Hi, I’m new here and I wanted to post a question I’ve had for a few years.
From what I remember from HS and college physics, and from TV science shows and such, I picture the Big Bang as an explosion, and the universe as an ever growing sphere. My question is, does the universe have a geometric center? If it does, can it be located (using the speed of gallaxies or something)?
No, because the universe is the big bang. - It is like asking where is the centre of the Earth’s surface.
Oh and because everything is moving away from everything else, wherever you are everything is moving away from you and it looks like you’re in the centre.
Everyone says that I think I’m the center of the universe. I believe the shape is solipsistic.
Lord Kelvin insisted that the only things we can claim to know are the things we can measure. As far as we can tell, and theories point to the same answer, everyone who measures the distances to the outer edges thinks they are at the center. It’s sort of like you are always at the center of the circle of the horizon.
I’m not sure that the shape of the universe (implied by assuming there is a center) is a valid question. That qustion sort of implies that the universe is an object contained within something else, but that isn’t the case.
Why can’t everything be moving away from everything else and there be a center. If everything was moving away from the center, wouldn’t everything still be moving away from everything else (unless the expansion was slowing, I guess)?
PC
In that case, the center wouldn’t be moving away from anything else.
Relativity doesn’t save you there either, because there’s acceleration going on, and you can tell who’s accelerating.
It’s my understanding that we are not necessarily moving away from galaxies in our own local cluster, and will collide with the Andromeda Galaxy in a few zillion years. But everything else we are moving away from.
OK, I get that after the “Big Bang” started, the “Universe” wasn’t just a shockwaves of primordial matter expanding into empty vacuum, it was the fabric of space ITSELF expanding into being.
And I’m lead to believe that the shape of the universe itself is “curved” like the surface of a balloon, and that’s why it can’t be said to have a “center.”
Now, for my truly stupid question…Even if the reason you can’t travel ALONG the “surface of the balloon” to get to a “center of the universe,” why can’t you travel, well, “down,” to what would be the middle of the hollow air-filled bit in a real balloon? I’m sure there’s a good reason why, I just haven’t heard anyone explain it. (That’s not some sort of a snide judgment on explanations I HAVE heard, BTW. I actually haven’t heard ANY explanations at all) This is all, of course, assuming that I haven’t wildly mis-understood* any explanations on this subject that I HAVE heard.
*Which is entirely possible, even probable.
Can anyone enlighten this poor, dim-witted, thread hijacker?
well, if the universe is infinite like I believe it is, then everything, and noting is the center… get my drift?
If it is like the surface of a balloon, which is two-dimensional, then we’d have to move in a direction perpendicular to the three we can comprehend to get to the center.
In a conventional explosion, you have a shock wave expanding out from the center. Outside the shockwave is unaffected. Inside the shockwave, you don’t observe expansion. This is different from what is observed in the universe.
The thing is the recession velocity is proportional to the distance, and this would be true no matter where you measure the recession from. So no point is distinguished by the expansion.
While I think that the exanding balloon analogy is a good analogy, you have to understand the limitations of the analogy. First it assumes a positively curved universe. I beleive that the current thinking is that the universe is open or negatively curved. Second it only illustrates a two dimensional universe imbedded in 3D space. The extra dimension is not real. Just as a two dimensional creature could not leave the surface of its 2D universe to get to the center, we could not leave the universe to find a center, because, by definition, the center cannot be reached.
I have heard that the end of the universe, as far as is detectable by our measuring or detecting tools is really the starting place we are at. Sorta like one of them there ‘whatchamacallit’ strips the has no end if you walk along it. 
Kinda like hitting yourself in the ass with a one way / straight line bullet that acts like a boomerang.:::
Thanks for the explanations. I guess I’ll keep the “center of the horizon” analogy, unless something better comes along 
The center of the universe is at the corners of Watt and El Camino in Sacramento. Do not tell anyone.
My understanding of this is as follows, however I could be way off base…
The balloon analogy is only valid as a representation of the surface of the universe. It’s a surface with no depth, so you can’t go “down” to the center, as there is no center in that example.
An explanation that I have heard that makes it easier for me to understand the whole concept of there being no center of an expanding universe, is to think of the “balloon” starting from such a small point (infinitely small, to be more accurate) that, for all intents and purposes, everything in that point is next to everything else in that point. So, when this point starts to expand (the big bang) there is no center point that everything is expanding from (remember it’s a surface that is expanding so don’t think of it expanding like a ball of matter, or like an explosion). Since the surface is expanding much like the surface of a balloon, and all points on the surface were at one time right next to every other point on the surface (as when the universe was just an infinitely small point), where do you say the “center” of that was? The trick is to not think of the balloon analogy as a 3D representation, as that makes it seem as though everything is moving away from a point in space and the “surface” of the universe is just the outer limit of the universe. I just have a lot of trouble trying to imagine the geometry of the universe, as it seems like the three blind men describing an elephant. Depending on what aspect you are talking about, the description varies wildly.
I think it gets confusing because I also hear the “raisin bread” analogy to describe how everything is moving away from everything else. In that example, you would think of all the galaxies as raisins in the dough, and as the dough rises, all the raisins seem to move away from each other. There isn’t any new dough being added and the raisins are not really moving through the dough, it’s just that everything is expanding equally. The only problem with that analogy is that it makes it seem like there is a “center” to the dough - just like the balloon analogy.
I think it is hard to comprehend because it seems that universe expansion acts like nothing that we can see in the world around us. Freaky, ain’t it?
Well, Vertigo, the problem with the balloon analogy or the raisin bread analogy is that they involve a person observing the object in question (in this case, the universe) from an external point of reference… but there is no “outside the universe” from which to observe it.
The way I describe it to myself… pick a direction and go straight for eternity. Eventually, you’ll wind up back where you started, despite having moved in a straight line.
Ermm…, OK. Imagine if you will that you are on the surface of a balloon…, or in the middle of a lump of dough…
I think it is kinda silly to say that you can’t try to understand the concept as a relative observer in those analogies. Heck if you want to pull them apart, there are lot more issues to go after than being on the “outside” observing something that has no outside.
And I think, for the conceptually challenged like myself, the example of going straight for eternity brings up more questions than it answers. To each his/her own, I guess.
Maybe. This is the case in a positively curved universe, and depending on the topology, it might (but probably isn’t) also the case in a flat or negatively curved universe. But so far as we know, the Universe is probably flat, and we have no indication as of yet that it has any nontrivial topology, so you probably won’t end up back at your starting point.
As for the balloon: It’s possible to get a 2-d surface of constant positive curvature by embedding it in 3-d Euclidean space, and this is what the surface of the balloon is. However, it’s just as possible to mathematically describe such a surface with only two dimensions, no reference at all to a third. For instance, you can measure the angles of a triangle. If the triangle is large compared to the size of the surface, then the sum of the angles will be significantly greater than 180[sup]o[/sup]. Alternately, you can measure the distances between four or more points to determine the curvature, or use a variety of other measures, all without leaving the surface. If the Universe is positively curved, then it might be wrapped around a center in some sort of higher dimension, but then again, it might not be, and we would have no way to tell the difference. On the other hand, we can determine that it’s curved.
So, would a better analogy be the interior of something like a Donut? If you travel along the “ring,” you’d get end up circleing back to your starting point, and if you traveled towards the donut’s “hole,” the space along the inside of the donut’s “skin” would curve, and you’d end up traveling along the inside skin until you got back to you starting point?
Well, lets hope I didn’t mangle my interperetation of both the Universe and baked goods too much.
Ranchoth
Ok, I don’t know how curvature and topology always get mixed up in these center of universe threads. Doughnuts, balloon surfaces, and saddles are all features those consequences of differential geometry (from GR) and actual topology (think, “the way things are shaped”). They really don’t have much to do with a “center” of the universe. So what if I come back around and meet myself in a rotating universe at the same time in space in one topological model? That doesn’t say a thing about a fiducial “center”.
The ONLY center to speak of is the observed center which is a completely arbitrary point (where the observation of the observed universe is made). Such is the center of the “observable” universe, but it has no physical meaning due to the Coperincan Principle.