Middle of the universe

[michel123456]

Basically, no matter where you go the universe looks the same. The large scale structure of the universe looks the same to someone in a galaxy that's 40 billion light years away as it does to us. It also looks the same to someone that's ten, twenty, or thirty billion light years away. The large scale structure of the universe looks the same to everyone - no matter where they are (it's homogeneous).

 

Likewise, all these widely separated observers would see that the large scale structure of the universe looks about the same no matter which direction they look (it's isotropic).

 

Yes.

With a little comment.

"Someone that's that's ten, twenty, or thirty billion light years away" can be 2 different things:

1. someone that is today so far away is supposed to observe roughly the same thing as we do today, but we cannot observe that guy, and he cannot observe us.

2. someone that was a long time ago so far away is supposed to having observed a different universe* than what we observe today**. We can see this guy in our telescopes, he cannot see us.

 

* a universe closer to the Big Bang and thus denser.

** I find that idea totally absurd, but scientific consensus says otherwise.

Edited June 7, 2011 by michel123456

[caharris]

In Guth's original theory of inflation he thought that the inflation field was the Higgs field. It was later determined by others that the Higgs field cannot fulfill the role of the inflation field.

 

As of now the inflation field is a separate scalar field of unknown origin, assuming that it exists at all.

That's what I get for only being halfway done with his book, eh?

[steevey]

As far as we can tell, the universe has no physical boundary and extends indefinitely or infinitely. The totality of the universe by that means cannot have a "center", however its possible there may have been a single point that all visible matter in the universe originated. This point cannot be concluded due to all the time that has past, the interactions with galaxies among each other and the size of the region where matter lies combined with the acceleration of galaxies away from each other. With the acceleration, every galaxy seems to be moving away from many other galaxy so that no single point can actually be traced back using straight intercepting lines of the trajectory of the galaxies. But, if you play that motion backwards, the matter in the universe does seem to come closer together. Researches strongly supported this when they found the Cosmic background radiation, the left over radiation from the earlier stages of the universe that mapped where much matter was located at that earlier point in time.

Edited June 7, 2011 by steevey

[csmyth3025]

As far as we can tell, the universe has no physical boundary and extends indefinitely or infinitely...

 

...its possible there may have been a single point that all visible matter in the universe originated...

I think the jury is still out on the question of whether or not the universe extends infinitely. As far as I know there isn't any conclusive evidence either way. It's my understanding that the Lambda-CDM standard cosomological model allows for the universe to be finite and unbounded or infinite.

 

I believe the generally accepted notion is that the '...single point that all visible matter in the universe originated..." was the entire universe at that time. If every point in the universe was at the same point when expansion began, then one could say that every point in the universe is the center of the universe. This is one of the reasons why the question "Does the universe have a center?" has no meaningful answer.

 

To use the old balloon analogy - Suppose you have a very small, but very elactic balloon the size of a pea and it's completely covered with shoulder-to-shoulder dots. You then blow it up to the size of the Earth. No matter which dot you point to and say "That's the center of all this expansion!", you would be right.

 

Chris

[DrRocket]

Perhaps the 'center' means the center of gravity of a grouping?

 

I imagine that the center of our solar system would be considered the sun. And I think we have a good idea where the center of our galaxy is. That makes me think we can point to the location of the center of our local group and the supercluster we are part of. Is it wrong to think that we could continue the process (assuming we had enough data to do so) of picking the center of ever larger groupings until we have identified the center of the universe? At what point does this process break down?

 

Is my confusion over understanding why the 'center' of the universe is not meaningful likely because I don't understand things like isotropic intrinsic manifolds?

 

The universe appears to be homogeneous on the largest scales, so mass is uniformly distributed, and there is no "center of mass".

 

A manifold is a topological space that is locally Euclidean -- locally just like ordinary space. The surface of a balloon, or of a globe is a 2-dimensional manifold, in this case a 2-sphere.

 

An intrinsic manifold is a manifold that is not explicitly embedded in anything larger.

 

Isotropic means essentially "the same in every direction".

 

You can talk of the center of the solar system, of the galaxy and of the local group, and you can continue as you expand to any number of subsequent steps, but that does not get you to a center of the universe. If you started with a point on the surface of a balloon and expanded in concentric circles you would always find the center to be the point that you started with. But the surface of a balloon is homogeneous and no point is distinguished from any other.

Edited June 8, 2011 by DrRocket

[Airbrush]

In our experience anything like the big bang would have a center. The big bang is supposedly not like an ordinary explosion which results in a shell of fragments flying outward in all directions leaving a central void. We do not see this anywhere in the universe. It has been suggested that the big bang could be explained by the balloon analogy. However that would make one visualize the universe like an expanding balloon which would have a definite center. Then one might guess that if our region of the universe is homogenious and isotropic, that could be explained as a "balloon" with a very, VERY, thick skin, in fact so thick that it would be at least as thick as the observable universe. Our observable universe would be very tiny compared to the entire universe. What we see in our region makes us suppose that there is nothing beyond the CMBR. This "thick-skinned balloon" idea fails, I believe, because for a unverse to be THAT large, the central void would be over millions of times the "thickness of the balloon's skin". A universe THAT large could not have originated only 13.7 Billion years ago. It would be at least millions of times older. But with things like cosmic inflation going on, maybe there are other kinds of cosmic inflation, which occurred at different intervals?

Edited June 8, 2011 by Airbrush

[DrRocket]

In our experience anything like the big bang would have a center. The big bang is supposedly not like an ordinary explosion which results in a shell of fragments flying outward in all directions leaving a central void. We do not see this anywhere in the universe. It has been suggested that the big bang could be explained by the balloon analogy. However that would make one visualize the universe like an expanding balloon which would have a definite center. Then one might guess that if our region of the universe is homogenious and isotropic, that could be explained as a "balloon" with a very, VERY, thick skin, in fact so thick that it would be at least as thick as the observable universe. Our observable universe would be very tiny compared to the entire universe. What we see in our region makes us suppose that there is nothing beyond the CMBR. This "thick-skinned balloon" idea fails, I believe, because for a unverse to be THAT large, the central void would be over millions of times the "thickness of the balloon's skin". A universe THAT large could not have originated only 13.7 Billion years ago. It would be at least millions of times older. But with things like cosmic inflation going on, maybe there are other kinds of cosmic inflation, which occurred at different intervals?

 

 

This is just wrong.

 

You have completely misunderstood the balloon analogy, which is quite common. It creates as much confusion as enlightenment.

 

The balloon is used to illustrate the idea of a manifold, without actually using the necessary mathematics. The idea is that the surface of a balloon is locally just like the usual plane -- the surface of a balloon is a 2-manifold. The problem lies in visualizing the balloon as embedded in 3-space. To make the analogy useful you have to forget about the balloon as being in 3-space and imagine that the surface of the balloon is all that exists. Then there is no "center" of that surface.

 

The universe is likewise an "intrinsic manifold". It is not embedded in anything larger. If it were, that something larger would be the universe. The universe is, by definition, the whole enchilada.

[Airbrush]

This is just wrong.

 

You have completely misunderstood the balloon analogy, which is quite common. It creates as much confusion as enlightenment.

 

The balloon is used to illustrate the idea of a manifold, without actually using the necessary mathematics. The idea is that the surface of a balloon is locally just like the usual plane -- the surface of a balloon is a 2-manifold. The problem lies in visualizing the balloon as embedded in 3-space. To make the analogy useful you have to forget about the balloon as being in 3-space and imagine that the surface of the balloon is all that exists. Then there is no "center" of that surface.

 

The universe is likewise an "intrinsic manifold". It is not embedded in anything larger. If it were, that something larger would be the universe. The universe is, by definition, the whole enchilada.

 

Thanks for your reply. How can what I wrote be "just wrong"? I only suggested that the balloon analogy is misleading to the casual astro-cosmo enthusiast, such as myself, who cannot twist their thinking into the surface of the balloon is "all that exists". Now I ask you to explain how the "thick-skinned balloon analogy fails. It allows for the observable universe to be homogeneous, isotropic and expanding. I proposed that 13.7 Billion years is too short a time for an expansion that is Millions of times larger than the observable universe. But I am not an expert. How would you determine the "thick-skinned balloon analogy" is not valid?

 

I don't know what you mean by "intrinsic manifold" or how it is relevant to my proposed concept. The observable universe is embedded in the entire universe. I am trying to explain the observable universe.

Edited June 8, 2011 by Airbrush

[DrRocket]

Thanks for your reply. How can what I wrote be "just wrong"? I only suggested that the balloon analogy is misleading to the casual astro-cosmo enthusiast, such as myself, who cannot twist their thinking into the surface of the balloon is "all that exists". Now I ask you to explain how the "thick-skinned balloon analogy fails. It allows for the observable universe to be homogeneous, isotropic and expanding. I proposed that 13.7 Billion years is too short a time for an expansion that is Millions of times larger than the observable universe. But I am not an expert. How would you determine the "thick-skinned balloon analogy" is not valid?

 

I don't know what you mean by "intrinsic manifold" or how it is relevant to my proposed concept. The observable universe is embedded in the entire universe. I am trying to explain the observable universe.

 

The balloon analogy is an ANALOGY. It is a (poor) substitute for the actual model from general relativity, which is a space-like slice of an intrinsic 4-dimensional Lorentzian manifold, most often assumed to be homogeneous and isotropic. The fact that you don't understand the preceding sentence is why popularizations use analogies.

 

You are trying to extend the analogy to a real model. That won't work. Real models are constrained by general relativity. Analogies are not supposed to be valid. They are ANALOGIES. In the case of the "thick-skinned balloon" it is not even clear what you mean by an analogy. It sounds more like a model. How that "explains" expansion or the size of the universe is a complete mystery.

 

The observable universe is a 3- ball, centered on the observer. There is no debate on that. The topology of the entire universe is unknown.

 

You should consider reading Alan Guth's The Inflationary Universe. It will not resolve all of your problems, but it will give you an idea of the nature of the real questions.

[npbreakthrough]

What is going on here?

??

[michel123456]

Never mind.

[Airbrush]

In the case of the "thick-skinned balloon" it is not even clear what you mean by an analogy. It sounds more like a model. How that "explains" expansion or the size of the universe is a complete mystery.

 

The observable universe is a 3- ball, centered on the observer. There is no debate on that. The topology of the entire universe is unknown.

 

I appreciate your knowledge on this. After some thought I want to change the name of my proposed model (not analogy) to the "coconut shell model". In this model our observable universe happens to be a tiny region embedded in the shell of the giant coconut. We see homogeneous, isotropic, expansion everwhere. The thickness of the coconut shell is greater than 90 Billion light years. The diameter of the giant coconut could be Trillions or Quadrillions (or perhaps Googols) of light years across.

 

Now would somebody tell me what's wrong with this model? And explain it like you are lecturing to a college Intro to Astonomy 101 class.

Edited June 9, 2011 by Airbrush

[Jacques]

Here.

For me the center of the universe is here.

[csmyth3025]

I appreciate your knowledge on this. After some thought I want to change the name of my proposed model (not analogy) to the "coconut shell model". In this model our observable universe happens to be a tiny region embedded in the shell of the giant coconut. We see homogeneous, isotropic, expansion everwhere. The thickness of the coconut shell is greater than 90 Billion light years. The diameter of the giant coconut could be Trillions or Quadrillions (or perhaps Googols) of light years across.

 

Now would somebody tell me what's wrong with this model? And explain it like you are lecturing to a college Intro to Astonomy 101 class.

In the ill-used ballon analogy people have a hard time understanding that the 2-D balloon surface represents the 3-D universe in which we live.

 

The 2-D surface of a balloon is imbedded in a 3-D world (the one in which we live).

 

The space manifold in which we live isn't imbedded in any sort of higher dimension as far as we know. It may have positive curvature (like a sphere), negative curvature (like a saddle) or just be flat (like a sheet of paper laying on a desk). As DrR points out, this manifold we live in is the whole enchilada. There are no outside dimensions (that we know about) and there is nothing outside of it.

 

Chris

[Spyman]

I appreciate your knowledge on this. After some thought I want to change the name of my proposed model (not analogy) to the "coconut shell model". In this model our observable universe happens to be a tiny region embedded in the shell of the giant coconut. We see homogeneous, isotropic, expansion everwhere. The thickness of the coconut shell is greater than 90 Billion light years. The diameter of the giant coconut could be Trillions or Quadrillions (or perhaps Googols) of light years across.

 

Now would somebody tell me what's wrong with this model? And explain it like you are lecturing to a college Intro to Astonomy 101 class.

Think about a "cogwheel tooth model", simply exchange "coconut" with "cogwheel" and "shell" with "tooth".

 

"In this model our observable universe happens to be a tiny region embedded in the tooth of the giant cogwheel. We see homogeneous, isotropic, expansion everwhere. The thickness of the cogwheel tooth is greater than 90 Billion light years. The diameter of the giant cogwheel could be Trillions or Quadrillions (or perhaps Googols) of light years across."

 

How can we decide which model is correct and which is false?

[MigL]

Airbrush, assume we lived in a version of your thick-skinned balloon universe, or 'coconut shell mode'. We now take a string and make a giant loop. We then grab the ends of the loop and pull it tight. In an isotropic universe where all things happen the same throughout the universe, we should be able to pull the loop right down to zero dimension. In your model, however, if the loop goes around the innermost shell ( or inner sphere ) we can only pull the loop down to the dimension of the inner shell.

 

Incidentally this also holds true for a torus and higher order topologies such that the 3-sphere is the only topology which seems to work. And unfortunately I forget what this property, exibited by the loop contraction, is called. Help me out here DrR, is it called simple connectedness, or am I thinking of something else ( math and especially topology aren't my strong suits )

[DrRocket]

I appreciate your knowledge on this. After some thought I want to change the name of my proposed model (not analogy) to the "coconut shell model". In this model our observable universe happens to be a tiny region embedded in the shell of the giant coconut. We see homogeneous, isotropic, expansion everwhere. The thickness of the coconut shell is greater than 90 Billion light years. The diameter of the giant coconut could be Trillions or Quadrillions (or perhaps Googols) of light years across.

 

Now would somebody tell me what's wrong with this model? And explain it like you are lecturing to a college Intro to Astonomy 101 class.

 

 

If I take your model literally the you are proposing that the observable universe is just a 3-ball in some enormous spherical shell. That could be, but the universe could be a 3-dimensional patch in ANY 3-manifold, and that opens up infinitelt many possibilities.

 

The usual assumption is the "cosmological principle" or "Copernican principle" that the universe is homogeneous and isotropic. That implies that space is a Riemannian manifold of constant curvature and is completely determined as follow; 1) in the case of positive curvature itis a 3-sphere 2) in the case of zero curvature it is Euclidean 3-space and 3) in the case of negative curvature it is hyperbolic 3-space. If the assumption of isotropy is relaxed then other possibilities, like a flat 3-torus open up.

 

If I take your model not so literally, then maybe is is just case 1.

 

Homogeneity and isotropy are not logical requirements, but they do seem to be consistent with observations on the largest scales. Relaxation of those assumptions generally is accompanied by rationale.

 

Your "thick-shelled coconut" is spherically symmetric, but not homogeneous -- given two arbitrary points there is not necessarily an isometry that carries one point to the other. Neither is it isotropic -- the radially direction is quite different from the azimuthal direction (there are loops in one direction but not the other). So, the question is what observational or theoretical evidence suggests such a model ? Ans: none.

 

There is a very good article by Geroch and Horowitz, "Global structure of spacetimes" in General Relativity An Einstein Centenary Survey, edited by Hawking and Israel.

Your library should be able to get a copy through interlibrary loan if it is not in their stacks.

 

 

Airbrush, assume we lived in a version of your thick-skinned balloon universe, or 'coconut shell mode'. We now take a string and make a giant loop. We then grab the ends of the loop and pull it tight. In an isotropic universe where all things happen the same throughout the universe, we should be able to pull the loop right down to zero dimension. In your model, however, if the loop goes around the innermost shell ( or inner sphere ) we can only pull the loop down to the dimension of the inner shell.

 

Incidentally this also holds true for a torus and higher order topologies such that the 3-sphere is the only topology which seems to work. And unfortunately I forget what this property, exibited by the loop contraction, is called. Help me out here DrR, is it called simple connectedness, or am I thinking of something else ( math and especially topology aren't my strong suits )

 

What you are describing is simple connectivity -- a vanishing first homotopy group. Unfortunately the "coconut shell" has the homotopy type of a 2-sphere and is simply connected. To distinguish an n-sphere from the Euclidean space of the same dimension you need to go to the n-th homotopy group.

 

A torus is not simply connected. The first homotopy group of an n-torus is the direct sum of n copies of [math]\mathbb Z[/math] But the usual argument against isotropy of the torus is that some geodesics are closed and others are dense (see "irrational flow on the torus"). I am not personally convinced that strict global isotropy is a good assumption. The model of av flat 3-torus is seriously considered in some circles (see for instance the "Pac Man" space in Brian Greene's The Hidden Reality (this is not an endorsement, and in general I dislike the book)).

[between3and26characterslon]

As the Universe revolves around me then I must be the centre.

 

I would like to try and help to explain the baloon analogy;

 

Let's say you have a length of wire and manipulate it into something representing the outline of a cube. You can see it's a cube because you see it in 3 dimensions. Now, being that it's just a wire frame, you can shine a light on it and cast it's shadow onto a wall. The shadow is only 2 dimensions but you have no doubt that it's a cube albeit a 2 dimensional representation of one.

 

In a similar way to described above the surface of a baloon is a 2d shadow of the 3d Universe, the baloon analogy would indicate a closed and finite Universe and is used to give something familiar and memorable to the layperson to identify with. An open and infinite Universe would be better described as a rubber sheet with the observable Universe being a certain radius from any given point. (on a side note, if the Universe is infinite, then some parts of it could be much older than ours?)

 

To the OP, everywhere is the centre of the Universe. If your friend thinks there is a definite centre then he probably also thinks there is an edge to it as well. So let's say your freind travels to the edge of the Universe, what is on the other side? Well by definition - NOTHING - not even space (or therefore distance) so let's say the edge of the Universe was an opaque membrane and you managed to climb through it what would you see. You would see you are standing on top of an enormous black sphere, however, you need distance to be able to see the sphere and as there is no distance between you and any part of this 'sphere' you would end up right back at the centre thus making everywhere the centre of the Universe.

 

I hope that's of some help.

 

EDITED for grammer and spelling

Edited June 11, 2011 by between3and26characterslon

[Airbrush]

As the Universe revolves around me then I must be the centre.

 

I would like to try and help to explain the baloon analogy;

 

Let's say you have a length of wire and manipulate it into something representing the outline of a cube. You can see it's a cube because you see it in 3 dimensions. Now, being that it's just a wire frame, you can shine a light on it and cast it's shadow onto a wall. The shadow is only 2 dimensions but you have no doubt that it's a cube albeit a 2 dimensional representation of one.

 

In a similar way to described above the surface of a baloon is a 2d shadow of the 3d Universe, the baloon analogy would indicate a closed and finite Universe and is used to give something familiar and memorable to the layperson to identify with. An open and infinite Universe would be better described as a rubber sheet with the observable Universe being a certain radius from any given point. (on a side note, if the Universe is infinite, then some parts of it could be much older than ours?)

 

To the OP, everywhere is the centre of the Universe. If your friend thinks there is a definite centre then he probably also thinks there is an edge to it as well. So let's say your freind travels to the edge of the Universe, what is on the other side? Well by definition - NOTHING - not even space (or therefore distance) so let's say the edge of the Universe was an opaque membrane and you managed to climb through it what would you see. You would see you are standing on top of an enormous black sphere, however, you need distance to be able to see the sphere and as there is no distance between you and any part of this 'sphere' you would end up right back at the centre thus making everywhere the centre of the Universe.

 

I hope that's of some help.

 

EDITED for grammer and spelling

 

Excellent presentation between! I especially like the part about what's beyond the edge of the Universe..."NOTHING - not even space (or therefore distance)...." However, if there was no distance between someone and any part of the "sphere" you would end up just inside the edge. How do you leap all the way back to the center?

 

Does cosmic inflation imply the possibility of infinite speed of expansion? Or just beyond the speed of light, but a finite speed?

[Dekan]

The universe is, by definition, the whole enchilada.

 

But there must be some outside space - to give the universe room to expand into.

 

Otherwise, the universe would have a fixed boundary. And always stay the same size. It couldn't expand.

 

Doesn't that show, that the universe isn't the whole enchilada?

[insane_alien]

But there must be some outside space - to give the universe room to expand into.

 

Otherwise, the universe would have a fixed boundary. And always stay the same size. It couldn't expand.

 

Doesn't that show, that the universe isn't the whole enchilada?

 

no. universe by definition means the whole enchilada.

 

an inability to imagine a lack of an outside does not mean that there must be an outside.

[Dekan]

no. universe by definition means the whole enchilada.

 

an inability to imagine a lack of an outside does not mean that there must be an outside.

Well if there isn't any space outside, what is the universe expanding into?

[insane_alien]

its not expanding into anything. its just expanding.

[Dekan]

its not expanding into anything. its just expanding.

 

Sorry, but I don't understand. If the universe is expanding, doesn't that mean the edge of it, is moving. How can it move, unless there's some space for it to move in?

[MigL]

Haven't you been following?

There is no moving edge. Your premise of an edge implies an outside. There is and, cannot be, either ( and even if there was, it could never affect us or be detected, so it's inconsequential ).

The universe ( means everything ) is all that we know and can ever know, and it is expanding by the average distance between objects of galactic scale getting larger.