# Effort to get us all on the same page (balloon analogy)

by marcus
Tags: analogy, balloon, effort
 PF Gold P: 3,685 To my mind the balloon analogy is a nuisance, gallaxies ect are not stuck to a surface, once one has read about the BA it takes some getting rid of.
 Astronomy Sci Advisor PF Gold P: 23,228 I said I would try to avoid abbreviations, but I need another one: CMB for cosmic microwave background. The balloon analogy teaches various things, but sometimes you have to concentrate in order to learn them. One thing it teaches is what it means to be not moving with respect to CMB. the balloon is a spherical surface and as it gradually expands a point that always stays at the same longitude and latitude is stationary with respect to CMB. Distances between stationary points can increase, and in fact they do. They increase at a regular percentage rate (larger distances increase more). In our 3D reality this is called Hubble Law. It is about distances between points which are at rest wrt CMB. In our 3D reality you know you are at rest wrt CMB if you point your antenna in all directions and get roughly the same temperature or peak wavelength. There is no doppler hotspot or coldspot in the CMB sky. That means you are not moving with respect to the universe. In cosmology being at rest is a very fundamental idea, we had it even before the 1960s when the CMB was discovered. Then it was defined as being at rest with respect to the process of expansion---you could tell you were at rest with respect to the universe if the expansion around you was approximately the same in all directions---not faster one one side of the sky and slower on the other, but balanced. It is the same idea but now we use the CMB to define it because it is much more accurate. Sun and planets are traveling about 380 km/s with respect to CMB in a direction marked by the constellation Leo in the sky. It is not very fast but astronomical observations sometimes need to be corrected for that motion so as to correspond to what an observer at CMB rest would see. Now let's take another look at the balloon and see what else it can tell us.
 Astronomy Sci Advisor PF Gold P: 23,228 Effort to get us all on the same page (balloon analogy) Now this will respond to Wolram's comment to some extent. What we observe is that the galaxies we have data for are moving very slowly with respect to CMB. Typically only a few hundred km/s! The whole Milky is only going about 500 km/s So as a mental convenience to make it easy on our brains, an excellent first approximation is to assume that all the galaxies ARE stationary wrt CMB and that they do in fact act like pennies stuck to the surface of the balloon. So a galaxy penny always stays at the same longitude and latitude on the balloon. And the balloon is inflating, which means that distances grow by a certain percentage every minute. That means longer distances increase faster, which is Hubble Law (recession speed is proportional to distance v = HD). And recession speed is not real speed in the sense that it doesnt change one's longitude and latitude. Distance between two galaxies can increase even though both are at rest with respect to the CMB, the universe, the process of expansion. Now photons of light CAN move across the surface from one longitude and latitude to another. We can think of them as moving some fixed speed like 1 inch per minute, where the inch of balloon surface is always measured at that current time. So if you send a flash of light off in some direction, once the photons have gotten a substantial distance from you there will be a percentage rate of increase of distance (a recession speed) as well as the light's own standard speed of one inch per minute. After a while the flash of light will be farther away from you than you would calculate if you just think say "five inches in five minutes" and don't take account of expansion. So that is another thing the balloon analogy can tell you. It can teach you to expect light to manage to get farther away than you expect, even though it is always traveling across the surface at one inch per minute. The material that emitted the light which we are currently receiving as CMB is now at a distance from us of 45 billon LY. The light managed to get here in slightly less than the age of the expansion which is 13.7 billion years. The material was much closer to us when the light started its journey. It didn't break any speed laws. Yet it seems to have come all the way across the balloon surface to us from a point which is now 45 billion LY away. This might strike you as paradoxical but it isn't really. The balloon analogy shows you how light can cover enormous distances is less than the expansion age. Perhaps this could be said more clearly if I had time to edit, and fix a lightyear scale on the balloon, in inches, and work out the arithmetic systematically. But I don't right now so maybe that can be for later, or someone else can volunteer to organize this part. Everybody should have a look at the Lineweaver Davis SciAm article that goes into this in depth, with pictures. I have a link to it in my sig---a version at princeton.edu.
Mentor
P: 8,316
 Quote by wolram To my mind the balloon analogy is a nuisance, gallaxies ect are not stuck to a surface, once one has read about the BA it takes some getting rid of.
This is a fine example why marcus' point of trying to avoid abbreviations in this thread is a good idea: what do you mean by BA?
PF Gold
P: 3,685
 Quote by cristo This is a fine example why marcus' point of trying to avoid abbreviations in this thread is a good idea: what do you mean by BA?
Balloon analogy, but you are correct keep things clear of abbreviations.
 PF Gold P: 3,685 So where is every one, i thought this should be mega important.
P: 622
 Quote by marcus ......what we need to focus on here in this thread is the INTUITION........ to properly understand the balloon analogy........a lot of the confusion we occasionally experience comes from getting that analogy somehow crossed up.
﻿I believe that a large part of the trouble many folk have in comprehending cosmology (here and elsewhere) is this:

In an effort to help ignorant folk like myself to ‘get on the same page’--- the ‘page’ on which
professional cosmologists describe their consensus — those who are very familiar with the
Friedmann - Lemaître - Robertson - Walker model tend consistently to over-interpret this
venerable and successful mathematical description of our evolving universe.

The pennies-stuck-on-a-balloon or raisins-buried-in-dough analogy mentioned here is an example. In it, an expanding balloon or rising dough models ‘space’, and pennies or raisins represent non-expanding structures held together by electromagnetic forces. But this analogy leads to much confusion. For instance:

Does space expand and should one think of it as a 'real' substance?
Do static structures experience disruptive stresses because the balloon or dough substrate is expanding?
Are non-expanding structures that cohere gravitationally, like galaxies or galaxy clusters, correctly thought of as pennies of raisins?
Matter is mostly empty space. So, does does all space expand? If not, why not? Where lie the boundaries beween expanding and static space?
Reference frames: every observer has her own personal frame-of-rest (the cosmic microwave background that she observes). Here the idea of this background as a substrate like rising dough may be useful. But is the radiation then like an personal ether?

These are simple confusions. But there are other subtle over-interpretations that can confuse. Many posts in this forum use ‘distance’ as if this were a simple concept in a cosmological context. It is not. Nor is ‘speed’ or 'velocity',
 Quote by Marcus here .... it gives recession speeds.
My bold.

And what about ‘space’? Not an easy concept to define (although your definition of space as “a bunch of distances” is a winner, Marcus. But then, what is distance?).

However, one shouldn't run before you can walk. So analogies and metaphors are useful and necessary aids to understanding. But it should be pointed out up-front that this is what they are. I think folk should be warned of the dangers of over-interpreting what for us human beings is a purely mathematical, and sufficient though perhaps not satisfying, description of change in our evolving universe.

I’ll go further: General Relativity, the foundation of modern cosmology, seems to me something not easily understood in the context of everyday experience. In particular, ‘expansion’ turns out to be not a simple concept. The Hubble flow may kinematically look like ‘motion’ in our local domain (where Special Relativity is adequate). But it is a quite different phenomenon.

Isn't it?
 P: 177 are a few questions from an entry level layperson acceptable here? if not then remove this post. 1. we are stationary but with the universe is expanding, the distance between us and distant galaxies is increasing due to expansion, so does that mean that the distance between us and the cmb is also increasing? 2. the cmb is 45b lyrs away, is that actual distance right now? the light we get from it was emittied 13.7 b yrs ago, was the cmb 13.7b light yrs away from us when that light was emitted? 3. we cannot say that space is a physical thing like the rubber of the baloon. We can only say that distance is increasing? 4. if distance is increasing does that mean that the three dimensional volume of the univers is increasing? 5. is the cmb analagous to the horizion that you would see if you were standing on the surface of the baloon?
Astronomy
PF Gold
P: 23,228
 Quote by TalonD are a few questions from an entry level layperson acceptable here? if not then remove this post. 1. we are stationary but with the universe is expanding, the distance between us and distant galaxies is increasing due to expansion, so does that mean that the distance between us and the cmb is also increasing? 2. the cmb is 45b lyrs away, is that actual distance right now? the light we get from it was emittied 13.7 b yrs ago, was the cmb 13.7b light yrs away from us when that light was emitted? 3. we cannot say that space is a physical thing like the rubber of the baloon. We can only say that distance is increasing? 4. if distance is increasing does that mean that the three dimensional volume of the univers is increasing? 5. is the cmb analagous to the horizion that you would see if you were standing on the surface of the baloon?
this is exactly the most helpful kind of feedback. something like this draft essay on what you can learn from the balloon analogy has to go thru editing and revision. questions like this are exactly what are needed to help guide revision.

3. what space is, physically, is something that physicists are working on---have a look at Frank Wilczek's new book Lightness of Being which is about the leading edge understanding of empty space. make your local librarian order the book. the link is in my sig. also read the SciAm article by Loll about the emergence of spacetime from a kind of chaos at the microscopic level---this is an unproven interesting conjecture which they simulate on the computer. we don't know yet what empty space is. but we do have a mathematical model for the increasing distances---that has to do for now.

4. yes, the instantaneous 3D volume of space can be defined and estimated in the case that it is finite, and recent satellite data gives a lower bound on the volume, and it is increasing in a perfectly normal way as the cube of the scale factor.
Of course if the 3D volume of space is not finite then it becomes more complicated to talk about it increasing. But if it is finite then we have this lower bound and it is easy to discuss. If you want a link to a reference, or simply to know the volume in cubic lightyears, please let me know.

5. what passes for the LOCATION OF THE CMB ORIGIN is a large spherical surface called the surface of last scattering where the stuff is that emitted the light we are now getting.
In the past we were getting CMB light from other stuff that is nearer, but that light has already gone by us.
In the future we will be getting CMB light from other stuff that is out beyond our current surface of last scattering---but that light is still on its way and has not reached us.

All the matter in the universe, including the matter we are made of, participated in radiating the CMB light. The CMB light that our matter emitted is now 45 billion away from us, where other people can catch some if they make antennas. Every patch of matter made CMB, it is just a question of TIMING to say where the matter is whose light you are currently receiving at this moment.

So your image of a horizon has some degree of rightness about it. Not a perfect analogy but it does tell the listener to focus not on the material stuff but on the mathematical object (the spherical surface, like the crcle of horizon on earth). there was a momentary onetime event when expansion was 380,000 years old and the glowing hot fog became transparent, and released its somewhat reddish orange light. Each photon of that light is now 45 billion lightyears from its point of origin.

1. you ask is the distance to the CMB increasing? the distances between all widely separated stationary things are increasing by Hubble Law, so the distance between us and the matter which sent us the CMB light we got yesterday is increasing as part of that general process. two approximately stationary patches of matter, their distance apart increases 1/140 percent every million years.

but something else is happening. the distance to the surface of last scattering is increasing in a more serious way. we only get the CMB light from some particular batch of matter once. it passes by. tomorrow we will get light from matter that is farther away than that batch whose light we got yesterday.

Question 2 was your best question of all.

==quote==
2. the cmb is 45b lyrs away, is that actual distance right now? the light we get from it was emittied 13.7 b yrs ago, was the cmb 13.7b light yrs away from us when that light was emitted?
==endquote==

No, the matter that emitted the CMB light which we are now getting was, when it emitted the light, at a distance of 41 MILLION lightyears from our matter.
You should get this number for yourself by going to Ned Wright calculator and putting in z = 1090. this is the redshift of the CMB light. It says that while the light has been traveling towards us the universe has expanded by a factor of 1090 (and the wavelength of the light increased by the same factor)

Since both our matter and the matter that emitted the light are stationary, and the distance between is NOW 45 billion, it must be that the distance THEN was 45 billion divided by 1090!
If you divide 45 billion by 1090 you will get 41 million.
therefore the distance to the matter then, when it emitted the light, was 41 million lightyears.

that's a pretty condensed explanation don't be discouraged if it doesn't satisfy you, keep asking, thanks for the above questions.
 P: 35 Marcus, in the following ===quote=== In the past we were getting CMB light from other stuff that IS nearer, but that light has already gone by us. In the future we will be getting CMB light from other stuff that IS out beyond our current surface of last scattering---but that light is still on its way and has not reached us. (I've capitalized the two words I'm interested in) ===end quote=== you use IS. Did you mean "was" for the first on and "will be" for the second? Frank
Astronomy
PF Gold
P: 23,228
 Quote by 81+ Marcus, in the following ===quote=== In the past we were getting CMB light from other stuff that IS nearer, but that light has already gone by us. In the future we will be getting CMB light from other stuff that IS out beyond our current surface of last scattering---but that light is still on its way and has not reached us. (I've capitalized the two words I'm interested in) ===end quote=== you use IS. Did you mean "was" for the first on and "will be" for the second? Frank
Hi Frank, good to hear from you. I think I will stick with IS, in this case. Think of concentric shells of matter. They all emitted their light at the same moment*. the inner shell we heard from yesterday (we got their light first), then today we got the light from the next shell, and tomorrow we will get the light from the next shell (light which is still on its way to us as we speak)

all this matter is stationary with respect to the process of expansion, so the shells stay in relation to each other. the inner shell matter IS closer to us than the next shell, and the outer shell IS farther from us. this comparative relation always was and is and presumably always will be.

so you could replace IS by some different words, but you don't need to because the sentence is true "as is"

*I've simplified the time scale, better to thing of million year time intervals than to think of days. just simpler to speak of yesterday today tomorrow.
P: 622
 Quote by marcus Sounds like no more questions this round. So maybe I will start trying to condense and put things in better language.....
Wish you well! Not an easy task.

You might like to look at the very careful way the concordance model is explained by 'outsiders', namely the distinguished particle physicists Quinn and Nir, in a recent popular book The Mystery of the Missing Antimatter. They choose a raisin-cookie analogy, perhaps because the balloon analogy is a bit shopsoiled. Remember that it dates from times when cosmologists were somewhat obsessed by the fascinating possibility of a spatially curved and closed universe --- aptly thought of as a 4-D version of the 2-D curved surface of a balloon. Spatial flatness rather deflates this analogy, as it were.

They also distinguish properly the difference between redshift and Doppler effect, helpfully clarifying superluminal expansion.
P: 622
 Quote by marcus ....the balloon is diagramatically more perspicuous However you could start a thread about the bread-dough analogy, oldman!
Thanks, but no thanks. Its really a matter of taste -- and I defer to yours. Perhaps it's a choice like that in some elections -- not between the good and bad, but between the bad and worse! But I'm being cynical!
 Astronomy Sci Advisor PF Gold P: 23,228 About analogies that's just being realistic, not cynical . We both know well, there are no perfect ones. I was glad to get the tip about Quinn and Nir. Thx. To move on to the main course, here is Ned Wright's page with the balloon analogy animation. http://www.astro.ucla.edu/~wright/balloon0.html It shows a computer animation of four-fold expansion during which the galaxies remain stationary (only the distances between them increase) and the photons move about and while the photons move they change color from blue, to green, to yellow, to red. as their wavelength increases. that is just false color because if light starts out blue and its wavelength expands by a factor of four it would be invisible infrared----but it is a pedagogical graphic that gives the idea of redshift. (also the yellow color comes out brown because of mixing with background or my browser limitations, but you get the idea) there is another Ned Wright balloon analogy animation here http://www.astro.ucla.edu/~wright/Balloon2.html It does not have dark energy, so expansion eventually slows to a crawl and actually turns around. I only watched the first half. Ah! now t=60 and it is heading for a big crunch! the photons are blueshifting from red to orange to yellow to green...etc.
 P: 177 "No, the matter that emitted the CMB light which we are now getting was, when it emitted the light, at a distance of 41 MILLION lightyears from our matter. You should get this number for yourself by going to Ned Wright calculator and putting in z = 1090. this is the redshift of the CMB light. It says that while the light has been traveling towards us the universe has expanded by a factor of 1090 (and the wavelength of the light increased by the same factor) Since both our matter and the matter that emitted the light are stationary, and the distance between is NOW 45 billion, it must be that the distance THEN was 45 billion divided by 1090! If you divide 45 billion by 1090 you will get 41 million. therefore the distance to the matter then, when it emitted the light, was 41 million lightyears." So if the light was emitted from a distance of 41 million light years but it took 3.7 billion years to get here then was the universe expanding faster than the speed of light at that time? is that what is meant by the inflationary period of expansion? And the expansion has slowed down now because it only takes 41 million yrs for us to see the light from a galaxy that is 41 million light years away right now. or maybe a bit longer because there is still expansion? most non physicists I have noticed seem to like to argue that if the universe is expanding then it must be expanding ito something, a higher dimension because that is normal everyday experience no doubt. Abstract counterintuitive concepts being difficult to accept if you aren't used to it. The physicists here then always say that extra dimensions aren't necessary, that the universe can expand without having another dimension to expand into. Do we know for certain one way or the other or is it just two different opinions? do we know or is there some prevailing opinion if the universe is infinite or finite? If it is finite and you traveled far enough in a straight line would you end up back where you started having gone all the way around? or is that an instance of where the baloon analogy breaks down?

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