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Is there any realistic, understandable, provable (even in some extent) explanation/model for the extent of the universe? What is its shape? and Why? I mean physical explanations not philosophical since the question is about a physical entity, the universe. When I try to think about this it is almost I cannot think at all!

note I am almost satisfied that there is no answer for this single question! I paid attention to all comments and spent time for watching the video-lectures recommended (I don't recommend) just to seek the answer, however, I got nothing about the question at all. The discussion and lectures are mathematical games far being physical to me. I respect all the science involved in cosmology but for this question even great researcher (some got Nobel prize recently) have nothing to say except playing with formulas and graphs. I was looking for physical meaning.

I lost my interest in this question.

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This is a couple of questions rolled into one; where this isn't always a problem, there are a couple of existing stand-alone question / answer sets that touch on these topics: here, here and here. With this, and the current format, it makes it difficult to close as a duplicate of any one of these questions - it is at risk of being closed otherwise, based on community input. –  Grant Thomas Oct 3 '11 at 15:24
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5 Answers

up vote 15 down vote accepted

I have split my answer into two different sections (shape and extent). Although the two are very much related in a physical and mathematical sense, they illustrate different aspects of the cosmology of our Universe. I have allowed myself to post a rather long answer, as I believe that the questions posed here cannot be answered easily by a simple statement. Instead it is my belief that the answers have to be approached from several directions, all rooted in mathematics and physics, but nevertheless converging on a more philosophical ground. Philosophical grounds are, after all, the common starting place for some of the most exciting theories within physics.

Shape

As for the shape of the Universe, this depends on the overall curvature (which is sort of the same thing). The curvature depends on the amount of matter and energy in the Universe, through Einstein's equations. The curvature and energy content can thus be estimated using a cosmological model (the typical one is $\Lambda$CDM). The curvature effects the expansion history of the Universe, which can be probed using large-scale structures, like galaxy clusters, and the cosmic microwave background radiation (CMB). Using this cosmological model along with observations, one can calculate the likelihood of the parameters in the model. Doing this one gets a curvature equal to, or very close to, zero. The analogue of which is a flat, smooth and infinitely extended bedsheet.

The curvature then appears as a natural parameter in the geometrical model of spacetime. How you choose to interpret this parameter is entirely up to you. Many people like to think of it in terms of analogies. The shape of the Universe, or spacetime in GR, is in mathematical terms analogous to the shape of other geometrical objects, like oranges or saddles. Since the spacetime in GR is 4-dimensional, the only physical analogies we can think of will be lower-dimensional equivalents hereof. Assuming that GR is the only theory we should use on large scales, these analogies are actually very accurate. This is not necessarily the case for more general cosmological theories, some of which propose so-called topological defects (eg. cosmic strings) and other geometrical objects which are not so easily visualized.

Thinking about the shape of the Universe is thus a very interesting and highly non-trivial endeavor.

Extent

The extent of the Universe is also a very interesting and non-trivial thing. Some solutions suggest that the Universe may be infinite. Others suggest that is is finite, but with no boundary (like the 2D surface of the Earth). Now there are a few natural ingredients, which turns the discussion in an even more interesting direction. These are: the finite speed of light, the finite age of the Universe and the expansion of the Universe. You might be familiar with Olbers' paradox. If not it is highly recommended reading.

Basically it illustrates our inability to see infinitely far away in the Universe. Thereby the term 'The Observable Universe'. This is actually more a statement about causality than of technological limitation. Because the speed of light is finite, and because the Universe is expanding, there is a limit to the distance to which we can send and receive signals. We can actually still observe some very distant objects, which emitted their light a very long time ago, but we will never be able to send a signal back to them, because the Universe is expanding too quickly, so the light will never be able to catch up with it, even if it travels for an infinite amount of time. This example illustrates the core of the matter about the extent of the Universe, which is horizons. Beyond these horizons we can never hope to reach, with our current understanding of things.

Another interesting consequence of the expansion of the Universe is that far away objects are sliding out of our horizon. Because of this you could say that the amount of content (of visible matter) in the observable Universe is actually decreasing, while the Universe in itself is ever-expanding.

The reason for bringing up this matter of horizons is that making statements about the Universe beyond the horizons (and out to infinity beyond) is on the verge of philosophy. This does not mean that one should stop thinking about it, or ask questions, quite the contrary. But it means that consensus is often hard to come by. Many exotic ideas exist about the nature of the un-observable Universe. These include multiverse, eternal inflation, local void, and many others. I suggest reading about those if this is of interest.

I'd like to end on a reference which is very much on the point of horizons. It features some very informative diagrams, which illustrate some of the different cosmological horizons that are present in our Universe. I should warn that they are not easily understood, even by professionals, but they convey a lot of information and insight, if given some time.

[1] T. M. Davis, “Fundamental Aspects of the Expansion of the Universe and Cosmic Horizons,” Feb. 2004.

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+1 This is a good explanation and a very well written, Thanks a lot for encouraging to think again about such interesting even so difficult subjects. –  Developer Jan 15 '12 at 14:53
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The universe, for the purposes of science, is the visible universe, as anything else is excluded by positivism. It's a sphere with us at the center, with a wall at 13.7 billion light-years away (as measured by defining "now" along a light-ray starting on the earth, going out in a backwards-oriented cone), at which the big-bang can be thought of as still banging, and this wall is like a black hole horizon, and which is flying outward at slightly less than the speed of light, and which will slow down more at some point in the future when dark-energy dominates, and then stop and just sit there, sucking everything away from us until the whole observable universe is a horizon surrounded almost empty de-Sitter space (just us and some neighboring galaxies) with a cosmological horizon spherical wall at a radius of something like 70 bn. lightyears.

That's it. That's the universe. Everything else is unwarranted extrapolation using models which are of dubious validity outside the cosmological horizon, and the extrapolations are not necessary, since they do not affect observations directly.

The shape of the universe is spherical around any observer, so we are not in a special place. This is a little counterintuitive.

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I believe he is asking outside of observable proof which as I have come to know and love. Science requires observable proof even in a future sense that at some point we will be able to observe what this question is asking,there is no possibility of observing the Actual bounds the only answer is physically there is a growing un-observable boundry somewhere outside "our" observation and being unable to measure that boundry against scientific principle in a scientific sense the universe is finite and growing at a finite rate that will be increased only with observable proof not speculation –  Argus May 20 '12 at 18:14
    
@Argus: Yeah I agree, but you could say it more clearly. –  Ron Maimon May 20 '12 at 18:26
    
as I am known to be unclear I figure it would be pertanint to maintain that sense of hap-hazardly combining many points of view and leaveing the observer to establish what it "Means" to them. To Appease Ron i would say "If you have no observations then all you have is Speculation, Not Science". –  Argus May 20 '12 at 18:31
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@Anixx: Not if you define the width properly (in the proper parametrization of past going light rays), then the universe is 13.7 billion light years in radius. The idea that it is bigger is through some ridiculous alternate parametrizations of the radius that do a mix of past-light-cone and "present" extrapolation using cosmological notions of time that I don't like and are somewhat incoherent. The best way is the obvious way--- age along a light beam. –  Ron Maimon May 21 '12 at 3:08
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@Anixx: It is not complete nonsense, it is absolutely correct. You just go out along a light-ray, and the distance is proportional to the time, until you get to the big-bang. This is the extend of the visible universe. There is nothing more to it. The reason this is mangled in other presentations is because people extrapolate the universe to give a nonsense "present radius". Why do you say it is nonsense? It is obvious that you go out 13.7 billion light years in any direction to the big bang (minus a little bit because of the dark energy). –  Ron Maimon May 21 '12 at 3:27
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If we could observe the universe in its entirety we would know its size. However, the extent of the universe remains unknown. The observable universe consists of what can be seen with telescopes and is currently limited by telescope technology.

The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years),3 putting the edge of the observable universe at about 46–47 billion light-years away.[4][5]

-Wikipedia (above link)

[edit] Here is a Model of how unimaginably huge the solar system is: If the sun were an 8inch bowling ball, Pluto would be the size of a pinhead 1 kilometer away. The Solar system extends as far as comets, which go 25 times as far away as Pluto - or 25km in the model.

The nearest star is 265.6 times as far away as comets, which in the model is 6,640 km. So if the sun were the size of an 8 inch bowling ball, the nearest star would be as far away as the earth's core (the earth's radius). The radius of the known universe is 10 billion times further away than the nearest star, which in our model is 66.4 trillion km away. The nearest star is 39.7 trillion km away, so if the sun were 8 inches in diameter the extent of the known universe would be 1 and a half times as far away as the nearest star. My brain is about to explode! Please up-vote before it does.

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I watched some movies recommended above and am not well satisfied with the explanations. Although one of the presenters recently got a Nobel prize but I see the explanation out of my understanding. I want a realistic picture since the universe is an existing entity. I see most explanations are mathematical based has no sense to me, there are only for modeling but don't look realistic, I think. I see your answer helpful (and will mark it as answer hopefully) and hope you can give me a bit more physical meaning of those numbers. What is behind those distances you mentioned? –  Developer Oct 6 '11 at 8:12
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humans can't visualise 28 billion parsecs, but that is the diameter of the observable universe - those numbers are entirely physical, just very very big. As @Joe said, that is as far as our telescopes can see, and as our telescopes get better we can see further. –  Rory Alsop Oct 6 '11 at 13:41
    
@Rory Alsop - Does this mean that the universe is infinite? The theory of curvature of time-space looks just theory far from being physical to me. If there is no clear answer yet I have to accept it, however physicist should avoid being trapped in mathematical abstract thoughts. Thanks anyway for your comments. I'm still have no imagination of what can be behind that distance you've said. This is big failure why human (at least me) cannot even imagine it! –  Developer Oct 6 '11 at 15:44
    
@Developer - no, it doesn't mean the universe is infinite (I don't know whether the maths suggests that or not) all it means is that we can see a long long way but that isn't right to the edge of the universe. –  Rory Alsop Oct 6 '11 at 16:15
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I believe I may be able to help you visualize this question(without math).

lets start with the universe at the beginning for example the entire universe is for simplicity 1 mile across.

The pressure at this size is extreme so everything is pushing on the thing next to it. So once these things push farther and farther the universe grows really fast at first because of the high pressure.

This can be visualized as a drop of water spreading out over the survace of a glass table thinner and thinner.

You are asking physically how much can that one drop "spread" the simpliest physical answer is there is no limit(infinite).

The water will spread forever as the pressure decreases you expect the speed of the spreading to slow but in a frictionless enviorenment there is nothing to slow down the water the original pressure keeps pushing increasing the speed again and again.

if you are at the beginning (or anywhere in the middle for that matter) the water on the edges will get so far away you can not see them anymore. So once out of your view you can not say with 100% accuracy how far away the Edges(Un-Observable Universe) are you can only describe physical horizon(Observable Universe).

See no math you can never get there but it is there somewhere.(so big it is not within normal reason to bring it into the equations hence = Infinite is just really saying "We do not have numbers that big". new math is used everyday and new thoeories are proven time and time again we need to revise our "Understanding" what we can imagine and what exists slowly start to aggree with each other as we find new ways to measure previosly "Infinite" ideas.

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to clarify newton did not see a gravitational equation fall from the tree. It was a physical object that he tacked an number on and another number for a bigger object"earth" and the math helped him explain the differences between these objects. Usually, the idea preceeds the math to prove it. if there was no algebra Einstein would still be a raving loonatic. Silly to say it like that but you are just guessing untill the math backs you up. –  Argus May 20 '12 at 18:04
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If we could figure out the source of 'matter' and 'space' in the universe we will get all answers regarding the Universe.

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protected by Qmechanic Jun 24 '13 at 10:48

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