[gjm11]

The answers from benihana and cromulent are correct, but the following may possibly be a useful thought experiment.

Imagine a balloon (our universe) with lots of ants crawling on it (photons moving at the speed of light). The ants can't walk any faster than 2mm/s. They can't take a message from any part of the balloon to any other faster than that. (And, let's suppose, there's nothing living on the balloon that moves faster than an ant.)

Now someone blows up the balloon; it gets much bigger. Its expansion moves the ants apart much faster than 2mm/s. But it's still true that if you have two points 10mm apart, no ant can get from one to the other in less than 5s.

Suppose the balloon starts off rather small, and then is blown up very rapidly: perhaps it grows abruptly from 100mm across to 1000mm across in less than a second. (The corresponding phenomenon for the universe is called "inflation"; it explains many otherwise puzzling things but no one knows for sure whether it really happened.) Then it may happen that the only ants that have been able to reach one particular place on the balloon (our observatories) have come from a smallish fraction of the balloon (the observable universe).

[phlux]

Epanding into what???

This is what I cant wrap my head around... into what is the universe expanding?

What if the universe is not a balloon - but a doghnut where the outer edges fold back into itself, the overall space it is in can be finite yet the surface is curved away. The light would travel along the curved surface of space-time and thus travel a greater distance than is linear.

[gjm11]

It's not expanding into anything (so far as we know), just expanding. A curved spacetime doesn't need to be, and so far as we know isn't, embedded inside a higher-dimensional not-curved thing in the way that a balloon is embedded in (more or less) flat three-dimensional space. (That sort of embedded picture is useful for the intuition, though.)

So, you might be asking, whatever does it mean to say that space, or spacetime, is curved? It means, e.g., that if you measure the radius and circumference of a small circle very very very accurately, the relationship isn't C = 2 pi r. If the circumference is "too small" then space is positively curved there, like a balloon. If it's "too large" then space is negatively curved there, like a saddle. The curvature might actually be different depending on the orientation of the circle, but if space is the kind of thing we think it is then you basically only need six numbers to tell the whole story at each point of space. (In two dimensions -- the surface of that balloon -- there's only one possible orientation for the circle, so you only need one number to describe the curvature at each point.) Note that you can measure this thing without needing to go "outside" space: the radius and circumference are measured within the space. Distances along the surface of the balloon.

Yup, the universe could be curved and topologically nontrivial in all kinds of interesting ways: it doesn't have to be like the surface of a balloon. (There's some intriguing but inconclusive evidence suggesting that the large-scale geometry of the universe is a "Poincare dodecahedral space": take a dodecahedron, and apply some magic to its faces so that when you try to leave it by one face you come back in by the opposite face.) And yes, light travels along the shortest paths it can within the universe, and doesn't take out-of-the-universe short cuts even if there are any.

[icegreentea]

Most experiments (usually regarding density of the universe, but also some looking at light from really far pulsars) points to the universe being flat. A flat universe also allows for the total energy of the universe to be zero, which is nice from a physicists standpoint, since then the big bang still obeys conservation of energy.