What is the universe expanding into?

Chris Smith put this simple query to astrophysicist Francesca Day...

Francesca - Yeah. This is actually a very good question. People often say that the universe is expanding and this means that there are galaxies in it are getting further apart. And the analogy people use is if you put a lot of dots on the surface of a balloon and those like the galaxies and as you blow the balloon up the universe is the surface of the balloon all the galaxies the dots get further apart from each other. And this is a very good analogy but it naturally begs the question well the balloon is expanding into the room. What's the universe expanding into. Unfortunately this is where the analogy ends. The universe the space we live in has the mathematical properties or some of the mathematical properties of the surface of the balloon but it has those properties without there being something it's expanding into. In math-speak we say that the manifold doesn't need an embedding in a higher dimensional space. That might not mean much to most listeners.

Chris - Oh I don’t know I thought that was really simple, thanks for that, moving on. In strict terms then does this mean that because one philosophical answer I heard to this question was to say well the universe is everything if that's your definition so therefore the universe is everything so it's not expanding into anything because it is everything. But that's sort of a get out clause isn't it because our human brains have not evolved to grasp but the idea that there could be something that's everywhere and everything all at once and getting bigger. But is everything. It doesn't quite sit with our perception of how the world works and the universe works.

Francesca - Yeah I think that might be the philosophical answer from my point of view as a sort of practising scientist I have some observations. I do some math which describes this expanding space which doesn't need to include what it is expanding into, the maths doesn't require that to be an outside the universe. And then I get an answer right, I compare the answer with data and it matches the data. So that's the sense in which the universe is expanding as far as I'm concerned.

Chris - If you look at where the universe is expanding is it expanding everywhere by the same amount and if I look at say the patch of space between the Earth and Mars, has that expanded a bit in the same way that the patch of space between us and the Andromeda galaxy has expanded, but obviously there's a lot more of that so it will have relatively speaking that would've expanded more.

Francesca - This is also a very good point. It's really only the bits between galaxies that are expanding the bit between galaxies are what governed by the overall motion of the universe within a galaxy the gravity of the galaxy is kind of holding stuff together so it doesn't expand and one way to see that is if absolutely everything was expanding by the same amount we wouldn't notice because anything we could use to measure it would also be expanding.