What is the Universe expanding into?

James - The Big Bang theory describes the universe's origins from an initial state of incredibly high temperature and density and its subsequent expansion. We think it's a good theory because it explains some of the features of the universe that astronomers have observed over the years, such as cosmic background radiation. And next week we'll have a special interview with Nobel Prize winner in physics, Brian Schmidt, who proved that the expansion of the universe is speeding up rather than slowing down. Stay tuned for that one. But you raise an interesting point, Anne. If the universe is by definition all that there is, as Paul writes on our forum, then nothing can exist outside of it for matter to be expanding into. Luckily, the University of Cambridge's public astronomer Matt Bothwell, is here to explain.

Matt - Unfortunately the answer is slightly counterintuitive. When we think of the universe expanding, we might think of a balloon being blown up or a cake or a loaf of bread rising in the oven. These are the metaphors we often use and it's good in one sense. We can think of a cake rising and it's an object that's expanding a bit like the universe, but it breaks down because outside the cake, there is the space of the oven. And as the cake gets bigger, it gets bigger and bigger into the oven. So what's outside the universe for it to be expanding into? And the answer really is that it's not expanding into anything. I think the trick is to not think of space and time as like a box the universe is in, but think of space and time as properties of the universe. When the universe is expanding, it makes space as it grows, but there's no space outside the universe that it grows into. It makes more space from the inside. So the universe itself is getting bigger, but there is no outside of the universe for it to get bigger into.

James - Halc builds on this point on our forum when they write the expansion of the universe is the metric expansion of space itself. Meaning that each volume of space is later larger. The universe was always infinite, never bounded, and has no centre, no edge. Thanks Halc. So Matt, if the likeness to a cake in an oven isn't satisfactory, are there any other useful comparisons we can use?

Matt - A nice analogy that you might like is to think of the number line. So the line of numbers that you learn in maths in school, you know, 1, 2, 3, 4, 5. If you take that number line and multiply the whole thing by two, you end up with 2, 4, 6, 8, 10. We've expanded the number line, but it's not expanded into anything. Just the spaces between the numbers have grown, just like the space between the galaxies is growing in our universe.

James - Thanks, Matt. I hope that somewhat cleared things up for you, Anne. Thanks for the great question and to Matt Bothwell from the University of Cambridge for helping us with the answer.