Expanding Universe Question

[smitadesai]

Yesterday some of us friends were sitting around and had this question over a cup of great coffee (or tea).

The discussion arose from a conversation about infinity.
The universe is expanding and so the question is it is expanding into what? What is that outer shell?

Any thoughts?

Thanks,
Smita

[Markus Schwarz]

Dear Smita,

the universe is not expanding into anything. While the expansion is often described by an inflating balloon in popular science books, this analogy fails when one asks your question. On cosmological scales the distance between two objects, galaxy clusters say, is indeed increasing with time. It can be described solely in terms of "intrinsic" quantities that don't require any extra dimension. This can well described mathematically. Unfortunately, it is not very intuitive and the balloon analogy is the closest I know of.

So far, there has been no evidence of extra dimensions, be they large or small.

Hope that helped!

[smitadesai]

Thank you, Markus.

[Chris Peterson]

the universe is not expanding into anything. While the expansion is often described by an inflating balloon in popular science books, this analogy fails when one asks your question. On cosmological scales the distance between two objects, galaxy clusters say, is indeed increasing with time. It can be described solely in terms of "intrinsic" quantities that don't require any extra dimension. This can well described mathematically. Unfortunately, it is not very intuitive and the balloon analogy is the closest I know of.

I don't think the analogy fails at all. The "outward" axis of the balloon is the dimension we call time. It expands in that direction, but we can only observe the spatial expansion of the surface- which is not itself expanding into anything. The same can be said for the actual Universe: it is expanding in time, but we cannot observe that (although we can understand it mathematically). What we observe is a 3D spatial expansion- again, like the balloon, an expansion that isn't into anything.