# Insights The Balloon Analogy ... the Good, the Bad, and the Ugly - Comments

1. Sep 16, 2015

### phinds

2. Sep 16, 2015

### Greg Bernhardt

3. Sep 16, 2015

### Geofleur

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.

4. Sep 16, 2015

### phinds

The "first" ? AAAACCKK ! You want MORE? I don't know anything else

5. Sep 17, 2015

### eaglechief

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"

6. Sep 17, 2015

### ShayanJ

I have the same feeling!

7. Sep 17, 2015

### timmdeeg

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'.

8. Sep 17, 2015

### JBA

This is what younget when someone tries to explain something that no one actually understands.

9. Sep 17, 2015

### phinds

I don't get you. What is it that "no one actually understands" ?

10. Sep 17, 2015

### eltodesukane

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?)

11. Sep 17, 2015

### eltodesukane

The point is that the so called "Hubble constant" is not a constant over time.

12. Sep 17, 2015

### phinds

And how would "contracting matter" explain the red shift of light from distant galaxies?

13. Sep 17, 2015

### Staff: Mentor

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?

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 not 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".

14. Sep 18, 2015

### JBA

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

15. Sep 18, 2015

### timmdeeg

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.

16. Sep 18, 2015

### phinds

OK. I agree w/ that. I wasn't sure if you meant what it doing as opposed to what causing it.

17. Sep 18, 2015

### JDoolin

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?

18. Sep 18, 2015

### phinds

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.

19. Sep 18, 2015

### JDoolin

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 * Distance

The 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, yields

Distance = velocity * time

which 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.

20. Sep 20, 2015

### Hornbein

I don't care for the balloon analogy. I like the "baking raisin bread" analogy.

21. Sep 20, 2015

### phinds

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.

22. Sep 20, 2015

### JDoolin

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.

23. Sep 20, 2015

### Staff: Mentor

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.

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.

24. Sep 20, 2015

### phinds

@JDoolin, 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.

25. Sep 21, 2015

### Haelfix

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..