> ...but the idea that space is expanding implies an outer "edge"

No, it doesn't.

For example: consider an infinite number of marbles in an infinitely long line, all of them are touching each other.

Now, get an infinite number of helpers stationed along the infinitely long line, and have them all move the marbles in front of them to the right and stagger them out in the process, so that there's now an inch between each marble.

You now have an infinite number of marbles an infinitely long line, but it's a longer line than it started out, because there's now an inch between all the marbles.

You will need infinite amount of energy to insert spaces between these marbles. Can you point us to source of this energy?

The theory of the Big Bang (or at least the variants with an infinite universe) includes infinite energy as part of the initial singularity.

A less brain melting way to imagine the "expansion without edge" thing is the classic balloon system. Imagine your entire universe exists across the plain of a semi-deflated balloon and then someone slowly begins to blow it up. The "space" in your 2D world is expanding, but there are no edges.

Which leads to the follow up question: if you travel far enough in one direction do you end up where you started? (well, no, not if space expands faster than c).

If it wasn't for the expansion, and if the universe had positive curvature, then yes.

As it stands, there's not only the expansion, but the universe also seems to be flat -- meaning it's spatially infinite, not just borderless. That observation is also compatible with a very, very small curvature, but in that case the size would still be so large that it wouldn't be practically possible to circumnavigate even if it wasn't expanding.

> * ...but the idea that space is expanding implies an outer "edge"

Space expension just means that the metric used to measure distance increases with time. It doesn't imply a border.

Consider the function equivalent to the Euclidian distance on a line : d(x1,x2) = sqrt((x2-x1)^2)

Now, you can consider the function : d2(x1,x2,y) = y * d(x1,x2)

If you view x1 and x2 as positions on one axis and y as a position on the other one, you can think of d2 as the distance between x1 and x2 expanding as you go higher in the plane.

Space expansion works in the same way but y is actually time.

Others have mentioned the "expanding ballon" analogy. But personally I find this more intuitive:

Imagine you are baking sweet rolls with raisins in them. When you are proofing your dough, the yeast makes the dough expand in each point, so all the raisins become further apart.

Now imagine starting with an infinite dough. As it is proofing, it will still be infinite, but there will be a measurable increase in the distance between raisins, and the speed of how quickly a specific raisin moves away from another one increases linearly with how far they are apart when you start measuring.

That's all "expansion of space" really is. We can see other stars moving away from our own, and the stars far away are moving away faster than those close to us. And since looking outwards is looking backwards in time, we can only see as far away as corresponds to the first stars that were formed.

And maybe you've heard that we can observe "expansion is slowing down". That is actually the fact that we measure a deviation from linearity in speed vs. distance.

Layman here. To your second bullet, yes, the universe can be infinite but also be expanding. A rough analogy I've heard is that there are infinite points on the surface of a balloon, but if you blow it up more, the distance between any two given points will be greater than it was, so it has expanded. I don't think we know if the universe is infinitely large or not though.

We don't know, but the universe appears to be perfectly flat, which would indicate it's infinite.

That could also just be due to inflation, mind.

[0,1] is not less infinite than [0,2]. Also the less does not need quotes, there is an ordering of infinities.

Edit: typo