Question: what was this big bang all about ?

Jon Copley answered on 25 Jun 2010:

It’s the idea that the Universe started, about 13.75 billion years ago, as a space filled with an incredibly hot and dense gas.

That space then expanded (a bit like an explosion – hence the name “Big Bang”), so that the energy and matter spread out. But there’s one important difference between the Big Bang and a normal explosion. In a normal explosion, the explosion spreads out into the space around it. But in the Big Bang, there is no space around it – space itself is one of the things that is expanding.

That sounds weird (and it is!) – but when people ask “what is the Universe expanding into?”, the answer is “nothing” – not even empty space. The Universe looks like it is expanding because the space between everything is getting bigger.

To get some idea of how that works, next time you have a party balloon, draw some dots on it with a pen before you inflate it. As you blow air into the balloon and it gets bigger, the dots on it will get further apart. But the dots aren’t moving on the surface of the balloon – instead, the balloon’s surface between them is stretching. The same kind of thing appears to be happening to the Universe – the space between galaxies is stretching.

Ok, so how do we know that the Universe is expanding like that, and that all started as the “Big Bang”? If we look at the light from distant galaxies, we can tell that they appear to be moving away from us. Now imagine what it would be like if you were one of the dots on your balloon, looking at the other dots as the balloon gets bigger – the other dots would seem to be moving away from you. That’s what we see when we look at other galaxies.

We can tell that other galaxies seem to be moving away from us because of something called “Doppler shift” in the light coming from them. That’s a bit like the change you get in the musical note of an ambulance siren as it moves towards you, and then passes you and starts moving away from you.

We can measure the same change in the “note” of light if it comes from an object moving away from us – that’s the “Doppler shift” (and I use the Doppler shift in the deep ocean too, to find my way around by bouncing sonar off things to see how I am moving relative to them).

Anyway, it turns out that the further a galaxy is from us, the more it is moving away from us. So this means that galaxies must have been closer together in the past – in fact, they must have all started close together, in the Big Bang.

We can also detect the “echoes” of the Big Bang in space – they’re not really sounds, but rather energy echoes, called the “cosmic microwave background”.

One of the first people to come up with something like the “Big Bang” as an explanation for what we see galaxies doing was Georges Lemaitre, in the 1920s – and he was a Catholic priest as well as a physicist. So he clearly didn’t think that ideas like this in physics disproved the existence of God!

In science, we can’t actually think about what was there “before” the Big Bang (or what lies “outside” the Universe), because in terms of the maths involved, there was no time before the Big Bang – and there is no “outside” for the Universe either! So we can’t use maths to ask those questions (and maths is the “language” of physics).

But one of the big questions in physics is what will happen to the Universe in the future – will it carry on stretching, and everything get so spread out that eventually there is very little energy anywhere (known as the “Big Freeze”, or “heat death of the Universe”)? Or, if there is just enough matter in the Universe, will it eventually start to come together again, in a “Big Crunch”? Physicists are trying to make the measurements of the Universe, how much matter is in it, and how it is expanding, to find out which is likely.