The Raisin Bread Analogy Question

[Julius Caesar]

read analogy here:

http://cmb.physics.wisc.edu/tutorial/hubble.html

Less like an explosion and more like raisins baking in raisin bread. No matter from which raisin you look, all other raisins appear to be moving away.

the question:

The universe is the very boundary of space time, so it isn't expanding into anything or is it? It is just expanding.

What does the "dough" in the analogy expand into?

 

[Chronos]

It is expanding into nothing - literally. 'Outside' the universe does not exist. The universe is not embedded in some pre-existing infinite emptiness. The raisin bread analogy fails when you observe it from outside the dough [you cannot observe the universe from 'outside' the universe]. It is only good so long as you are inside the dough. The only way you can tell the dough is expanding is by watching the raisins.

 

[Julius Caesar]

It is expanding into nothing - literally.

Nothing? That does not seem logical. You might as well have said that a perpetual motion machine is possible. Something cannot expand into "Nothing".

My thought is that our universe is just one bubble amongst many bubbles within an ocean of something...something that is not "nothing".

 

[phinds]

Nothing? That does not seem logical. You might as well have said that a perpetual motion machine is possible. Something cannot expand into "Nothing".

If you are looking for things to be intuitively "logical" in cosmology and/or quantum mechanics, you are in for some rude disappointments.

My thought is that our universe is just one bubble amongst many bubbles within an ocean of something...something that is not "nothing".

And you could well be right but this is at preset a theological statement (not falsifiable) and an opinion, not science.

Certainly there are theories along those lines but their validity as theories is not on a par with the theory of relativity or the theory of evolution.

 

[Julius Caesar]

yes indeed I'm afraid it will quite possibly always remain a theological statement (not falsifiable), however if we deal in probabilities I can say that something expanding into nothing is less probable than something expanding into something correct?

 

[Vorde]

however if we deal in probabilities I can say that something expanding into nothing is less probable than something expanding into something correct?

Yes, however there is no evidence to support the idea that there is something 'outside' of our universe. And while it can be intellectually stimulating to think about it, because there is no evidence (or way to find evidence), it makes more sense to assume that there is nothing outside of our universe and go from there, because if we can't detect/measure/interact with it, it mine as well not exist.

 

[Julius Caesar]

there is no evidence to support the idea

Vorde, there will probably NEVER be evidence to support my idea. However the idea that there is "nothing" outside our universe seems a bit childish to me. You might as well tell me that there is a hidden realm called heaven and hell :)

 

[Drakkith]

Vorde, there will probably NEVER be evidence to support my idea. However the idea that there is "nothing" outside our universe seems a bit childish to me. You might as well tell me that there is a hidden realm called heaven and hell :)

Just realize that current models of the universe never say there is nothing, they simply don't require that there be anything to expand into. (As far as I know at least) Also, you may be unaware that a simple 3d volume is not sufficient to describe the universe, so an analogy or thinking about it that way will ultimately fail. Models of the universe take into account spacetime, not just space. When you get down to the details, there is no way to visualize what is happening since it is primarily described through math where the results on non-intuitive and not through something like a picture. This is similar to Quantum Mechanics, in that QM is also described using complex math and any attempt to visualize what is "really" happening is futile.

Edit: Also, is thinking that there is "nothing" really any different than considering the universe infinite, being in a multiverse, or being finite? Not in my opinion.

 

[Vorde]

The problem with your thought (at least in my view, there are definitely people out there who would disagree with this) is not your idea, but the concept of an area outside of our universe in the first place.

Any point in our universe can be represented by 4 coordinates: x,y,z,t (lets ignore higher dimensions for now because they don't change the problem). Now you can locate any point in the universe like that. So when someone says "outside our universe", they are referring to a point, which by definition cannot be located using that coordinate system. The question is, it is even meaningful to talk about an area that cannot be mathematically located?

EDIT: I didn't see the Drakkith post, he summed it up better than I did.

 

[Julius Caesar]

The question is, it is even meaningful to talk about an area that cannot be mathematically located?

Meaningful? Interesting choice of words. Is it meaningful to imagine a creator that gets angry? I wonder... However if the analogy of the raisin bread, which is taught at Berkeley (where I first heard it) is propagated then the logical question must be pondered: what does the dough expand into? It may not be meaningful, however the question must be asked. Dwell on it... for it is all we can do.

 

[Vorde]

It doesn't have to be asked however. Imagine this:

You are in a two dimensional universe that is governed only by the following rules:
-There are no deformities in the smoothness of the plane in all directions.
-There are only 2 dimensions of movement (and therefore of location), Up/down and left/right.


Now there are 2D people in this universe, and an adventurous 2D object decides to go as far 'left' as he can. Eventually he finds that he returned to the place he started. This is done many times, and each time the same thing happens. Now the scientists of this world analyze this, and come to the conclusion that this is proof they lived in a curved universe.

Now as 3 dimensional people, we look at that and we can easily imagine it as a surface of a sphere, you go in one direction far enough and you come around to the other side. And looking at that, you may be tempted to ask, well what's outside of the sphere? What happens when you lift up from the sphere?

However it is not a meaningful question, you simply cannot do that. This universe is defined only having those two dimensions, so outside of the surface of this 'sphere' (or inside for this matter) is simply not a place.

Do you see why when applied to our universe this prevents the exterior of the universe from being a meaningful place, even if we can't picture it as well?

 

[Drakkith]

Meaningful? Interesting choice of words. Is it meaningful to imagine a creator that gets angry? I wonder... However if the analogy of the raisin bread, which is taught at Berkeley (where I first heard it) is propagated then the logical question must be pondered: what does the dough expand into? It may not be meaningful, however the question must be asked. Dwell on it... for it is all we can do.

There's nothing wrong with the question. The raisins in dough is an analogy, a way of visualizing it, nothing more. When you ask about the crust and what the dough is expanding into, then the answer you get is that the universe has no crust and is not expanding into anything according to current knowledge. The raisin bread isn't going to be 100% correct because it is JUST a way of teaching the subject, of making people understand it. It is not the model itself.

To take the question further, the answer to what's beyond the universe, or if there is an edge or beyond to the universe, the answer you get is "we don't know, but our models don't need one to work". That's all.

 

[Julius Caesar]

"we don't know, but our models don't need one to work"

great answer and of course the correct one for now. We simply don't know. To say it's "nothing" is wrong. The answer is: We don't know. Period. However if we do KNOW that the universe is expanding then it is more probable it is expanding into SOMETHING rather than NOTHING. That's my point. Thanks guys for the great discussion!

 

[Drakkith]

great answer and of course the correct one for now. We simply don't know. To say it's "nothing" is wrong. The answer is: We don't know. Period. However if we do KNOW that the universe is expanding then it is more probable it is expanding into SOMETHING rather than NOTHING. That's my point. Thanks guys for the great discussion!

You have no way of knowing that, nor do you understand what is meant by "expansion". Science has shown that on the scale of the very large and the very small things do NOT work as we think they should. I see no reason to say that it is more probable for the universe to be expanding into anything. In fact, I would say that there is more reason to believe it isn't, as the models of various curvatures of the universe don't place the universe within something else yet still show it can expand. Don't try to base your view on "common sense" as that will fail you repeatedly when you learn about science.

 

[Julius Caesar]

nor do you understand what is meant by "expansion".

Oh Really? Do tell what this years Nobel prize was all about? I await your explanation of "expansion" and into what said "expansion" is "expanding" into...


2011
The prize was awarded with one half to:

SAUL PERLMUTTER and the other half jointly to BRIAN P. SCHMIDT and ADAM G. RIESS for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.

http://nobelprizes.com/nobel/physics/

 

[DaveC426913]

great answer and of course the correct one for now. We simply don't know. To say it's "nothing" is wrong. The answer is: We don't know. Period. However if we do KNOW that the universe is expanding then it is more probable it is expanding into SOMETHING rather than NOTHING. That's my point. Thanks guys for the great discussion!

No, it's not that we don't know.

Our models of the universe are logical and consistent without there needing to be anything outside it.

There is no reason to invent anything outside. And "because the idea bothers me" is not a reason.

The mistake you're making is assuming that the universe (which is as unique as it is possible for a phenomenon to get) should have some precedent in your experience that you can draw on to try to understand it. But seriously, are there other universes that you've encountered that you're comparing this to, such that this one's topology "doesn't make sense"?

The universe is not obliged to meet anyone's idea of "common sense".

 

[Julius Caesar]

No, it's not that we don't know.

But you don't know do you ;)

 

[DaveC426913]

But you don't know do you ;)

The nature of the scientific method is that we observe, and we build models that describe what we see.

Our models describe what we see. And they do it pretty well.As for "we don't know" well, we don't know that God doesn't exist either. We also don't know that the late Carl Sagan doesn't have an invisible fire-breathing dragon in his garage, or that a teapot is not orbiting Jupiter.

But the scientific method has nothing to say about it until and unless our world needs the existence of a God (or an invisible dragon, or a space-faring teapot or an outside-the-universe) to accurately describe its workings.

See?

 

[phinds]

The universe is not obliged to meet anyone's idea of "common sense".

Yeah, Dave, I tried to point that out in post #4 but he's not going to listen to that point of view cause he knows he's right.

 

[Julius Caesar]

The nature of the scientific method is that we observe, and we build models that describe what we see. Our models describe what we see. And they do it pretty well.

Very true and well said however we see that the universe is "expanding". The nobel prize was just awarded for this. What good sir is it expanding into? I ask you in good faith and await your answer.

 

[DaveC426913]

What good sir is it expanding into?

Please read https://www.physicsforums.com/showthread.php?t=537037", starting at post #2.

 

[Julius Caesar]

pretty pathetic reply. seriously... you could have been a tad bit more creative. Especially considering we're dealing with nonsense here.

 

[DaveC426913]

What's pathetic is that you are just asking the same questions again because you didn't like the answers you got.

Your question has been answered. If you want more discussion, you will have to
1] absorb what you have learned so far
2] formulate some new questions based on 1].If you think science is nonsense, why are you bothering to hang around on a forum dedicated to science? There are plenty of fora out there that will happily give you answers more to your liking.

 

[Julius Caesar]

Nope I was fine with the answers I got. We don't know. I'm comfortable with that. Not going to get anything more substantial than that if I sit on this board for a million years. What got me was your comment DaveC426913: "No, it's not that we don't know." I guess that's what got me posting more. Other than that I'm cool. I guess you know it all right Dave?

 

[DaveC426913]

Nope I was fine with the answers I got. We don't know. I'm comfortable with that.

Sure. As long as you're comfortable with invisible dragons and orbiting teapots too.

 

[Julius Caesar]

Dave, I can see why you have 13k posts. Because you like to argue. But if you want to go down that path you say:

"No, it's not that we don't know.

Our models of the universe are logical and consistent without there needing to be anything outside it.
There is no reason to invent anything outside. And "because the idea bothers me" is not a reason."

OK. Sure. But when we talk of "expansion"... nobel prizes awarded for it just within days etc. What is the said "expansion" expanding into? If you'd like to reply with more silly double-speak feel free to do so.

 

[Drakkith]

From wikipedia here: http://en.wikipedia.org/wiki/Expansion_of_the_universe

Over time, the space that makes up the universe is expanding. The words 'space' and 'universe', sometimes used interchangeably, have distinct meanings in this context. Here 'space' is a mathematical concept and 'universe' refers to all the matter and energy that exist. The expansion of space is in reference to internal dimensions only; that is, the description involves no structures such as extra dimensions or an exterior universe.[6]

See this article and try to understand what a metric is and then come back to this. It really isn't as simple as "we don't know". Why? Because we DO know things. A great many in fact. You are basing your opinion on some absolute notion of "knowing" that isn't realistic.

 

[cepheid]

Julius Caesar,

First and foremost I would sincerely recommend that you lose the attitude. Physics Forums is a great resource in which some very learned people give freely and generously of their time and knowledge to try to explain science. If you keep an open mind, you might come to learn and understand things which you previously did not. If you don't, and if you just choose to belittle people out of your ignorance, you will simply get banned. Do you want this?

I think what Chronos was trying to say in his first sentence below would have been better worded as, "it is not expanding into anything." If you had only paid closer attention to (and thought more about) everything else that he said after that (emphasized by me below), then I think we could have avoided a lot of heartache in this thread:

It is expanding into nothing - literally. 'Outside' the universe does not exist. The universe is not embedded in some pre-existing infinite emptiness. The raisin bread analogy fails when you observe it from outside the dough [you cannot observe the universe from 'outside' the universe]. It is only good so long as you are inside the dough. The only way you can tell the dough is expanding is by watching the raisins.

What Chronos was trying to explain was that "expansion" in this context should not be thought of as an expansion of some universe with finite boundaries into some larger space within which it is embedded. That is not what is meant by expansion, and the existence of this larger surrounding space is not required. Instead of thinking of it as an expansion into some other space, think of it as an expansion OF space itself. In other words, when we say that the universe is expanding, we mean that distances between objects are getting larger, because the amount of space between those objects is increasing. This occurs in a uniform way, such that every point in the universe expands away from every other point. If you want to think of this as new space being "created" where there was none before, then that's certainly one way of looking at it or thinking about it (and I'm going to come back to this issue of "how to think about it" later).

I can certainly run you through a few lower-dimensional analogies that would explain how this uniform expansion works. Consider a 1D universe consisting of a ruler with evenly - spaced tick marks. We'll say that the initial space between tick marks is 1 "unit." The way the expansion occurs is that, after one time interval, the amount of space between adjacent tick marks doubles so that now the distance between adjacent tick marks is 2 units. After another time interval, maybe it has doubled again to 4 units. An interesting feature of this type of "uniform" expansion (every point expands away from every other point) is that from the point of view of any observer on any tick mark, it appears as though he is at the centre and all other points are moving away from him. But in reality, there is no centre and no edges. Maybe that's because it's a straight and infinite ruler, in which case geometry in this universe is "flat" (Euclidean). Or maybe the two ends "curve" and join together to form a circle. However, in that case, I want to emphasize that the space inside the circle and outside the circle has no meaning in the analogy. The 1D curve is the universe...everything that exists. Observers in this hypothetical universe can only move within the universe i.e. along the line.

We can extend this analogy to a 2D universe consisting of a plane with evenly-spaced grid points. We'll say that the initial space between grid points is 1 "unit." Just as in the 1D case, the squares in the grid get larger and larger with time, representing the expansion of space itself. Also just as in the 1D case, from the point of view of any observer at any point on the grid, it appears as though he is at the centre and all other points are moving away from him. But in reality, there is no centre and there are no edges. Maybe that's because it's a flat, infinite, 2D Euclidean plane. Or maybe the 2D surface curves to form a closed spherical surface (which always has no centre and no edges). This is the famous "balloon analogy" in which the 2D surface of the balloon represents the universe, and dots on the surface of it move away from each other as the balloon inflates. Again, I want to emphasize that the space within the sphere and outside the sphere has no meaning in the analogy. The 2D surface is the universe...everything that exists. Observers in this hypothetical universe can only move within the universe i.e. along the surface.

The raisin bread analogy is the 3D version of the above 2 examples. Maybe it's an infinite 3D space, or maybe it somehow "curves" to form a closed "hypersphere" (don't ask me to visually represent that). Either way, it has no centre and no edges. Any observer at any grid point within the space thinks that he is at the centre and all other points are moving away from him. The cubical sections that the grid divides the space into get larger and larger with time, representing the expansion of space itself.

I want to try to drive home a very important point: physics does not consist of vague and qualitative descriptions of what is going on (contrary to what you seem to think). Physical theories are mathematical models that describe nature. These models must:

1. be quantitative
2. make predictions
3. be testable
4. match up with and explain our observations really well.

At the end of the day, if 4 isn't satisfied, we're not going to keep that theory around, we'll throw it out. Physics is ultimately an experimental science. In the context of this thread (cosmology and the large scale dynamics of the universe), General Relativity is the relevant and applicable theoretical model. And General Relativity defines very precisely what is meant by the thing we refer to as "expansion." That precise definition is (and must be) mathematical. The descriptions in English that I and others have been giving you are, at best, crude attempts to describe these precise statements of General Relativity in words. (That's why I said above that they are merely "ways of thinking about it"). Not only will these verbal descriptions not be as precise and meaningful, but we will also be forced to attempt to interpret them using our intuition, which is based on our everyday experience. That doesn't always work. More often than not, our intuition fails us, and our everyday analogies are inadequate to the task of comprehending what the model says about the nature of the universe. PLEASE ponder this previous sentence carefully before you discount cosmology just because you can't make sense of the not entirely adequate verbal summaries of its results that we are giving you. If you want to criticize cosmological theory itself, then you'll have to understand it first, not just simple descriptions of its results.

 

[DaveC426913]

Thanks cepheid. I was gearing up for a long explanation, but it looks like you threw yourself on that grenade.

I'll just make it very short since you covered most of the points.

The balloon analogy illustrates a 2D universe like our own 3D, but with one dimension thrown away. As the balloon is inflated, its surface area increases (expands). i.e. if you were a 2-dimensional ant living at the surface of the balloon, you would measure your entire universe (in all its glorious 2-dimensions) expanding. Note that the 2D surface of the balloon does not expand into anything - it simply increases in its (2) dimensions.

Now, the obvious objection is "yes, but the balloon itself is expanding into 3D space". While that is true, it is an artifact of our analogy. It turns out that an extra dimension is not necessary to have this expansion. i.e our 3D universe is expanding, but that does require there to be a 4th dimension that it is expanding into.

If you could walk far enough around the 2D balloon's surface, you would arrive back at your starting point. Yes, there is an implication that our 3D universe may wrap around, i.e. if you could go far enough across our universe, it is possible that you might arrive back at our starting point. Though this is not the only solution.

Does this help clarify how it is possible to have an expanding universe that is not expanding "into" anything?

 

[DaveC426913]

Dave, I can see why you have 13k posts. Because you like to argue.

You came here with a question. An army of volunteer professionals has been happy to answer your question. You don't like the answer, so you argue it.

Indeed, I have acquired a reputation here for my patience with people who resist learning.

 

[phinds]

You came here with a question. An army of volunteer professionals has been happy to answer your question. You don't like the answer, so you argue it.

Indeed, I have acquired a reputation here for my patience with people who resist learning.

Julius Caesar, I want to most emphatically support BOTH part of what Dave just said. I have complimented him in the past on his incredible patience with people like you who seem steadfastly determined to be argumenative and to totally ignore all evidence and all explanations presented to them by folks who CLEARLY know far more that they do. Forum rules prohibit my telling you what I really think of you.

 

[marcus]

It is expanding into nothing - literally. 'Outside' the universe does not exist. The universe is not embedded in some pre-existing infinite emptiness...

Nothing? That does not seem logical. You might as well have said that a perpetual motion machine is possible. Something cannot expand into "Nothing".

My thought is that our universe is just one bubble amongst many bubbles within an ocean of something...something that is not "nothing".

Just realize that current models of the universe never say there is nothing, they simply don't require that there be anything to expand into...

Dave, Chronos and Drakkith are right. I think Julius is stubbornly not listening to them because he is misled by the English sentence "The universe expands."

To a many a layman, which is evidently what Julius is, "expanding universe" gives the wrong mental picture. Popularizers and science journalists should never have used that infectious phrase.

People should be told, instead, that distances expand, in our universe according to a definite largescale pattern.

The standard math model (different from the words and always-misleading analogies used to convey it to public) does not have a boundary. We have no evidence that a boundary exists, so we cannot justify including boundary in the model. So there is no OUTSIDE the universe---this is not even definable within the standard math model.*

All we know, or think we know, is this pattern of largescale distance expansion. It is predicted theoretically by the well-tested general law of changing geometry (gen. rel.) which tells us that large distances between points at rest relative to the average bulk matter HAVE to change. Distances HAVE to change according to a certain simple rule in order for the law to work, and expansion is one solution: uniform percentage rate. Furthermore that is what we observe! It not only comes out of the well-tested equations of geometric gravity, it is confirmed by what we see. And there is no boundary.

It would mess up the math and add unnecessary complication to add a boundary to the picture. Not only is one not needed, but it would be dishonest to pretend one without any evidence, and it would mess things up.

The verbal mistake was probably made in the 1930s or whenever the public was given the phrase "expanding universe". It evokes the image of being outside the U and the U having a definite boundary, like a piece of metal being heated and expanding. For unsophisticated listeners saying "it expands" inescapably calls up the idea of something with a definite shape expanding into the surroundings. That is not what the model says.

*There are alternative schemes which have boundary but lack supporting evidence, have not been thoroughly worked out, and are not used in normal cosmology work. One of the boundary-rich alternatives is the bubble picture you get in "eternal inflation" scenarios.
It's fine to speculate about such, but one can hardly say they are required logically

 

[DaveC426913]

A concise and elegant way of putting it.

"Distances within our universe are increasing."

 

[cepheid]

A concise and elegant way of putting it.

"Distances within our universe are increasing."

I said this too! It's buried in there, somewhere, honest!. Still, marcus probably was much more eloquent.

 

[bigmig]

I don't want to reinvigorate the arguments above but from a non-specialist viewpoint the question 'expanding into what' (and I really do appreciate the geometric nature of this and am comfortable with the extra space/space itself answer, I sort of prefer inflation) is the usual question, but rarely is it turned round to 'expanding out of what'. Which seems the logical corollary. And would warrant a similar answer I should think.
More significantly perhaps following this line of thought, does the math (which is beyond me) give any indication of how small a system can be and still maintain the situation that any 2 observers would 'see' themselves getting further apart? I realize we can only see the recession outside of bound systems usually spanning large distances.
However, within gravitationally bound systems I understand that 'inflation' is still present but has anyone deduced a lower limit to the size this operates at? I guess this would incline to the discreteness or otherwise of space at the smallest level - Planck size? So would the Planck size increase by continuum or up by energy levels. Bit of a thought experiment. The idea is that every observer sees themselves as the centre of inflation/distance increase, and centre implies points equidistant to either side, so how small can this system go, the Swartzchild limit?

 

[Drakkith]

Hmm, is it that gravity holds things together AGAINST expansion, or is it that once enough mass is in a volume of space the gravity simply causes expansion NOT to occur at all?

 

[bigmig]

Yes we know the inflation factor is much weaker than any other attraction even gravity and therefore, despite a small effect gravity holds systems more or less in Newtonian motion. What I struggle to conceptualize is how this spacial/distance inflation plays out as you move from intergalactic to interstellar and so on down to quantum scales.
I have read some work that attempts to rationalize continuity and discreteness at smaller scales by working with differentiable 'non-continuous' objects (but my understanding of this and the math is weak).
Despite the divergence of the math with everyday experience I think it still important that things should be reasonably explainable in terms of more familiar concepts.
Even though a 4th (or more) spatial dimension is unimaginable it is sort of conceptualisable with some practice and dumping preconceived notions, so is infinity etc. But at the end of the day, all this heavy physics has to explain what we see, perceive and expect at the human scale.
But the inflation of space where every 'observer' sees themselves as the centre, sort of 4th dimensional expansion - surface of hypershere etc, may not be what's happening at smaller scales.
Or as you say 'drakkith' does the interaction of matter/gravity with space change things. So if particles are introduced into space then the normal forces would resist a spatial inflation that is happening around them, or their presence negates the inflation. But if that's the case then it seems to me this variable inflation around matter would create a discontinuity/tension in space? I feel I'm barking up the wrong tree here and that's why I could do with a model. Help!