How do we know the Big Bang happened?
Most scientists accept that the Universe began with a Big Bang. But how do we know about something that happened so long ago?
The Universe is expanding
When we look at the night sky, the stars we can see are within our own galaxy. But there are also some fuzzy patches which we need a telescope to see clearly. These are other galaxies like our own - but they are much, much further away than the stars.
If we look closely at these galaxies, we'd expect that some would be moving towards us and some would be moving away, But in fact, almost all galaxies are moving away from us - some at very high speeds indeed.
We know the galaxies are moving away because of an effect called red-shift - similar to the way that a car sounds more high-pitched as it approaches you, and low-pitched as it moves away.
If most galaxies are moving away from us, it follows that the Universe is expanding. (It does not mean that Earth is at the centre of the Universe. Imagine the Universe like a fruitcake expanding in an oven. Every point is getting further away from every other point, wherever you happen to be.)
If the Universe is expanding, that suggests it used to be smaller and more compact. And if you go back far enough, then there was a moment in the past when all the matter in the Universe was packed into a point and exploded outwards. That moment was the Big Bang. We can even work out when it happened, from the current size of the Universe and the speed at which it is moving - about 14 billion years ago.
We can see the afterglow of the Big Bang
We can't see it with the naked eye, but some of our telescopes can. Our eyes actually only see a fraction of the light in the Universe. As well as visible light, there are other kinds of light, such as X-rays, infrared light, ultraviolet light, radio waves and microwaves. They have shorter or longer wavelengths than visible light.
After the Big Bang, the whole Universe was flooded with incredibly bright light. As the Universe has expanded, that light has been stretched so that it is now microwaves.
A microwave telescope can see this ancient light from the very beginning of the Universe. In fact, a view through a microwave telescope shows the whole sky filled with a glow, day and night. Unlike light coming from a star, it is the same wherever you look, and wherever you are. This glow is called the Cosmic Microwave Background.
We can see gas clouds in the early Universe
Looking out into space is like looking back in time. That is because light from objects that are far away takes longer to reach us than light from objects nearby. If an object is a million light years away, we are seeing it as it looked a million years ago.
Modern telescopes are so powerful that they can view objects many billions of light years away, close to the time of the Big Bang.
If the Big Bang did happen, then we'd expect those distant views to reveal clouds of gas which had not yet turned into stars and galaxies. And those gas clouds should be made of very different stuff to the modern Universe. That is because most of the chemical elements in the modern Universe are made inside stars. The earliest gas clouds should contain almost no complex elements. Instead they should be made almost entirely of the most basic elements, hydrogen and helium.
Gas clouds were recently found in the distant Universe. Some of them are around 12 billion years old. Even at this incredible distance, we can tell what they are made of by using a technique called spectroscopy to analyse light that passes through them.
This shows that they are mostly hydrogen and helium - exactly as the Big Bang theory predicts.
What happened before the Big Bang?
If there was a Big Bang, what caused it? And what existed before? This is one of the biggest questions in science today. Some scientists think that instead of expanding just once, the universe has grown and shrunk many times.
For others, this question doesn't need an answer. Stephen Hawking has suggested that the idea of time before the Big Bang is like the idea of a point North of the North Pole. Time begins with the Big Bang, and that's that.
The Big Bang features in the second episode of Stargazing Live on BBC Two at 8pm on Wed 9 January. Get more involved with Stargazing Live or catch up on the series at the above link.