If the universe is expanding, what is it expanding into? Does the concept of ‘expanding into’ even exist?
It’s not expanding into anything. By definition space is all there is.
Yes, this is a hard concept for almost everyone.
Related question, is there any difference between the universe expanding and everything in it shrinking?
Yes. Space can expand but matter appears to be fixed. Additionally, if the earth shrank to half its size, that’s a matter of 4000 miles. That would hardly explain the expansion of space over 10 billion light years or 600 billion trillion miles. And all of our measurements of space would fail if we assumed that the measure of a mile itself changed over time.
Could one say that the Universe is expanding into “nothing?” In other words, creating nothing into something?
Whatever’s happening, it’s happening faster than the speed of light, which does my head in almost as much.
Yea, except the creation isn’t happening at an “edge” but is equally spaced out through the universe. Every unit of space gets more…err…space in it per unit time. So the end result is that objects appear to move apart at a rate proportional to their distance.
The tried and true metaphor to explain it is to picture galaxies as points on the surface of a balloon. The surface of the balloon doesn’t have an edge, but when you inflate it, the points of away from each other, and the further the distance between the points (along the surface of the balloon), the faster the move apart.
unfortunately, we can hold a balloon and see this occurring. We can also understand the concept of the balloon now occupying a region that was occupied by air. However, it’s hard to understand a balloon just occupying more something without the something being there.
Ugh, science is hard.
Here’s another way to think of it: Suppose you have an infinite sheet of graph paper. At one time, the squares on the graph paper are 5 millimeters on a side. A little while later, they’re 6 millimeters, then 7 millimeters, and so on. It seems clear that the graph paper is expanding, even though it’s always infinite and doesn’t have an edge.
Even nothing would be something. You could define it and its properties and tell when it vanished. There is not even nothing outside of space. Space is everything.
Yes, this is unlike anything in our everyday experience and so even analogies have their limitations. It’s really more a metaphysical understanding than a physical one.
But as I’ve said before, why do people think they should be able to understand the universe? That we think we have a bare grip on its properties is pretty amazing. For our tiny human brains to truly and deeply comprehend things that are orders of magnitude bigger, smaller, older, hotter, or weirder than anything humanity has ever been exposed to in its history is asking too much.
Are you saying space exists outside of the universe? If not, I still don’t understand.
So the universe was infinite in size even when it was just a split second old?
If it IS infinite, then yes it was infinite from the beginning. It may not be, in which case the term would be “finite but unbounded”. Like the surface of a sphere; it’s finite, but you won’t find an edge.
In a related question, does it mean that the subatomic space between the electron and the nucleus is expanding as well, while the particles remain the same size?
Nothing exists outside the universe. Including nothing.
I don’t know how to describe it any other way. I realize it’s counterintuitive because we sit inside space. We don’t have anything to compare space to.
Try it just way. Everywhere we look is all there is. Sometimes we look away and when we look back there’s more over there. Where did the more come from? From the inside. Does that mean there’s an outside? Nope. There’s just inside and then there’s more inside.
No, everything that’s bound together by some force isn’t going to expand away from each other. There is, however, a theoretical scenario, known as the Big Rip, in which a progressively accelerating expansion eventually tears all matter apart – basically, the mechanism is that the point beyond which things recede faster than light speed from each other comes ever closer to any given observer, eventually shrinking the size of the observable (and hence, interacting) universe to below atomic levels, so that everything recedes from everything else faster than light speed.
It’s perhaps more intuitive to say that ‘outside the universe’ is just not a meaningful location – imagine, for instance, the universe as a two-dimensional plane: there’s nothing above of below it simply because there’s no above or below; there’s just the two dimensions of length and width. Outside the universe is kind of like that, or like ‘north of the northpole’ – it’s not that it’s a place where there’s nothing, it’s simply not a place.
Nothing. Which is literally “No Thing”. There are no things that exist outside of the Universe, except possible other universes.
But it is very probably not infinite.
Which would mean that you cannot go 20 light years beyond the edge of the Universe, because there is no space-time in which to travel beyond the edge.
Now, assuming that you could find the edge, what would you find there? Your guess is as good as anyone elses. An expanding particle wavefront, most likely, creating or dragging space-time in it’s wake. If you passed it and kept going, you would yourself be that expanding wavefront.
So basically, the space is expanding between the particles, but the atomic forces are keeping the atom together as usual. Yet, there’s the possibility that the expansion of space will eventually hit a breaking point, come a googol or so years, and rip all matter apart at light speed?
But your ruler would always show the sides as 5mm because your ruler would be expanding at the same rate, right?
-Joe
It depends a bit on how exactly dark energy works. If it’s a simple cosmological constant, that essentially amounts to a repulsive force proportional to the distance between two objects; the net effect of this is that the equilibrium state of a bound configuration is a bit larger than it otherwise might have been, with ‘bit’ here meaning ‘undetectably little’. Most observations, as far as I’m aware, favour this option. If this is an accurate model of dark energy, there won’t be any shredding, since a cosmological constant is static in time and space (else, it wouldn’t be much of a constant).
But if dark energy is allowed to vary, taking on different values at different points in space at different times, things might look different. Such models are known as ‘Quintessence’, a special case of which, called Phantom Energy, would lead to the Big Rip scenario mentioned above.
For this kind of stuff, though, Chronos seems to be the resident expert, so for any additional questions, he might better be able to help, drawing on his advantage of, you know, actually knowing this stuff. 
Doesn’t mean I’m not always happy to give it a shot, though.
Where do other universes, the multi-verse concept fit in to all this?