balloonanalogy

Learn Inflation Balloon Analogy Misconceptions

This article was written with considerable input from several senior members of the PF community.

The Balloon Analogy is a simple-minded way to help describe (but not completely describe) two facts of cosmology that are difficult for many people to see, namely that the universe is expanding uniformly and that there is no center (and no edge).

The analogy is disliked (often intensely disliked) by serious physicists because it causes at least as much confusion as it is intended to avoid and is often badly misunderstood and/or incorrectly extrapolated to ridiculous points of view and this article is intended to help with that problem.

Presented here is the simple-minded balloon analogy and then a discussion of cosmology and why the balloon analogy is so flawed but also what it does help explain and how. There are many other aspects of cosmology that can get drawn into discussions of the balloon analogy, but I have for the most part resisted discussing any that are not immediately relevant to the analogy, lest I get carpal tunnel syndrome while having this article turn into a textbook on cosmology (which I am not qualified to write anyway).

What the Balloon Analogy is intended to describe

(1) The universe is expanding OUTSIDE of systems that are gravitationally bound, or bound by other local forces (e.g. strong and weak forces). That is, things the size of a local cluster of galaxies and smaller (like the Milky Way, Earth, you, me, atoms, and so forth), do NOT expand.

(2) The expansion has no center and everything is moving away from everything else, with things farther from each other receding faster from each other than things closer together.

THE ANALOGY

Think of each gravitationally bound system as a penny, and glue a bunch of these pennies onto a balloon that is only slightly blown up.

Now we blow up the balloon more. ALL of the pennies move away from each other uniformly, and those that are farther away from each other move away from each other faster than those that are closer together. If you choose ANY penny on the surface, it sees ALL of the other pennies moving away from itself and it sees the ones farther away moving away faster than those close by. No penny is the center of the expansion. There IS no center to the universe.

That is the way the universe works and that is what the analogy is intended to show. The fact that all the pennies move away from each other and that ones farther away move away faster and that there is no center.

What DOESN’T work in using the balloon analogy and how it can be more usefully thought of

FIRST: NO CENTER there is no center. Only a portion of the surface of the balloon is to be considered in the analogy. This is difficult for some people to get their head around because it is so obvious that the balloon is really a 3D object with a center. Well, yes it is, BUT NOT IN THE ANALOGY. Only the surface counts in the analogy, so if you insist that there IS a center, you are completely misunderstanding and misusing, the analogy and you are likely arriving at one of those false conclusions that make physicists grit their teeth at the balloon analogy. So don’t! There is no center (to the balloon surface in the analogy or to the universe)

SECOND: SIZE/SHAPE The analogy should ONLY consider a portion of the balloon’s surface — it does not make any statements about the size or shape of the universe (other than it is getting bigger). Forget that the surface of the balloon is curved. That’s NOT intended to be representative of the actual universe. It is actually more reasonable to think of a flat sheet of rubber that is being stretched equally in all directions. That would be a better analogy, but you’d have to confine the analogy to only a section of the sheet. Edges would NOT be part of the analogy. The analogy is not intended to comment in any way on the shape of the universe, whether it is open or closed, flat or curved, or ANY of those things. Those are NOT part of the analogy. The universe not only has no center, it has no edge, but that does not imply that it is necessarily infinite, it could be finite but unbounded (like the surface of the Earth, for example). Also, the balloon, as we humans look at it and consider only the surface, is a 2D object (the surface is) but we recognize that it is embedded in a 3D world. The analogy is NOT intended to imply that our 3D universe is embedded in some 4D space (I’m not talking about 4D space-time but 4 dimensions of distance) — that just isn’t part of the analogy.

THIRD: LOCAL EFFECTS The pennies don’t change size (gravitationally bound systems don’t expand and nothing inside of them expands), they just get farther apart and none of them are at the center. There IS no center. The most reasonable way to UN-simplify the balloon analogy in this regard would be to consider that the construct is not a balloon with pennies glued to it but rather that it is a balloon in which, somewhat magically, pennies have been embedded such that the circumference of the pennies melds with the rubber and there IS no rubber inside the area of the pennies. When the balloon expands, the pennies get farther apart but THEY do not expand. Even this UN-simplification of the analogy has a problem because actually, dark energy does exist inside of gravitationally bound systems, it just doesn’t have any effect (see the comments at the bottom of the page).

FORTH: NO STRETCHING The surface of the balloon “stretches” and this leads to weird discussions of the “stretching” of space or the “expansion” of space. To further UN-simplify the balloon analogy, what you REALLY need to think of is the construct described in “THIRD” directly above, BUT … take away the actual balloon material and just think of things happening to the pennies as though the balloon WAS there (so as to maintain the pennies’ motion in the analogy). In other words, what cosmology REALLY says is not that space stretches or expands but rather just simply that gravitationally bound systems keep getting farther away from each other. It is DISTANCE that is changing, not space. This is another of those things that are badly served by the balloon analogy. There is of course, in some sense, “more” of something in between galactic clusters as the distance increases, but just what it is that there is “more” of gets to be a theological/philosophical discussion that gets WAY beyond the balloon analogy. I refer you to Metric Expansion of Space

FIFTH: COSMOLOGICAL TIME There are sometimes attempts to bring cosmological time into the analogy by considering that the 3D center of the balloon represents the singularity and the balloon (the universe) expanded from there. This is an excellent example of stretching the analogy WAY beyond where it is intended to go (pun intended).

 

EXPANSION(S)

There are really three “expansions” that people get mixed up about.

INFLATION — In a very early and very tiny fraction of a second, just after the singularity, the universe “expanded” in an incredibly massive burst. This is known as INFLATION, not expansion, although it IS an expansion as the term is used in the English language. This inflation is NOT a 100% confirmed fact, and there are differences of opinion about it among physicists (and cosmological models), but it is by far and away the best model to explain the universe as we understand it. This “inflation” was in some ways very similar to the current accelerated expansion, and there is some lively discussion in physics circles as to the possibility of a relationship between the two, but that’s getting a bit beyond the balloon analogy.

EXPANSION — After inflation, the universe settled down to a more sedate rate of expansion and THIS is what is normally meant by the EXPANSION of the universe. It is still going on today. This expansion has NOTHING to do with “dark energy” (see the paragraph directly below), although dark energy has been present, as far as is known, since the beginning of the universe AND has been having an effect all along although it did not overwhelm gravity until about 8 billion years after the singularity. There really have been two things going on all along, the expansion I’m talking about here, AND the contribution of “dark energy”, but see directly below for further discussion.

ACCELERATION of the expansion — Up until sometime in the late 1990s, it was assumed that gravity was slowing down the rate of expansion (as it was understood in the paragraph above, with no reference to “dark energy” and that eventually, it would either reach a steady-state and just go on at a constant rate forever, or much more likely would reverse direction and contract (the Big Crunch scenario). When the first possible measurements were made, much to everyone’s surprise, it was found that not only is the rate of expansion NOT slowing down, it is ACCELERATING. This acceleration is NOT what is normally meant by “expansion” (see the paragraph directly above), it is the ACCELERATION of the expansion. It is attributed to a force that is not understood, and that has been given the name “dark energy”. The “dark” just means we have no idea what it is (just what it does), and this is TOTALLY unrelated to “dark matter” (which is called “dark” because it is LITERALLY dark … it does not reflect or emit any electromagnetic radiation at any frequency, including visible light). So “dark energy” has been around all along, we now understand, and about 6 billion years ago, it started to overcome gravity and cause the expansion to start accelerating.

 

OTHER NOTES ON COSMOLOGY

Dark energy and dark matter have NOTHING to do with each other and all statements I have ever seen to the contrary have been utter nonsense. It would likely have saved hundreds of thousands of keystrokes here on The Physics Forum if “dark matter” had been more appropriately called “Zwicky matter” and “dark energy” had been called “vacuum energy”

“Dark energy” is so staggeringly weak on local scales (a planet for example) that it has an effect that is described by some as (various forms of the phrase …) “utterly negligible” and by others as “non-existent”. I’m in the “non-existent” camp and my favorite analogy is that it is like an ant pushing on a house. It’s not that the ant has such a tiny effect that it is not measurable, it is that the any has no effect at all. The ant IS pushing on the house, but it cannot to any extent at all overcome the forces that keep the house on its foundations.

Expansion, even with acceleration, is so staggeringly slow on small scales that it might as well not be happening. Over cosmological distances, it has a huge effect, but here’s my favorite analogy to show local effect. Even though the universe is expanding, it’s still going to be hard to find a parking place. This is just a simple-minded way of thinking about the local effects of the expansion. If you could go out into intergalactic space and magically draw a set of parking place lines, it would be about TWENTY BILLION YEARS before they had moved far enough apart to let you park a second car. Now, I’m willing to circle the block a couple of times to get a parking place, but twenty billion years is just too much. I’d be late for the movie.

On the other hand, it has such a huge effect over cosmological distances, that the galaxies at the edge of our observable universe are receding from us at something like 3 times the speed of light. No speeding tickets are issued, because they are not moving faster than light in our frame of reference, they are just getting farther away. It’s like a boat being carried down a fast river. To someone on the boat, it’s traveling 10 mph but to someone on the bank, it’s moving 25 mph. It’s the 10 mph that has to be compared to the speed limit; the 25mph is irrelevant to the speed limit.


NOTE: measures of distance and time in cosmology, as well as the shape/extent of the universe and the fact that “space” is really “space-time”, are all very complex topics, and my simplistic ways of talking about them in this article are just that … simplistic. My point here was to produce a fairly modest, but correct (with some simplifications) analysis of the balloon analogy without, as I noted at the beginning, writing a text on cosmology.

Also, my use of the pop-science terms “expansion/expanding” should not be interpreted to mean that space is a “thing” that can bend/stretch/expand. Things get farther apart (Metric Expansion) and things with no force applied to them move on “bent” (Euclidean) lines which are actually straight lines in the math that describes space-time (Riemann Geometry)

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  1. phinds says:

    [QUOTE=”Edriven, post: 5248424, member: 572993″]I’m not sure how I changed the subject. I was responding to a post discribing acceleration of the universe. My point is, that red shift tells us that the universe is accelerating. Therefore the sun can not be in a balanced free fall because it is accelerating. Acceration means that forces are not equal and solar thrust is pushing stars apart. It’s my idea that solar thrust is the energy described in dark matter. What better place to get energy from than the stars? What would change the direction of solar thrust? Gravity and collision of other matter.[/QUOTE]
    Ah, good. Now I understand your question.

    No, red shift does NOT tell us that the sun is accelerating. You are making a very common mistake of confusing recession velocity with proper velocity. Google “Metric Expansion” for more discussion.

  2. Edriven says:

    [QUOTE=”phinds, post: 5248416, member: 310841″]You seem to keep changing the subject. What is it that you want to know, exactly? Please be as precise as you can with your question.[/QUOTE]
    I’m not sure how I changed the subject. I was responding to a post discribing acceleration of the universe. My point is, that red shift tells us that the universe is accelerating. Therefore the sun can not be in a balanced free fall because it is accelerating. Acceration means that forces are not equal and solar thrust is pushing stars apart. It’s my idea that solar thrust is the energy described in dark matter. What better place to get energy from than the stars? What would change the direction of solar thrust? Gravity and collision of other matter.

  3. phinds says:

    [QUOTE=”Edriven, post: 5248338, member: 572993″]Thanks for the explanation. But the difference in the space station and the sun is that the space station is not accelerating away from the earth. Due to red shift we know that we are accelerating away from other stars. Right?[/QUOTE]
    You seem to keep changing the subject. What is it that you want to know, exactly? Please be as precise as you can with your question.

  4. Edriven says:

    [QUOTE=”PeterDonis, post: 5248269, member: 197831″]Free fall means zero proper acceleration–it means the object feels no force. It is weightless.

    It’s in orbit about the center of the galaxy, yes. But that’s perfectly consistent with it being in free fall, just as the International Space Station, in orbit about the Earth, is in free fall–it feels no force, and is weightless.[/QUOTE]
    Thanks for the explanation. But the difference in the space station and the sun is that the space station is not accelerating away from the earth. Due to red shift we know that we are accelerating away from other stars. Right?

  5. PeterDonis says:

    [QUOTE=”Edriven, post: 5248173, member: 572993″]Could you please explain this free fall to me.[/QUOTE]

    Free fall means zero proper acceleration–it means the object feels no force. It is weightless.

    [QUOTE=”Edriven, post: 5248173, member: 572993″]I thiught sun was in orbit at 828,000 mph and completes one orbit in 230 million years.[/QUOTE]

    It’s in orbit about the center of the galaxy, yes. But that’s perfectly consistent with it being in free fall, just as the International Space Station, in orbit about the Earth, is in free fall–it feels no force, and is weightless.

  6. Edriven says:

    Thx for helping me with this topic. I thought there is no way that it could be applied evenly because of solar busts and storms happen unevenly Most of its power or wind goes in space but when a another star is “near” it’s solar winds push on our entire universe.

  7. Edriven says:

    [QUOTE=”phinds, post: 5247958, member: 310841″]Thanks.I disagree. It is a limited analogy, not a bad model, and as long as you understand the limitations of the analogy it is quite a good one.[/QUOTE]

  8. Edriven says:

    [QUOTE=”PeterDonis, post: 5247934, member: 197831″]No, they’re not. They are in free fall; there is no thrust being applied to them.[/QUOTE]

  9. phinds says:

    [QUOTE=”Hornbein, post: 5248000, member: 489043″]Right. It took a while for the “raisins” to form.
    [/QUOTE]Yes, it did, but that has nothing to do with the raisin bread analogy, which is just a way of talking about what the “raisins” are doing now and in the future.

  10. Hornbein says:

    [QUOTE=”phinds, post: 5247995, member: 310841″]I think we must be talking about different “baking bread” analogies. The one I’m talking about has noting to do, really, with all that you just said. Rather, it is an alternate to the balloon analogy and talks about the universe NOW, not starting just after the singularity.[/QUOTE]

    Right. It took a while for the “raisins” to form.

    I was thinking I couldn’t fill up a whole article with the raisin bread thing. But I guess I can if I try.

  11. phinds says:

    [QUOTE=”Hornbein, post: 5247989, member: 489043″]Hmmm, how would I go about doing that? The key thing that people miss is that the early Universe had no empty space whatsoever and stayed that way for quite a long time. I think it would be entertaining to trace an imaginary eye witness of the Universe looked as it grew. At first it is unimaginable, then like starting at the center of the Sun and traveling outward. Then some empty space appears. The color goes down the spectrum to dark red then the whole thing turns black though still very hot. It appears that it will stay black forever, then the previously negligible force of gravity very slowly saves the day. Giant suns form and quickly explode to make iron and such. Neutron stars collide to produce the heavy elements. Rocky planets form.[/QUOTE]
    I think we must be talking about different “baking bread” analogies. The one I’m talking about has noting to do, really, with all that you just said. Rather, it is an alternate to the balloon analogy and talks about the universe NOW, not starting just after the singularity.

  12. Hornbein says:

    [QUOTE=”phinds, post: 5233738, member: 310841″]I agree, it’s a good analogy. Why don’t you write an insights article on it? I’m sure Greg would be happy to have that.[/QUOTE]

    Hmmm, how would I go about doing that? The key thing that people miss is that the early Universe had no empty space whatsoever and stayed that way for quite a long time. I think it would be entertaining to trace an imaginary eye witness of the Universe looked as it grew. At first it is unimaginable, then like starting at the center of the Sun and traveling outward. Then some empty space appears. The color goes down the spectrum to dark red then the whole thing turns black though still very hot. It appears that it will stay black forever, then the previously negligible force of gravity very slowly saves the day. Giant suns form and quickly explode to make iron and such. Neutron stars collide to produce the heavy elements. Rocky planets form.

  13. phinds says:

    [QUOTE=”Mordred, post: 5236664, member: 351508″]Good job Phinds , Myself I would add that one function of the balloon analogy is to help students understand the Cosmological principle, then adding a brief description on the terms homogeneous and isotropic.

    Another thing to consider is that the angles between any three points of measure also do not change.

    Just a couple of points to consider adding.[/QUOTE]
    Yeah, I thought about it, but as I said in the article, there are just too many other things that could be brought into the discussion and it was already a bit longer than I would have liked.

  14. phinds says:

    [QUOTE=”Edriven, post: 5247812, member: 572993″]Beautifully written. [/quote]Thanks.[quote] It is a bad model. [/QUOTE]I disagree. It is a limited analogy, not a bad model, and as long as you understand the limitations of the analogy it is quite a good one.

  15. phinds says:

    [QUOTE=”Edriven, post: 5247812, member: 572993″]Beautifully written. It is a bad model. I wanted to touch briefly on dark energy. How does a rocket move in space. It burns fuel and creates thrust. If rocket wants to go faster, it must burn longer. Or use bursts to add speed. Basically longer burn equals acceleration. What have the stars been doing? Are they not pushing away from one another? SOLAR THRUST. the opposite force of gravity.[/QUOTE]
    To the extent that there is any thrust created by stuff being ejected from stars, it averages out to being uniform over all directions so net thrust is zero. So no they are not, as Peter has already pointed out, thrusting away from each other.

  16. PeterDonis says:

    [QUOTE=”Edriven, post: 5247812, member: 572993″]What have the stars been doing? Are they not pushing away from one another?[/QUOTE]

    No, they’re not. They are in free fall; there is no thrust being applied to them.

  17. JDoolin says:

    [QUOTE=”Chronos, post: 5235598, member: 10970″]The problem with science is it forces you to rely on the ‘known’ to explore the ‘unknown’. We have no reliable way to test the ‘known’ aside from empirical evidence. Our ‘knowns’ currently appear pretty reliable given the vast body of empirical vetting they have managed to survive. That should not give us any false sense of confidence. No aspect of theoretical knoledge is invulnerable to future experimental results. We should, however, not abandon our hard fought knowledge without thoroughly well confirmed experimental evidence.[/QUOTE]

    You can also use the “unknown” to explore the unknown.

    Sometimes it makes sense to introduce a false or uncertain hypothesis for exploring the unknown.

    For instance, we imagine what the world would really be like if the world were flat, and conclude that we should be able to see mountains thousands of miles away. We try to imagine a geocentric solar system and realize there is no mechanism which holds the planets or sun in orbit around the earth. But to my knowledge, this isn’t done with “The Big Bang Theory.” Instead of asking, honestly, at what a “real” Big Bang would imply, one is generally met with a chorus of “Everyone knows the Big Bang wasn’t really a big bang.”

    It’s like if we were discussing heliocentric vs. geocentric models of the solar system, but the heliocentric people insisted on calling their model “geocentric” then adding “geocentric isn’t really geocentric.”

    Only after you give the ideas distinct names can you honestly make a comparison between them. Otherwise, you will have many different people all using the exact same words, and all having completely different meanings.

  18. Mordred says:

    Good job Phinds , Myself I would add that one function of the balloon analogy is to help students understand the Cosmological principle, then adding a brief description on the terms homogeneous and isotropic.

    Another thing to consider is that the angles between any three points of measure also do not change.

    Just a couple of points to consider adding.

  19. Chronos says:

    The problem with science is it forces you to rely on the ‘known’ to explore the ‘unknown’. We have no reliable way to test the ‘known’ aside from empirical evidence. Our ‘knowns’ currently appear pretty reliable given the vast body of empirical vetting they have managed to survive. That should not give us any false sense of confidence. No aspect of theoretical knoledge is invulnerable to future experimental results. We should, however, not abandon our hard fought knowledge without thoroughly well confirmed experimental evidence.

  20. JDoolin says:

    [QUOTE=”phinds, post: 5233989, member: 310841″][USER=268035]@JDoolin[/USER], you seem to have the impression that my article is targeted towards people who know a lot of physics. Nothing could be further from the truth, and all of the things that I discuss address issues (yes, sometimes in simple terms) that amateurs DO have as witnessed (as Peter pointed out) by a large number of threads here on PF, to say nothing of elsewhere.

    Even the very terminology you use is unknown to the target audience, so I do not consider your objections to be relevant to the article.[/QUOTE]

    I wish that my comments could be seen as something other than objections. I don’t “object” to your statement that there is no such thing as cosmological time. I don’t “object” to your statement that space does not stretch. To the contrary, I agree with you.

    But in our agreement, I don’t think [I][U]we[/U][/I] agree with the standard model of cosmology.

    But I’m not trying to establish, right now, whether we are right, or we are wrong. What I’m trying to get at is whether these are actually two distinct models, or if they are the same model.

    I think they are two completely different models, and a lot of people don’t realize that they are different. The point is that the differences show up strikingly in your analysis of the balloon model, and I thank you for that!

  21. JDoolin says:

    [QUOTE=”PeterDonis, post: 5233927, member: 197831″]Only if there is dark energy present. An FLRW model with only matter and radiation present is like what you are calling a “kinematic” model; there is no force pulling things apart, only inertia from the initial big bang.[/QUOTE]

    Is the Kinematic model just an example of the FLRW model universes, or is it something entirely different? Have a look at the following descriptions–they seem to imply that the FLRW models, there is NO INERTIA from the initial big bang event.

    [QUOTE][URL]http://www.indiana.edu/~geol105/images/gaia_chapter_1/big_bang_was_not_a_fireworks_dis.htm[/URL]

    “the completely wrong impression that the event was like an explosion and that the universe is expanding today because the objects in it are being flung apart like fragments of a detonated bomb.”[/QUOTE]

    [QUOTE][URL]https://4gravitons.wordpress.com/2014/12/05/the-three-things-everyone-gets-wrong-about-the-big-bang/[/URL]
    “The problem here is that, despite the name, the big bang was not actually an explosion.”[/QUOTE]

    [QUOTE][URL]https://answers.yahoo.com/question/index?qid=20130811144132AAgWtf0[/URL]
    “How do we know that the big bang wasn’t really an explosion?”[/QUOTE]

    [QUOTE][URL]http://physics.stackexchange.com/questions/66910/why-is-the-big-bang-the-biggest-explosion-in-the-universe[/URL]
    ‘The big bang is not an explosion in the conventional sense of the word. The big bang corresponds to an exponential expansion of spacetime and it is this incredible rate of expansion that can be dubbed “explosive”.'[/QUOTE]

    However, I can also see that taking the description of the scale factor of the universe.

    [tex]H(t) = frac{da/dt}{a}[/tex]

    [tex]H(t) = frac{da/dt}{a}=1/t[/tex]

    [tex]a = frac{da}{dt} t[/tex]

    Mathematically, then, you could argue that the kinematic universe is an example of the FLRW metric with Hubble Parameter equal to the reciprocal of the age of the universe.

    However, even if the math is the same, the FLRW metric explains that changing scale factor between objects as a stretching of space (or a change in distance, if that seems more comfortable) between comoving objects, so there would be no inertia. The kinematic model would explain that changing scale factor as a velocity between non-comoving objects, so there would be inertia.

    So no, I don’t think the kinematic model is an example of the FLRW metric. It is an entirely different theory, based on entirely different assumptions.

    [QUOTE]You may not be confused by that description, but many, many people are, as evidenced by the copious threads here on PF caused by such confusion. For one thing, “stretching space” invites the hypothesis that something is doing the stretching; even with dark energy present, the small force it exerts isn’t exerted on “space”, it’s exerted on comoving objects.[/QUOTE]

    Confusion usually comes from a failure to define your terms. If I define “stretching space” to mean [B]an increase in distance between comoving objects[/B], then the FLRW metric definitely describes stretching space.

    Perhaps in other minds, the word “stretching” has a different connotation. For instance, in order to stretch something, you need two hands, pulling away from each other, and a substance in between. But there’s no substance in space, and there are no hands on either side, so it can’t stretch.

    I think that the idea of stretching space [I]should[/I] invite the hypothesis that something is doing the stretching. The alternate hypotheses are that there is no stretching, or that the stretching represents a phenomenon without any cause. And then the hypothesis that there IS stretching should lead you to a prediction that there should be a negligible but finite repellant force between comoving objects, which would be a function of their mass, their distance, and the age of the universe.

  22. Haelfix says:

    I happen to not like the Balloon analogy, as one wastes as much time correcting the mistakes as one would to simply explain the correct mathematics.

    A few problems.
    1) Topology. The primary problem the balloon analogy leads too is that the student immediatedly visualizes a spherical universe. Then the teacher has to explain that based on our current measurements of certain values (the sign of the cc, the amount of omega matter etc) we actually seem to prefer a universe with a simpler topology that more closely resembles R^4. Then we say something about the lack of a spacetime singularity in the middle of the sphere, that the big bang happened everywhere simultaneously and not at a specific point, that proper distance doesn’t necessarily mean that galaxies were all squished together etc. Of course, this statement is also incorrect, or rather uncertain. The correct statement is that we do NOT know at this time what the topology of the universe is. It is perfectly consistent with data to have a universe with a nontrivial topology (although not as it turns out, something identically spherical). If we live in such a universe, then there are loops that can and will contract to a point, and you really do have a pileup of galaxies in principle. Likewise, you also run the risk of having circumlocution (the non detection of mirror galaxies fortunately allows us to put constraints on this effect) and you could in principle have a spacetime singularity at a point (although again we really should talk about geodesic incompleteness and the possible existence of a horizon if cosmic censorship applies).

    Which gets to the broader point. GR puts some constraints on the global structure, but it is fundamentally a metric theory. It is absolutely vital to not mix up metric expansion (really the clocks and rulers e.g. the definition of distances between points) from how those events/points are arranged in the more fundamental point set topology. The sooner the student gets this point, the better things will be

    2) Questions surrounding the physical properties of the elastic substance of the balloon will come into question. This of course is a disaster as it is exactly the opposite of what we want to show, as the elastic potential will have the wrong sign and completely different properties (it will heat up when you pull it apart, etc).

    3) The analogy is fundamentally Newtonian. It is likely one of those strange and quirky mathematical accidents, but the FRW solution happens to be originally derived for Newtonian cosmology without the need for GR. There we really do have an expanding spherically symmetric ball of radiation that behaves exactly as one pictures in the analogy (without CC). But we know this is wrong. It is wrong the second one wants to put in inhomogeneities into the equations, at which point you have a solution that behaves nothing like what you might guess based on your Newtonian intuition. There you really need the full power of GR.

    Which gets to the last point. Normally we correct the balloon analogy with the raisin bread analogy to avoid the messy question about why we don’t feel atoms in our body or say planetary orbits blowing up under the influence of expansion, and the teacher says that that local gravitational interactions are much more important and that the model only represents long range phenomena, but then the student instantly wonders how a theory of spacetime (gravity) is suddenly chopped up into regimes of validity. Which gets back to doing things correctly by introducing legitimate GR corrections in the form of inhomogenieties.

    Anyway, it gets complicated correcting all the mistakes, so I prefer limiting the analogy as much as possible around bonafide physics students..

  23. phinds says:

    [USER=268035]@JDoolin[/USER], you seem to have the impression that my article is targeted towards people who know a lot of physics. Nothing could be further from the truth, and all of the things that I discuss address issues (yes, sometimes in simple terms) that amateurs DO have as witnessed (as Peter pointed out) by a large number of threads here on PF, to say nothing of elsewhere.

    Even the very terminology you use is unknown to the target audience, so I do not consider your objections to be relevant to the article.

  24. PeterDonis says:

    [QUOTE=”JDoolin, post: 5233760, member: 268035″]In an FLRW model, there is a negligible force pulling things apart[/QUOTE]

    Only if there is dark energy present. An FLRW model with only matter and radiation present is like what you are calling a “kinematic” model; there is no force pulling things apart, only inertia from the initial big bang.

    [QUOTE=”JDoolin, post: 5233760, member: 268035″]As I described in my last post, the cosmological scale factor is generally presented as a changing scale of the FLRW universe as a whole. I would be hard-pressed to find a better description for that than “stretching space”.[/QUOTE]

    You may not be confused by that description, but many, many people are, as evidenced by the copious threads here on PF caused by such confusion. For one thing, “stretching space” invites the hypothesis that something is doing the stretching; even with dark energy present, the small force it exerts isn’t exerted on “space”, it’s exerted on comoving objects.

  25. phinds says:

    [QUOTE=”Hornbein, post: 5233720, member: 489043″]I don’t care for the balloon analogy. I like the “baking raisin bread” analogy.[/QUOTE]
    I agree, it’s a good analogy. Why don’t you write an insights article on it? I’m sure Greg would be happy to have that.

  26. phinds says:

    [QUOTE=”JDoolin, post: 5231675, member: 268035″]The balloon analogy is primarily used to explain how redshift is NOT caused by relativistic Doppler recession of distant galaxies. [/QUOTE]
    That has not been my experience at all. I have always found the balloon analogy used to simply give a graphic demonstration of how it is that the universe is expanding uniformly from every point and that there is no center. I’m not familiar with its use regarding any discussion of Doppler shift.

  27. phinds says:

    [QUOTE=”JBA, post: 5231491, member: 570081″]What is actually driving the observed acceleration and expansion/inflation of our universe.[/QUOTE]
    OK. I agree w/ that. I wasn’t sure if you meant what it doing as opposed to what causing it.

  28. timmdeeg says:

    [QUOTE=”eltodesukane, post: 5231329, member: 394501″]The point is that the so called “Hubble constant” is not a constant over time.[/QUOTE]
    That’s right and therefore it might be better to use the term Hubble parameter, which means the ‘rate of expansion’.

    In short, during the epoch of inflation the universe expanded exponentially (driven by the cosmological constant only) and thus the ‘rate of expansion’ was constant (roughly). Since then it is decreasing and will be approaching asymptotically a constant value in the very far future again, due to the dominating dark energy then, at least according to the current model.

  29. JBA says:

    [QUOTE=”phinds, post: 5230949, member: 310841″]I don’t get you. What is it that “no one actually understands” ?[/QUOTE]

    What is actually driving the observed acceleration and expansion/inflation of our universe.

  30. PeterDonis says:

    [QUOTE=”eltodesukane, post: 5231301, member: 394501″]Many of those interpretation problems do not appear if we consider matter contracting instead of space expanding.[/QUOTE]

    Perhaps not, but you now have a whole new set of problems. Such as, if matter is supposedly contracting, why is the size of the Earth not changing?

    [QUOTE=”eltodesukane, post: 5231301, member: 394501″]All we can measure are ratios of distances.[/QUOTE]

    On cosmological scales, perhaps this is true, since converting between the various cosmological distance scales is basically taking ratios of different indirect distance measurements. But ultimately all of those cosmological distance ratios are calibrated to distances that are [I]not[/I] measured as ratios. I used the size of the Earth as an obvious example above, but perhaps a more relevant example for this discussion would be distances to stars measured by parallax. That gives an absolute reference for distance that cannot be interpreted as “contracting”.

  31. phinds says:

    [QUOTE=”eltodesukane, post: 5231301, member: 394501″]Many of those interpretation problems do not appear if we consider matter contracting instead of space expanding.
    Those are equivalent point of views as far as we know.
    All we can measure are ratios of distances.
    (If fraction a/b is increasing, is a increasing or b decreasing?)[/QUOTE]
    And how would “contracting matter” explain the red shift of light from distant galaxies?

  32. phinds says:

    [QUOTE=”JBA, post: 5230927, member: 570081″]This is what younget when someone tries to explain something that no one actually understands.[/QUOTE]
    I don’t get you. What is it that “no one actually understands” ?

  33. timmdeeg says:

    Hi phinds, I think you article is extremely helpful to the ‘interested layman’.
    The only thing I would recommend to reconsider is the wording ‘the rate of expansion’ is slowing down or is accelerating, resp. This could confuse the layman who knows about the Hubble constant, which isn’t accelerating. The expansion of the universe is either accelerating or decelerating. Or perhaps, but I’m not sure, it’s more precise to say the universe expands at an increasing rate, in order to avoid the term ‘the rate of expansion’.

  34. Shyan says:

    [QUOTE=”phinds, post: 5230278, member: 310841″]The “first” ? AAAACCKK ! You want MORE? I don’t know anything else :smile:[/QUOTE]
    I have the same feeling!

  35. phinds says:

    [QUOTE=”Greg Bernhardt, post: 5230031, member: 1″]Nice first Insight [USER=310841]@phinds[/USER]![/QUOTE]
    The “first” ? AAAACCKK ! You want MORE? I don’t know anything else :smile:

  36. 1oldman2 says:

    As a "layman"(barely) I find the balloon analogy and related conversations very useful and interesting in regards to my understanding of the principles discussed.I must add at this point that I'm no scholar, we won't be collaborating on any papers and the Nobel prize is safe from me. in fact if intelligence is relative then compared to yours mine would be measured on the Planck scale.My point is when someone takes the time to develop good analogies such as the balloon one here then a much broader segment of society is able to learn these concepts and hopefully get interested and excited about the sciences in general. Good job !! looking forward to more writing in this manner.

  37. Edriven says:

    Beautifully written.  It is a bad model.  I wanted to touch briefly on dark energy. How does a rocket move in space. It burns fuel and creates thrust. If rocket wants to go faster, it must burn longer. Or use bursts to add speed. Basically longer burn equals acceleration. What have the stars been doing? Are they not pushing away from one another?  SOLAR THRUST. the opposite force of gravity.

  38. JDoolin says:

    I'm not sure if any further discussion is coming, so I'll try to make my observations a little clearer.You've listed five misconceptions of the balloon analogy.  I think only the first two you listed are actual misconceptions.  The last three are actual features of an FLRW model universe, but not a kinematic model.(First: No Center)  I agree, this is a misconception. The two (spatial) dimensional balloon surface with a center in (spatial) 3D space might imply that the universe is a three-dimensional structure with a center in a 4D space.  (Second: Size/Shape) I agree; another misconception.  The shape of the balloon might imply that you can get back where you started by going far enough in a straight line.(3) (Third: Local Effects).  I disagree that this is a misconception.  There is a big difference between an effect that is negligible, and an effect that doesn't exist.  An ant, pushing on a house, still exerts a force.  If the house were sitting on a frictionless plane, it would accelerate.  In an FLRW model, there is a negligible force pulling things apart; but it is so tiny that gravity and electrical forces hold it together.  In a kinematically expanding universe, there is no force pulling things apart.  You just have inertia, from the initial big bang event.(4) (Forth: No Stretching)  I disagree that this is a misconception.  As I described in my last post, the cosmological scale factor is generally presented as a changing scale of the FLRW universe as a whole.  I would be hard-pressed to find a better description for that than "stretching space".  Again, in a kinematically expanding universe, you could say "no stretching".(5) (Fifth: Cosmological Time) The balloon analogy highlights a very important difference between a kinematic model of the universe and the standard model.  In a kinematic model, every particle is literally touching at the moment of the big bang, and they separate because of their velocity.  In the balloon analogy, every particle was already separated by a distance on the balloon surface, but the scale factor of the universe was equal to zero, so the balloon, itself, was contracted to a point.  That behavior at the singularity is an essential difference between a kinematic model universe and a FLRW metric universe.

  39. JDoolin says:

    Here, look at this page on wikipedia regarding the cosomological scale factor:  https://en.wikipedia.org/wiki/Scale_factor_(cosmology)The article begins with the assumption that the scale factor exists… and comes down to v = H d;  e.g. velocity = Hubble's Constant * DistanceThe implications here is that Hubble's constant is… well, a constant.  And the velocity and distance relationship is… well; weird.However, a simple modification to the equation; stating that Hubble's constant is the reciprocal of the age of the universe, yieldsDistance = velocity * timewhich you teach to students in Junior High.In my experience, very smart people are very uncomfortable with the idea of DIstance = Velocity * Time being applied at cosmological scales.  They will definitively say "No, that is NOT it."  And usually, they will use some version of the balloon analogy to make their point. However, here, you seem to have debunked all aspects of the balloon analogy which would have actually conflicted with the kinematic description.My point, I guess, is that as soon as you invoke that scale factor, a(t), then you are strongly implying that space is (or at least could be) stretching over time–perhaps in an unknown and unpredictable way.

  40. JDoolin says:

    I think there really are more than one model out there that you have to address.  It's really not fair to just say "A lot of people think that the center of the balloon represents the big bang singularity, but that's just not true."The balloon analogy is primarily used to explain how redshift is NOT caused by relativistic Doppler recession of distant galaxies.  However, I think if you take the balloon analogy, but only the TWO features you said were true, and "caveat" the FIVE features you said were false, there is no material way that your model actually conflicts with the idea of Doppler recession velocities.So is there a valid feature of the Balloon analogy that actually contradicts with Doppler recession?

  41. eltodesukane says:

    Many of those interpretation problems do not appear if we consider matter contracting instead of space expanding.Those are equivalent point of views as far as we know.All we can measure are ratios of distances.(If fraction a/b is increasing, is a increasing or b decreasing?)

  42. eaglechief says:

    well done and thumbs up! I sometimes find myself in sticking too close to the ballon model, but it helps in the beginning.perhaps in future one comes back to the balloon-model by taking it more by word like "what behaves on the surface like a balloon might behave in the inside like a balloon, as well"

  43. Geofleur says:

    Nice article! I just used the balloon analogy today with my students, and I didn't say any of those wrong things. But I worry now that my students would draw those spurious parallels themselves. I may have them read this.

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