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

• Insights
JDoolin
Gold Member
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.
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.

http://www.indiana.edu/~geol105/images/gaia_chapter_1/big_bang_was_not_a_fireworks_dis.htm

"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."
"The problem here is that, despite the name, the big bang was not actually an explosion."
"How do we know that the big bang wasn't really an explosion?"
http://physics.stackexchange.com/qu...ig-bang-the-biggest-explosion-in-the-universe
'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".'
However, I can also see that taking the description of the scale factor of the universe.

$$H(t) = \frac{da/dt}{a}$$

$$H(t) = \frac{da/dt}{a}=1/t$$

$$a = \frac{da}{dt} t$$

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.

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.
Confusion usually comes from a failure to define your terms. If I define "stretching space" to mean an increase in distance between comoving objects, 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 should 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.

JDoolin
Gold Member
@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.
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 we 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!

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Chronos
Gold Member
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.

1oldman2
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.

JDoolin
Gold Member
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.
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.

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.

PeterDonis
Mentor
2019 Award
What have the stars been doing? Are they not pushing away from one another?
No, they're not. They are in free fall; there is no thrust being applied to them.

Gold Member
2019 Award
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.
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.

Gold Member
2019 Award
Beautifully written.
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.

1oldman2 and bapowell
Gold Member
2019 Award
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.
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.

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.
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.

Gold Member
2019 Award
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.
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.

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.
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.

Gold Member
2019 Award
Right. It took a while for the "raisins" to form.
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.

Imager
Gold Member
@phinds

Thumbs up! As beginner, I find the analogy very helpful and I look forward to your next Insights article!

1oldman2
Could you please explain this free fall to me. I thiught sun was in orbit at 828,000 mph and completes one orbit in 230 million years.

I agree just wish we had a better model

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.

PeterDonis
Mentor
2019 Award
Could you please explain this free fall to me.
Free fall means zero proper acceleration--it means the object feels no force. It is weightless.

I thiught sun was in orbit at 828,000 mph and completes one orbit in 230 million years.
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.

1oldman2
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.
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?

Gold Member
2019 Award
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?
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.

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.
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.

Gold Member
2019 Award
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.
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.