I know the galaxies are moving away from us (faster and faster) but when we say the universe is expanding, is that really only pertaining to the galaxies expanding or is the space itself expanding? I know we don't yet know what the boundary is, but I just wanted clarification around the commonly used phrase "Universe Expansion".
Yes, the idea is that the universe actually is expanding -- the "fabric of space" if you will. The common analogy is to think of baking a loaf of raisin bread where the batter is the universe and the raisins are galaxies. As the bread expands while baking, the actual universe is getting larger, and the galaxies are just getting carried along for the ride.
Another common analogy if you want to picture the universe as two dimensions expanding into three (because we think the universe is three dimensions but warped in higher dimensions) is to think of a balloon. Take the balloon, uninflated, and draw dots on it (those are your galaxies). Now blow the balloon up. The surface of the balloon is the universe, and it's the universe that's expanding while the dots (your galaxies) are moving away from each other because of it.
That's really the basic story in a nutshell, though I know it can be hard to understand since we think of the universe as being empty, so how can emptiness expand? There is actually a pretty good Wikipedia article on the expansion of space that has some nice conceptual diagrams illustrating how this works, and the best part -- no equations! :-)
$\begingroup$ Despite hearing it so often, I've never been convinced by the balloon analogy. Don't the dots also expand? If a dot covers 1/100 of the balloon when you draw it, doesn't it still cover 1/100 once you've inflated the balloon? If so, then I feel like the analogy implies that the "galaxy" is expanding too. Of course, maybe I have insufficient experience in inflating balloons... $\endgroup$– WarrickJul 11, 2011 at 7:52
1$\begingroup$ @Warrick Yes, in fact, the dots do expand. Even the space between an atom's nucleus and its electron cloud expands. The galaxy (distance between stars) is expanding, too. However if you do the math you'll find the dots won't cover 1/100 at the start and at the end. $\endgroup$– ghoppeJul 11, 2011 at 20:31
$\begingroup$ @Warrick in short, the greater the distance, the greater the rate of expansion. $\endgroup$– ghoppeJul 11, 2011 at 20:32
1$\begingroup$ @Warrick: The analogy is imperfect since yes, the dots do expand. The better analogy is the raisin bread one, but that doesn't illustrate quite the same thing (2D expanding in 3D). ghoppe is somewhat incorrect, though. At present, galaxies do NOT expand, as they are held together by gravity which is stronger than the expansion force. Same with clusters. There WILL come a time, though, when gravity is NOT strong enough and galaxies will actually be dragged apart by the expansion of space. Don't remember off-hand what that time-frame is, but it's at least in the billions of years away. $\endgroup$ Jul 12, 2011 at 4:22
$\begingroup$ Instead of drawing dots on the balloon, tape coins or something on it. They won't expand as you inflate it - there's your analogy. $\endgroup$– ThrivethJun 7, 2013 at 11:57