How do we know the Big Bang actually happened?

Where did we come from?

It’s the oldest question in human history. Everyone’s asked it, from philosophers and poets to mathematicians and physicists.

Most scientists think that everything that we know and experience around us began at a moment known as the Big Bang, 14 billion years ago. But how can we have any clue about something that supposedly happened so long ago?

From speeding galaxies to ancient gas clouds, there is evidence that we can detect today – the remnants of the Big Bang, that tell a clear story about the origins of our Universe.

We can see the universe 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. These are other galaxies like our own – but they are much, much further away than the stars.

Almost all of these galaxies are moving away from us – some at speeds of hundreds of thousands of kilometres every second.

If most galaxies are moving away from us, it means that the Universe is expanding. If the Universe is expanding, then in the past it must have been much smaller. Go back far enough, and there was a moment when all the matter in the Universe was packed into a point and expanded outwards. That moment was the Big Bang.

We can even work out when it happened from the speed of the galaxies: about 14 billion years ago. We can't actually see the galaxies moving, but the clue is in the light coming from them – it is redder than it should be.

Picture of distant galaxies
We can see that almost all galaxies appear to be moving away from us.

Why galaxies look redder than they should

The tell-tale signature of a galaxy speeding away from us can be detected in the same way that we can tell when a police car passes us with its siren on. When that happens, the siren sounds lower pitched to us, because the waves are stretched.

Light is also made up of waves, so the same is true of very fast-moving objects like galaxies. If a galaxy is moving away from us, the light waves are stretched. That makes the light seem redder. The faster the galaxy is moving, the redder the light.

Capturing 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 stretched into 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. This glow is called the Cosmic Microwave Background.

Maggie Aderin-Pocock explains that ripples in the Cosmic Microwave Background hint at the starting conditions that seeded stars and galaxies. Image: ESA/Planck.

How starlight differs from microwaves

Unlike the light coming from a star, the Cosmic Microwave Background is the same wherever you look, and wherever you are in space.

We can peer back in time

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 have not yet turned into stars and galaxies.

Astronomers have recently found gas clouds like this in the distant Universe. Some of them are around 12 or 13 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.

As Big Bang theory predicts, these ancient gas clouds are made of very different stuff to the modern Universe. Most of the chemical elements in the modern Universe are made inside stars. Because the gas clouds come from a time before stars, they consist almost entirely of the most basic elements, hydrogen and helium.

Picture of an ancient gas cloud discovered in 2012 near a quasar 13 billion light-years away, seen here as a faint red dot in the centre.
An ancient gas cloud was discovered in 2012 near a quasar 13 billion light-years away, seen here as a faint red dot in the centre.

Learn more about this topic:

How will the Universe end?
Did comets kick start life on Earth?
Does Einstein’s general theory of relativity still matter?