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## Where is the center of the universe?

 Quote by thetexan Are you saying that space can be thought of as the surface of a sphere? I have seen countless examples of the dots on the balloon analogy where the three dimensions are given as layers on an onion. Each dot on each layer getting farther away from each other per layer and the dots getting father away on different layers....every dot in the onion getting farther away. There is no need for a surface analogy for the dots on the onion analogy to work.
These are, in my opinion, a dangerous set of analogies to begin to make. It leads to even more misconceptions than the original balloon analogy! What's trying to be done here, is that people are trying to equate the third dimension in the onion example with time. I.e. the two-dimensional spheres (described by two angular coordinates), and then the radial direction is time. As time increases, the radius of the two dimensional spheres too increase. I personally don't find this very helpful in understanding anything, and think it causes more confusion than good. You have to know the limitations of your analogy before you try to push it very far at all.

 Here is a simple question. Prove, simply, that the universe is something other than what it seems to be...a big 3 dimensional bread loaf. If one cant, then there is the possibility of a center.
Look, if you're going to argue 'you can't prove that X isn't true', then you can leave this forum right now. We're scientists, that's not the way science works. We construct models, which give good predictions, and we have faith in these models because they work. The big bang (complete with no center and all!) is one of these. We can never 'prove' it is true. We do not aim to. But there is an incredible amount of predictive power in the big bang model, and has so far been in accordance with every observation we have made. This is why we put so much stock in the theory, and describe it as if it were 'true'. But you have to understand that it's just a model, like all other physical theories.

You're welcome to come up with a full mathematical model describing a universe with a center or whatever you like, include unicorns if it pleases you. Show that it matches observational data, and publish in a peer reviewed physical journal. Then maybe we can talk about it. Until that time, we'll stick to the theory which has made countless verified predictions.
 The simple answer is yes, I want to understand whatever makes sense scientifically without doing violence to common sense. Im not the expert to determine which is which so I rely on you guys. Im sure it must be frustrating for you with people like me. Im sorry. I dont mean an explosion in the sense of a violent release of energy sending everything out in all directions. I mean that something, whatever it is, expansion, gunpowder or otherwise sent everything out in all directions. In that sense it resembles an explosion (to my thinking) in that everything, space, objects, whatever, was sent outward from the previous location which was a small point. I understand that space itself expanded and in doing so gives takes everything with it. I dont understand why what ever you call it, doesnt result in the same thing....that is....lots of stuff out there....where....it used to be all bunched up here in this little point. What ever you call all of that you have the beginning condition, that of everything in a little point followed by all of that stuff spread out everywhere. That must mean it went from here to there which implies at least the idea of a trajectory as seen from a far off vantage point. I know the idea of a out of system vantage point make seem non-sensical but it helps me to visuallize what happened, if indeed, this is all close to true. There are two points that I cant grasp, the idea of space being a surface of something and why the big loaf of bread idea is not correct. Are you saying that the universe is not 'containable' in a big box as a finite object? Yes, I want to understand. tex

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 Quote by thetexan ... I mean that something, whatever it is, expansion, gunpowder or otherwise sent everything out in all directions. In that sense it resembles an explosion (to my thinking) in that everything, space, objects, whatever, was sent outward from the previous location which was a small point...
I'm trying to think where to begin. Other people may jump in (as some have earlier) and that would be fine. I'll think of something---perhaps not the best.

You know the problem with the "explosion" image is that it occurs at a point in space. that's the basic reason the image is so bad and causes newcomers such confusion.

The universe is ALL SPACE so how could it be concentrated in some "location" located in some other space? There is no other space.

I'm always going to talk about the simplest most standard cosmo model---there are more elaborate fancy models with extra dimensions and crazy extra universes but 9 out of 10 cosmologists never bother with them. The simple standard model is what is used to calculate times/distances with and to compare with observational data.

So the universe at this moment is 3D and it is all space. There is no surrounding space and no edge or boundary to it. It is all space and all existence. That's how the standard model treats it, and it works. Simplest that way.

WE DON'T KNOW whether the volume, at this moment, is finite or infinite. We are getting closer to finding out, but it could still go either way. This confessed ignorance is probably another obstacle to communicating at popular level. Probably TV producers don't want to have a scientist come on the program and say we don't know something as simple as that. So the popularizers fudge or gloss over.

Because it isn't known for sure that the universe is finite, cosmologists of necessity must keep updating both versions. Sometimes showing work in duplicate. They use both the finite version of the standard model and the infinite volume version. More often the infinite version but not so exclusively that it represents a onesided commitment.

In the finite volume case the full universe would in any case be many times larger than the observable portion--- so effectively, for all practical purposes, it gives the same numbers, looks about the same as the infinite case, and fits the data more or less equally well. Better instruments should eventually decide which version to use.

In a 2D analogy there would be two cases: an infinite flat plane or else the surface of a very large balloon so large that it seems flat to the 2D creatures living on it. For them it might as well be an infinite flat plane.
And keep in mind when thinking of any 2D analogy that whichever it is, it is ALL OF SPACE. All existence is concentrated there, there is no above or below the flat plane or inside or outside of the balloon surface.

It takes some care and concentration to think about those 2D analogies.
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Let me know if there are any urgent questions about this so far. I will get back to it. But I'll take a break now.
 Recognitions: Gold Member Thetexan, imagine this. Take ALL of the observable universe, about 40 billion light years in radius and cram it into a cube that is 1 meter in diameter to represent the very early universe. Next to this cube is another cube of the same size that contains all of the universe between 40 billion and 120 billion light years from us in our current time. (Each cube is 80 billion light years across in our time) Do this on each side of this cube. Now make more cubes that contain everything from 120-200 billion light years from us. Now continue to do this. Forever. This is what the very very early universe is thought to have been like. A very very dense state, but still infinite in size. So, now we have a infinite amount of 1 meter cubes everywhere. Now start time up and watch what happens. Each cube expands. From the point of view of ANY particular cube every other cube is being pushed away from it. The further away a particular cube is from your frame of reference cube, the faster it recedes from you, as every cube in between yours and that one is expanding, causing the recession velocity to add up per cube. AKA a cube that is 10 cubes away will recede twice as fast as one 5 cubes away. A cube 100 cubes away will recede 10 times faster than the number 10 cube. So, there is no center, no explosion, nothing like that.
 Recognitions: Gold Member Science Advisor Drakkith, I think part of the trouble is that Discovery Channel viewers are apt to misunderstand the word "singularity" because it sounds like "single point". So they think the cosmologists are telling them that the universe expanded from a single point. As you know if the universe now has an infinite volume of space then it always did. It began expanding from an already infinite volume. As in your image, with your many boxes. One of the original meanings of "singularity" is ODDITY. [Sherlock Holmes might say "Watson, do you not find it singular (i.e. odd, abnormal) that the watchdog did not bark on the night of the alleged robbery?"] In physics, a singularity means a breakdown of a manmade model, anywhere it stops giving meaningful numbers. This does not have to happen only at a single point. A singularity can occur simultaneously throughout a large region. But when people hear the word "singularity" they think of the universe expanding outward from a single point located somewhere in space. It reinforces the "explosion" mistake.

 Quote by thetexan The simple answer is yes, I want to understand whatever makes sense scientifically without doing violence to common sense.
Then I am sorry for you, because the universe does not really respect 'common sense'. Common sense are those things you've learned will happen in a medium small, low-gravity, slow moving realm of the universe. When you get into the very small, and/or high gravity, and/or relativistically moving universe, 'common sense' no longer applies.

So to understand scientifically what's happening in the universe, you have to suspend common sense. It still works fine in it's realm of application, but it doesn't apply very far.

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 Quote by alexg So to understand scientifically what's happening in the universe, you have to suspend common sense...

But I think it's a matter of degree. And people's common sense differs from person to person.

So their common sense restricts people different amounts.

I feel very comfortable with a 3D space that has no edge or boundary, and has a finite volume. I find that commonsensical.

I dont try to imagine it from the outside (it has no surface or surrounding space) I like to imagine it from the inside. Living in such a space, circumnavigating it, shining beams of light in it.

It is commonsense notion for me because the 2D analog is so familiar. The surface of the earth, which is finite area (as the other is finite volume) and has no edge. "Great circle" routes being the "straight lines" because the shortest distance,etc. etc.

But some people's common sense is more restrictive in that department of imagination, more of an impediment. On the other hand there are places where my common sense is undeveloped and handicaps me, where another person might find something intuitive that I don't.

So people differ.

There may be a lot here that Tex eventually will find commonsensical. And of course because it is a mathematical science based on fitting math models to data, there will be plenty more that he will not. But that's to be expected. Just a question of degree.
 It's a hard concept to understand and visualize let alone convey. I like to think of the balloon analogy, but instead of all of the cosmos spread across the entire balloon, our universe is a single dot (not a point) on the surface. Now blow up the balloon a billion times. We can't see any curvature from our tiny perspective because the balloon is just too big, however our dot of a universe still stretches along with the balloon. From the perspective of our universe it will seem to expand from all directions. The balloon analogy would suggest that there could be an inside and an outside, which is not true. At least, we don't know of it. If there is, it would be a 5th dimension. It would also suggest that the space-time would not be expanding uniformly. Which is not what we observe. It appears to be uniform expansion everywhere we look. Being uniform, there's no way for use to tell what direction the expansion is, if there is, so as far as we know there is no center. Any place you go in our universe, it appears to be the center. To help clarify, some people refer to the raisin bread analogy, where the raisins represent matter and the rising bread is space-time, but it still leaves the same implication because I think most people visualize as if they were looking down upon it from the outside. Instead it's better to think of actually being a raisin in that bread. From your perspective, you can't see the boundaries. For all intents and purposes, there is no boundary because as fast as you try to move through the bread, the bread is expanding even faster. Still, there is another question. If space-time is expanding and we extrapolate back to the beginning the result is that all of matter was extremely dense at one time. This makes one wonder what the universe is in, because if it was once smaller and more dense, then it suggests that the universe is suspended in something. Well it is, it's suspended in space-time!
 Well, I dont want to get folks mad or in an argument. I think I still cling to what seems to me to be logical explanations. Clearly, it's gone well beyond intuitive. What I need is a good book that takes a layman and brings into the current theory with something I can understand. I dont think Im illiterate since I have taught college advanced computer programming and and am an air traffic control instructor as well as a former commercial pilot. So I guess I need help understanding. And I do want to learn this stuff. So thanks for your all's help in advance. tex
 First, guys this is my first post. I have been reading on here for awhile and the pure amount of information on this website has blown me away. Thank you to all of you for teaching me so much already. I have a question or two that goes along with Tex's questions so I hope you don't mind me jumping in here. So I understand the 2D balloon analogy is just a comparison to the universe and not a theory to explain the universe. But I heard on this forum that some study was done (I don't remember the acronym) that showed that the universe was either nearly flat or exactly flat. If the universe is not flat and had some curve does that mean if you traveled in one direction would you eventually end up in the same place? For example the surface of the balloon can kinda be considered infinite because if you start in one direction you will continue on forever. Even though you will pass over your starting point many times. Is this how an infinite universe can be considered? Or am I way off base here? If there is no curve to the universe and what I said above is not true (which I suspect it's not) I have a hard time understanding how there can be an infinite universe without a boundary. Also, could someone try to explain the difference between a positive curve and a negative curve? I've seen that referenced a couple of times but I have no idea what the difference is. Thank you.

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 Quote by TallyNole But I heard on this forum that some study was done (I don't remember the acronym) that showed that the universe was either nearly flat or exactly flat. If the universe is not flat and had some curve does that mean if you traveled in one direction would you eventually end up in the same place?... Also, could someone try to explain the difference between a positive curve and a negative curve? I've seen that referenced a couple of times but I have no idea what the difference is...
Thinking in 2D analogy, the surface of a sphere has positive and a saddle surface has negative. The way you measure is draw a triangle (with great circle segments, as if lightrays would follow the surface---shortest distance "geodesics" for the sides of the triangle)

If the angles add up to more than 180 degrees it is positive, like the sphere.
If the angles add up to less, then it is negative curvature, like the saddle. If angles add to exactly 180 then it is zero curvature, i.e. flat. It might for example be an infinite flat plane.

As a pilot, Tex will have no trouble imagining this triangle that adds to 270 instead of 180. Fly east along equator to longitude 0, (I.e. greenwich meridian) then turn left 90 and fly up the longitude line to the north pole then turn left 90 and follow the 90W longitude line back to the equator, finally turn left 90 again and follow equator back to where you started. You have made a triangle and you only turned a total of 270 degrees (three right angles).

On an ideal flat plane going around a triangle involves a total turning of 360 degrees. So another way to express positive curvature is to say there is a DEFICIT ANGLE. It is roughly proportional to the size of the triangle or other figure that you are going around. The larger the triangle (within reason) the larger the deficit. Little triangles have almost no turning deficit because they think they are flat or nearly.

The study you heard about was probably WMAP and they combined a lot of data and got an ERROR BAR around zero for the curvature. Future studies will narrow it further. So far all we know is it could be a little positive or a little bit negative, or exactly zero. But it certainly is nearly zero in any case.

If it turns out to be positive then the most natural model is the 3D analog of a sphere,and the answer to your question is YES you could circumnavigate such a thing as long as it was not expanding. Expansion, if too fast, would defeat your attempt to circumnavigate.

A 3D sphere is sometimes called a 'hypersphere" or a "3-sphere" or "S3". Dont worry about the jargon, it is just the 3D analog of the 2D surface of the earth or of a balloon. It is considered FINITE volume. It has no boundary.

A finite thing can either have a boundary or not have a boundary.

A finite thing does not have to be immersed in any larger something. It might exist entirely on its own.

The geometry of a thing does not have to be seen from the outside, it can be explored and probed and measured and experienced from the inside by the creatures that live there. This is good because there may not be any outside vantage point. So a lot of geometry is done from an intrinsic perspective. Like going around triangles and other loops and seeing how much you turned.
 Recognitions: Gold Member Science Advisor Tex, you have impressive qualifications. Commercial pilot---traffic-control instructor. It makes sense to me that you would not have a hard time learning some modern differential geometry (the geometry of curved surfaces and 3D spaces often as experienced from within by sort of calculus methods.) I can't say anything for sure but it would not surprise me if you lucked out, found a decent textbook suitable to your needs, and showed up in a few months knowing some differential geometry. That is the math basis of General Relativity (ask in the Relativity forum what they think is a good entrylevel diff. geom. text. get it from the library, don't buy, in case it is not right for you) General Relativity is the math basis of cosmology. Except that cosmology is much much simpler because we have approximately evenly distributed matter so that the curvature is approximately uniform, and we have the almost uniform ancient light (called CMB) which is like a kind of navigation aid because it tells you when the universe thinks you are at rest, sitting still with zero Doppler hotspot in your sky, at rest relative to ancient light. So conceptually cosmology is way easier than GR. However GR is the math basis. cosmology is a simplified special case with extra tools. I would suggest you start a thread in Relativity forum that just says what is a good entrylevel GR book? Get a moderator's attention. they like to be helpful. Ask if you need some prerequisite diff. geom. or if the GR text is selfcontained. What can you lose? You don't HAVE to take their suggestions but at least you can scope it out that way. But if you DONT find a book that is right for you, you can still learn quite a lot just by talking with people here in an informal motley un-academic way.

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 Quote by thetexan The simple answer is yes, I want to understand whatever makes sense scientifically without doing violence to common sense.
You have to let go of that common sense to understand quantum mechanics, relativity, or cosmology. Your common sense is a result of repeated experiences in one very, very narrow spectrum of physical interactions. Most people's common sense is markedly Aristotelean. Even Newtonian physics goes against common sense to some extent. The quantum world, the relativistic world, and the universe as a whole are far removed from the experiences that formed the basis for your common sense. Your common sense doesn't apply. It's one of the first things a young physicist has to learn.

You yourself have already had to learn to let go of your common sense. You mentioned that you are a pilot. Flying is anything but intuitive, at least to most people. That's why so much training is needed. There are some situations in flying that completely defy common sense.

In quantum mechanics, relativity, and cosmology, almost everything completely defies your common sense. That doesn't mean it's wrong. It is your common sense that is out of whack. Your common sense is orders of magnitude outside of its range of applicability.
 Recognitions: Gold Member Science Advisor Tex, here is a free 9 page article by a world-class academic James Hartle: http://arxiv.org/abs/gr-qc/0506075 click on PDF He gives his thoughts about teaching Gen Rel to undergraduates. How to motivate it, make it real, not abstract. He lists a bunch of APPLICATIONS of GR that will make it interesting to undergrads. And other people I think. Hartle has a textbook called "Gravity" that sells for $70. It would be at a local college library in the math or physics section. I'm getting curious. Does he use his own philosophy about applications to make his textbook interesting? Another essay by a reputable guy that i personally find less interesting, Robert Wald, giving his thoughts about teaching Gen Rel:http://arxiv.org/abs/gr-qc/0511073 He lists and evaluates the available textbooks and gives high marks to Geroch, General Relativity from A to B. This is only$9.50 and has no math. I didn't like what I saw of it. But Wald is a reputable guy and said it is an excellent introduction. He could be right and I wrong. Wald himself also has a GR textbook. Personally I think Hartle is more creative and has more interesting ideas about how to motivate and explain. I would say forget the Wald article, and forget his Geroch recommendation and I would read the Hartle article and I'm actually tempted (not to buy but) to track down his textbook at some library. I'm curious. Just my personal attitude. There is a free online textbook by Sean Carroll that some people I've talked to like. I'll get a link. http://relativity.livingreviews.org/...es/lrr-2001-1/ This is a short clearly written online COSMOLOGY text. It was written 2001 but has been updated until around 2008, a lot of simple math formulas. Mathy but nothing terribly hard. And anyway its free. You can read it online in HTML or download it PDF. No pictures. Here is a "no nonsense" short intro to GR and cosmology in 24 pages by Carroll: http://preposterousuniverse.com/grnotes/grtinypdf.pdf Here is a 230 page set of lecture notes by Carroll written in 1997 http://arxiv.org/abs/gr-qc/9712019 basically a free GR textbook! He is a talented communicator. But my mind keeps going back to that 9 page article by Hartle
 Recognitions: Gold Member Science Advisor We have reached the point where it appears new physics is required to answer the remaining 'big questions', IMO. Scientists have never widely accepted the idea that existing knowledge is a 'horizon' on our ability to understand the universe, AFAIK. To paraphrase Sherlock Holmes - the mission of science is to eliminate the impossible. Whatever remains, however improbable, includes the truth - The universe is a cruel, yet irresistable mistress.

If our universe has to have a center it would exist in a higher dimension and not in our 3d space.

 Quote by thetexan Well, I dont want to get folks mad or in an argument. I think I still cling to what seems to me to be logical explanations. Clearly, it's gone well beyond intuitive. What I need is a good book that takes a layman and brings into the current theory with something I can understand. I dont think Im illiterate since I have taught college advanced computer programming and and am an air traffic control instructor as well as a former commercial pilot. So I guess I need help understanding. And I do want to learn this stuff. So thanks for your all's help in advance. tex
What exactly are you having trouble understanding?

 Quote by TallyNole First, guys this is my first post. I have been reading on here for awhile and the pure amount of information on this website has blown me away. Thank you to all of you for teaching me so much already. I have a question or two that goes along with Tex's questions so I hope you don't mind me jumping in here. So I understand the 2D balloon analogy is just a comparison to the universe and not a theory to explain the universe. But I heard on this forum that some study was done (I don't remember the acronym) that showed that the universe was either nearly flat or exactly flat. If the universe is not flat and had some curve does that mean if you traveled in one direction would you eventually end up in the same place? For example the surface of the balloon can kinda be considered infinite because if you start in one direction you will continue on forever. Even though you will pass over your starting point many times. Is this how an infinite universe can be considered? Or am I way off base here? If there is no curve to the universe and what I said above is not true (which I suspect it's not) I have a hard time understanding how there can be an infinite universe without a boundary. Also, could someone try to explain the difference between a positive curve and a negative curve? I've seen that referenced a couple of times but I have no idea what the difference is. Thank you.
Positive curvature would be like the outside surface of a sphere and negative curvature would be like the inside. The angles of a triangle would add up to more than 180º under positive curvature and less than 180º under negative curvature.

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