# Center of the universe

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#### killium

##### Guest
About the center, they always tell that there is no center. Yet, they are telling the universe is 150billions light years across (if i remember right).<br /><br />If something has a lenght, the center is at half of this lenght no ? How can we "mesure" the lenght of something and, at the same time, try to convince poeple that there is no center ?<br /><br />Maybe there is something i didn't get right ? <div class="Discussion_UserSignature"> </div>

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#### jcdenton

##### Guest
150 billion light years? I think you mean 1/10th of that.<br /><br />When astronomers say there's no centre of the observable universe, they mean that there's no uniform pattern in which the observable universe is laid out around a given point. Of course there's a <b>geometric centre</b> of the observable universe but that's besides the point.<br /> <div class="Discussion_UserSignature"> </div>

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#### Leovinus

##### Guest
Ok, so where is the geometric center of the universe? <div class="Discussion_UserSignature"> </div>

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#### jcdenton

##### Guest
Since the universe is in constant expansion (every point is expanding from every other point), we don't have a proper point of observation therefore we cannot properly tell where the geometric centre of the observable universe is. <div class="Discussion_UserSignature"> </div>

S

##### Guest
It's out there but not as we know it, sorry showing my Star Trek age <img src="/images/icons/laugh.gif" />

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#### igorsboss

##### Guest
The geometric center of the observable universe in which I reside is the bridge of my nose.

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#### killium

##### Guest
Maybe we cannot tell where it is but, we cannot tell that there is NO center either. The center must be somewhere, i mean there is a 3 dimensionnal coordinates that tells the center is "here". Moreover, if the universe is stretching in every direction, the center is immobile.... <div class="Discussion_UserSignature"> </div>

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#### jcdenton

##### Guest
That new size estimate of the universe is new information to me. Anyway, the universe doesn't quite contract like a balloon where the air is expanding outward from the centre, it expands more like a loaf of raisin bread in an oven (old analogy). Imagine each raisin is a galaxy, or galactic supercluster. When you pick a raisin, you'll notice all the other raisins have spread further apart, with the furthest receding the fastest. That's what universal expansion is like, meaning the centre is always changing (even if you stood there for a moment).<br /> <div class="Discussion_UserSignature"> </div>

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#### heyscottie

##### Guest
The problem is that the center of the observable universe, as igorsboss noted, is right here.<br /><br />The universe is expanding directly away from Earth in all directions. That puts Earth at the center.<br /><br />But we have gone beyond the geocentric models of the universe now, haven't we? If we start with the idea that it is so highly improbable for Earth or even the Milky Way or even anywhere within the local group to be the center of the universe, we run up against a problem when we notice that we ARE at the center.<br /><br />How can this be? We are at the center, but we don't believe we are "special" enough to really be at the center. How can we rectify the situation?<br /><br />Easy: ALL points in the universe are the center. That is to say that every point can make an equally valid claim about being in the center. But in reality, none of them is prefered.<br /><br />The reason this is possible is that the Big Bang is not an explosion in space -- it is an explosion OF space. In short, it happened and is happening everywhere at once in the universe.<br /><br />Scott

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#### killium

##### Guest
"The problem is that the center of the observable universe, as igorsboss noted, is right here"<br /><br />I agree that the center of the "observable" universe is here. But what about the whole universe ?<br /><br />"The universe is expanding directly away from Earth in all directions. That puts Earth at the center. "<br /><br />I don't see why this puts the Earth at the center. Every point will have that impression that everything is expanding from it, without other references.....that doesn't change the fact that somewhere exist the geometric center. The universe is still expanding today, but it is no longer a point.<br /> <div class="Discussion_UserSignature"> </div>

M

##### Guest
Going back to the universe-as-a-balloon analogy, there certainly is no physical "center" on the curved surface of the balloon, only the illusion of being centralized.You might respond, then, that the center lies within the balloon, at the middle. But the surface and the center of the balloon existed together only at the balloon’s origin, when it first began to inflate. In a sense, then, there is a center to the universe, but it exists billions of years in the past, when everything was localized in one place. The center is the moment of the big bang.Just as the center lies in the past so does the so-called edge of the universe. Astronomers sometimes refer to the "observable edge" of the universe, but what they mean is the limit of what they can see of the universe with their telescopes. Like the center, the edge is more correctly a boundary in time.

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#### igorsboss

##### Guest
<font color="yellow">ALL points in the universe are the center. That is to say that every point can make an equally valid claim about being in the center. But in reality, none of them is prefered.</font><br /><br />Yes!<br /><br /><font color="yellow">The main clue to understand is that you are always in "your" centre. You cannot detach from yourself.</font><br /><br />Excellent!<br /><br /><font color="yellow">there is a 3 dimensionnal coordinate s</font>stem<br /><br />Since spacetime is warped by gravity, and mass is distributed nonhomogeneously (lumpy), simple 3-dimensional geometry fails to describe the details of the distant universe. Three dimensions yields a decent approximation, but only for short distances from the observer.<br /><br />General relativity is a better theory, but unfortunately the whole theory and everything it describes are both way, way, over our heads.<img src="/images/icons/wink.gif" />

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#### lewcos

##### Guest
There "has" to be a center of the universe. Everything has a center....look at all the galaxies spinning with a center...everything spins with a center, planets, stars, moons and I'm sure the universe does if we could step outside of it and view it.

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#### jcdenton

##### Guest
We don't know if there's an "outside" of the universe. <div class="Discussion_UserSignature"> </div>

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#### lewcos

##### Guest
"We don't know if there's an "outside" of the universe. "<br /><br />I agree, we don't know very much about anything.<br /><br />I would think that since everything else spins, why not the universe?<br /><br />Perhaps we are one of many universes that all spin around a "center" universe, and all those universes are spinning around an even larger one. Perhaps there is no end to how large we can go or how small we can go.

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#### rpmath

##### Guest
<font color="yellow">There "has" to be a center of the universe. Everything has a center...<br /></font><br />And where is the center of Earth surface?<br />Can you signal it in a map?<br />You need to add a 3rd dimension to set a center here, and all the surface points are at the same distance… <br /><font color="yellow"><br />look at all the galaxies spinning with a center...everything spins with a center, planets, stars, moons and I'm sure the universe does if we could step outside of it and view it.<br /></font><br />You can use a 4th dimension to set a center for the universe.<br />I think this dimension can be seen as time. <br />You can see past as down on Earth:<br />If you are at different points of Earth surface that I am, your “down” will signal in a direction different to mine, but both will signal to Earth center.<br /><br />I think the past for different observers will signal in different directions too, but them all will signal to Big Bang.<br />The limit of the universe is the now, the inside is the past, and the outside is future.

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#### worthj1970

##### Guest
I'm curious how all this works. In life everything has a center. The earth's shell doesn't, but only an idiot would say that the earth has no center; it does without question. So if everything we know has a center (and I challenge anyone here to show something that doesn't), why do we assume the universe doesn't? Is it simply that it's conveniently enigmatic to think so? Remember that for thousands and thousands of years the earth was the center of the solar system and universe (and was also square). Why is it so difficult to see the universe as being similar to practically everything else that we can observe?

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#### worthj1970

##### Guest
Not really. Everything has a center. There might not be anything there, but there's a center. If the universe as we know it started as a bang, then there is a central point from which everything expands. (To me this is where the baloon analogy falls apart; a baloon is expanded from on end.)<br /><br />It's confusing to speak of the universe as this great big blob. There are parallels out there (irregular galaxies come to mind), and maybe in the grand scheme of things that's the exception and not the rule. But even in that case there's a central point. So I simply cannot accept that there is no center to the universe, because there is nothing in the universe that we know that has no center.

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#### worthj1970

##### Guest
Much like the edge of a galaxy, I suppose, and there could possibly be similar universes and so on infinitely (I'm not sure why we feel there has to be a finite-ness to things). The reality is that is one thing we'll probably never know, unless by some chance we're somehow poised near that edge and someday can travel to it.

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#### nacnud

##### Guest
Ok try this, the balloon analogy (BA) only works for two dimensions. For a two dimensional person running around on the surface it appears that space is expanding, unbounded and has no centre. Now apply that to the three dimensional universes that we inhabit, however remember that there is not necessarily an extra dimension that is being ignored as in the BA the universe is expanding into and that travelling in one direction long enough may not bring you back to your starting point. Not everything has a centre.<br /><br />Any better?<br />

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#### rpmath

##### Guest
<font color="yellow">I'm curious how all this works. In life everything has a center. The earth's shell doesn't, but only an idiot would say that the earth has no center; it does without question. <br /></font><br />Yes, it does… but the center isn’t in the 2 dimension surface, it is in a 3rd dimension.<br /><font color="yellow"><br />So if everything we know has a center (and I challenge anyone here to show something that doesn't), why do we assume the universe doesn't? Is it simply that it's conveniently enigmatic to think so? <br /></font><br />If the universe is finite but boundless, you need a 4th dimension to find a center, there is no center in the 3d surface.<br /><br />Let’s start from 1d:<br />In 1d you have only line segments, and each segment must have a center. But if you put together both ends of a segment you have a boundless 1d continuum. <br />You don’t have a center of the segment, but you can find the center of the 2d shape it creates.<br /><br />In 2d you can have many bounded shapes, and all of them must have a center, you cannot find a 2d shape without a center. But if you warp a 2d shape in a 3d space and close its bounds, your shape become the 2d surface of a 3d solid, and you need at least a 3rd dimension to find its center…<br /><br />In 3d you can do the same thing…<br />You can have many 3d solids, all of them with a center, but if you warp one of them in 4d, and close its bounds you can have a 4d thing whose surface is a limited, but boundless 3d space.<br />You will need a 4th dimension to find its center…<br /><br />Are you a 3d guy and cannot see in 4 dimensions?<br />well… I am one too… <img src="/images/icons/smile.gif" /><br />I cannot see 4d things but the math seems to work… so I believe it.<br /><font color="yellow"><br />Remember that for thousands and thousands of years the earth was the center of the solar system and universe (and was also square). Why is it so difficult to see the universe as being similar to practically every</font>

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#### rpmath

##### Guest
<font color="yellow">Ok try this, the balloon analogy (BA) only works for two dimensions. <br /></font><br />No, It can work with other number of dimensions too…<br />You can inflate a 1d circle in a 2d space.<br /><font color="yellow"><br />For a two dimensional person running around on the surface it appears that space is expanding, unbounded and has no centre. Now apply that to the three dimensional universes that we inhabit, however remember that there is not necessarily an extra dimension that is being ignored as in the BA the universe is expanding into <br /></font><br />Well… may be an extra dimension is not required to represent the curved universe, but it can be used. An easy way to assign a 4th dimension to the universe is calculating its age, like:<br />r = UniverseAge * c <br />in this case r is like the age of the universe, and the center of the universe is at r = 0 (the Big Bang).<br />You can use other equations to define a 4th dimension, like:<br />r = UniverseAge * v + a * UniverseAge<sup>2</sup> / 2<br />or whatever you need to make the expansion proportional to r, like in the expanding balloon.<br /><font color="yellow"><br />and that travelling in one direction long enough may not bring you back to your starting point. <br /></font><br />I cannot imagine some finite unbounded thing where traveling long enough in one direction you wont return to the starting point… but it will be for things with an open dimension<br /><font color="yellow"><br />Not everything has a centre.<br /></font><br />I was trying to imagine something where I cannot add an extra dimension to define a center,<br />But it can always be done:<br />Just define r=1 for each point of some center-less thing, and you have a center…<br />Well… r in this case is useless but it can be defined…

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#### nacnud

##### Guest
<font color="yellow">No, It can work with other number of dimensions too…<br />You can inflate a 1d circle in a 2d space. </font><br /><br />yep but its not a ballon anymore, thats what I meant<br /><br /><font color="yellow">Well… may be an extra dimension is not required to represent the curved universe, but it can be used... I was trying to imagine something where I cannot add an extra dimension to define a center, </font><br /><br />I agree what I mean was that the extra dimention is not physicaly needed for the universe to exist, it is not needed for the universes to expand into. Anyway you've explained it better than I did, hats off to you<br /><br />

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#### kmarinas86

##### Guest
The number of corners on a circle is zero or infinity... which one? try undefined.<br /><br />What is the center of the universe... is it everywhere or nowhere? perhaps the center of the universe is undefined.<br /><br />In a fractal universe, everything would be relative by the proportion of the size of one object to the other. The proportion of this would be one times a infinitely long rational number. Actually, I think no number is not rational. If we were not constrained by the limitations or obligations of actually writing the number down or computing it, a rational number going on forever in the order to 10^10^100 ^ 1000... .................0.0000000.....00.0.0000.........0000000... digits would concievably come with the number pi (no matter how long)... if a rational number, as defined, can have an arbitarily large length of numerator and denominator... then every number is a rational number... but of course, if it is arbitrarily long, we can call it irrational.

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