I just found this forum this morning, eventually got enough nerve to post a new thread in the relativity section, then came here when I saw this subforum because of questions I've long had about space, the universe, and cosmology. I've spent the last 4 or 5 hours reading this thread, with occasional breaks to read responses to my post in the relativity section, and to respond to them.
So, anyway, first, thank you so much for this thread! It is an excellent introduction. I guess that at some point it would help to condense it into an FAQ of some sort, but I fear one would lose a lot of the insights available from reading through it all.
I have two questions to add, which have puzzled me for a long time. I think this thread has answered one of them, but I want to be sure. I haven't recognized any answer to the second question, but perhaps I've missed something. Given the time it has taken to read through the thread, I haven't gone through the additional 'exercises' recommended as of yet, except to read the SA article about misconceptions about the universe. If one of the other sources will answer either or both of my questions, please just point that out.
First a bit of background. Some aspects of the 'generally accepted' current understanding of the universe I have no problem conceptualizing. For example, that our 3D universe has no edge(s), that there doesn't have to be "something" outside of it to expand into, and that there doesn't have to be a 4th (spacial) dimension to expand 'into' (although there could be a 4th spacial dimension--I think: I am confused by the arguments that stable atoms, etc., cannot exist in more than 3 spacial dimensions).
So, first, the question I think you've answered. Does the universe expand everywhere equally, and in particular, "here"? I have read in rather unreliable other places that there is no expansion where matter is present in substantial quantities, such as within our galaxy. If I have understood this thread properly, however, the correct answer is that the universe, or 'space', whatever that may be, is expanding everywhere, but that locally (anywhere) binding forces continuously bring back matter to its previous size, be that matter individual atoms, or, for example, our bodies, our solar system, or our galaxy itself. Do I have that right? If so, does that 'rule' also apply to our Local Group, or are the galaxies that make it up too far separated for the gravitational forces among them to cause the whole Local Group to continuously 'spring back'?
Or, in other words, for example, does the Andromeda Galaxy approach us at a speed based precisely on the gravitational attraction (and momentum) of it and the Milky Way galaxy, or is the speed slightly reduced by the expansion of space, although not enough to make a practical difference?
More generally, at what point does the strength of gravity become too weak to cause matter to 'spring back' to the shape it had at any given point before the current moment's spacial expansion? (I have no idea if I've phrased that question accurately or even meaningfully.) Or, let me ask it another way: although all galaxies are gravitationally attracted to all other galaxies, obviously most of them are too far separated to overcome the expansion of space, or else there would be no expansion. But how far is too far? Between any two galaxies? Between Local Groups? Between Super Clusters? Or perhaps we don't know?
I'm not happy at all with how I tried to phrase my first question. Let's see if I can do any better on my second. I think it's a tougher question, but more easily asked. Most simply, if the universe is closed (finite), mustn't it have a center? We may not be able to locate it, it may not be within our 3 spacial dimensions, but doesn't it have to exist somewhere?
I understand that if the universe is open/infinite, then the concept of a 'center' is meaningless. I also understand that there is a distinct possibility that the universe is open (even if intuitively, I don't like the idea, and find the idea of a finite universe much more satisfying).
Yet everything I've read states (with insufficient proof, it seems to me) that the universe has no center. Since the universe is larger, presumably much larger, that that portion of it we can see within our horizon dictated by the speed of light (and expansion, etc.), I understand that we presumably have no way of identifying where the center of the universe is. But unless there's some aspect of solid geometry I don't understand (which may be true, of course), a finite universe still has to have a center, no?
Again, that center might be in a 4th, or higher, spacial dimension, if such dimension/s exist/s. Just as the center of the balloon in the balloon analogy is not findable by the folks on the balloon surface, since the center is in the 3rd dimension.
[One side comment: I suggested the center could be in the 4th or 'a higher' spacial dimension because of an analogy from the balloon ultimately. Let us consider a one dimensional world by taking just one line drawn on the balloon... a great circle initially for convenience. This one-dimensional world would have its center in the second dimension, namely at the center of the balloon, but along the plane which bisects the balloon along the great circle. So the center is just 'one dimension' beyond the world itself. But now, take that great circle and make it irregularly wavy along the surface of the balloon. It is still one dimensional (a line), but it's center could only be found in the 3rd spacial dimension, somewhere offcenter of the balloon.
Well, I don't know if my analogy is accurate, but I thought I'd toss it out as well, just to find out.]
I apologize for being so verbose, but if I could get responses to my two questions, I think I'd be much more comfortable with my ability to conceptualize the universe than I have been in many years.