is not just science - it provides a model for my spirituality, too. There are numerous good introductory books about cosmology (see Bibliography), but the one that influenced me most was a work of semi-fiction, called "Deep Time" by David Darling. The "hero" of this novel is a proton, created out of the energy a few minutes after the Big Bang, and the story of the Universe from the point of view of this proton! The proton eventually ends up in a gold atom deep within the Earth, which is eventually mined and beaten into the plaque that went on the Voyager space mission. The story carries on for many trillions of years in the future - far, far longer than the Universe has existed to date.

Seen from this point of view, the Universe expanded and cooled. Eventually (three minutes after the big bang), the Universe was "cool" enough for matter to condense out of this primeval soup. Shortly after, hydrogen atoms formed. By a series of processes, these coalesced together to form stars, exploded under certain circumstances to form planets and heavier elements, such as our Earth. These atoms formed molecules of varying degrees of complexity, including DNA. This DNA replicated to form other forms, which we now call "life forms" - some of these became plants, some dinosaurs and so on - until eventually we end up with the planet, plants, animals and people that we have today.

So the Universe expanded, and everything in it came from this one point - including us. We're not separate from the Universe at all - we're part of it. In fact, we ARE the Universe in a very real sense - the Universe has become conscious, because we - a collection of particles of matter - are looking at it and asking questions about it. The Universe has become self-aware. This self-awareness need not be limited to humans, of course - whether or not you accept that animals may have the capacity to ask such questions in their own way, the Universe may well have created other life forms elsewhere in the Universe, that are asking the same sort of questions that we are.

One scientist, Frank Tipler, has taken this a step further with publication of his "Physics of Immortality" (see Bibliography). This astounding book is subtitled "Modern Cosmology, God and the Resurrection of the Dead". Frank Tipler is a respected cosmologist, and does not have a religious agenda - he still considers himself to be an atheist, and even has a chapter in the book entitled "Why I am not a Christian".