# Explaining expansion.

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I don't understand your objection. In the balloon analogy as I understand it only the surface of the balloon exists. It doesn't have sides so no pressure can be brought to bear on them.

Or are you talking about distortions to the surface? And, if so, what is their real world equivalent? I'm probably missing something!

Olly

Agree with olli here, its about the elasticity of space time, not the volume of the balloon.

A galaxy's mass stretches the space time fabric towards the core. (These are the distortions on the surface) This is essentially gravity in action, back to the analogy if you put two weights on the balloon they will roll towards each other as they put downward pressure on the fabric. (Downwards in the real world happens in the 4th dimension)

Therefore galaxies(and all other matter) attract each other(and all other matter)

But at sufficiently large distances the tension in space time (created by the stretching effect of expansion) overcomes pull of 2 masses and they drift apart.

If it was not for this tension all matter would implode back into a singularly.

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It doesn't work for folks who simply cannot shed the rest of the balloon from their conception. I know this from trying the analogy. It doesn't make them foolish, it just doesn't sit with they way som

As an astronomy provider I'm often asked to discuss the expansion of the universe. 'But what does space expand into?' is the common objection and I usually reply that it doesn't expand into anything.

In the BB theory I'm sure that there is no immeasurable external void. Such a void would be the universe. I think this is the 'explosion misconception' in action. It imagines the universe expanding in

Not an objection Olly, I've always struggled to get my head around this topic of something that has no boundary.

As a student I had a physics master who baffled me with a similar analogy to the coins/balloon, he used a ball with blobs stuck on it, as he inflated it the blobs moved further apart.

All went well until I pressed a point on the surface with my thumb.

I've never recovered.

All the best,

Rich

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i dont like the ballon model, imo it visually complicates it.

a) curved

b  )has an inside and out

c) is inside a room, its an object with paramaters

Edited by Earl
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i dont like the ballon model, imo it visually complicates it.

a) curved

b  )has an inside and out

c) is inside a room, its an object with paramaters

It doesn't work for folks who simply cannot shed the rest of the balloon from their conception. I know this from trying the analogy. It doesn't make them foolish, it just doesn't sit with they way some people think so you need an alternative, which is what I was trying to find.

When I taught English a key idea in poetry was that you have to distinguish the sound of a word from its meaning. For some people this presents no problem whatever and they cannot understand how anyone could fail to understand this distinction. For others the sound of the word seems to refuse to separate itself from its meaning and I spent a long time trying to devise demos that worked for those who didn't get it.

This is what makes teaching interesting and why I opened the thread.

Olly

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I like it Olly.

I think a lot of the problem is also caused by the fact that whenever a TV programme demonstrates the Big Bang, they always have some spectacular explosion or other....not helpful.

The other thing that gets to me, and you see this in all of the explanations of the Big Bang and aftermath, is the.....'x seconds after the Big Bang, the universe was the size of a pea' or some similar statement.

This is simply wrong.....the universe has always been the size of the universe...it's in the meaning of the word. There is nothing external to measure it against, so such statements are meaningless. All you can say, is that over time, the relationship between the bits of the universe has changed....either they've all mysteriously got smaller and are contracting, or the space between them is expanding ( no idea how you'd be able to tell the difference...perhaps someone here could explain). As far as we are aware, it's the latter.

I have a 'matrix' type question based on your response... Might it be the case that particles don't actually move, it's simply the space between them changes?

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I have a 'matrix' type question based on your response... Might it be the case that particles don't actually move, it's simply the space between them changes?

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This is the whole point. In the expansion of the universe the galaxies are not flying apart becuse they are being accelerated  but because space itself is expanding.

Olly

Edited by ollypenrice
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This is the whole point. In the expansion of the universe the galaxies are not flying apart becuse they are being accelerated but because space itself is expanding.

Olly

Might it also explain why galaxies collide? How exactly does gravity interact with such changes in the spacetime?

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The balloon works model for me. The idea of rigid coins glued to the surface (rather than stretchable, drawn-on, "galaxies") even better. Maybe if the balloon were made of infinitely strong material, we would see the ultimate "Big Rip" too! But such things can only represent PART of reality - "Perfection" is a tad optimistic. Representing higher dimension in 2 | 3-D space has always fascinated me though.

Hypnotic, but not overly revealing...

http://members.wolfram.com/jeffb/visualization/stringtheory2.shtml

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Might it also explain why galaxies collide? How exactly does gravity interact with such changes in the spacetime?

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Galaxies only collide if they are fairly local to each other. The expansion of the universe (the Hubble Flow) is not very significant on small scales but accumulates with distance. (The more space there is to expand, the more it expands.) So galaxies local to each other can fall towards each other because their mutual gravitational attraction is greater than the expansion of the universe in the small space separating them. When a lot of space separates them that space does a lot of expanding and, to boot, their gravitational grasp of each other is much weaker.

Olly

Edited by ollypenrice
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The balloon works model for me. The idea of rigid coins glued to the surface (rather than stretchable, drawn-on, "galaxies") even better. Maybe if the balloon were made of infinitely strong material, we would see the ultimate "Big Rip" too! But such things can only represent PART of reality - "Perfection" is a tad optimistic. Representing higher dimension in 2 | 3-D space has always fascinated me though.

Hypnotic, but not overly revealing...

http://members.wolfram.com/jeffb/visualization/stringtheory2.shtml

If the balloon were made of material less than infineitely strong then I think the term 'big RIP' might be perfectly appropriate! It would apply nicely with three full stops as well; R.I.P.

Heheh,

Olly

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Galaxies only collide if they are fairly local to each other. The expansion of the universe (the Hubble Flow) is not very significant on small scales but accumulates with distance. (The more space there is to expand, the more it expands.) So galaxies local to each other can fall towards each other because their mutual gravitational attraction is greater than the expansion of the universe in the small space separating them. When a lot of space separates them that space does a lot of expanding and, to boot, their gravitational grasp of each other is much weaker.

Olly

I think what I was wondering was; if space can expand, could it not shrink too?

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I think what I was wondering was; if space can expand, could it not shrink too?

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Certainly, until quite recently, cosmologists considered that one endgame for the universe might be the BB running in reverse and imploding again. 'Bouncing' universes have also been considered. (Successive expansions, collpases and rebounds.) Research by Saul Perlmutter and colleagues has, however, suggested that its rate of expansion is now accelerating, arguing against the re-collapse model.

Olly

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These topics are much easier to grasp when being explained by amateurs as the terms and examples used are non scientific.

I am learning more from these threads than I ever did at School.

Thanks

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I must admit this is a fascinating question. I think of it as space is expanding into nothing. The problem is the concept of nothing is a difficult one to try and understand let alone explain. A vacuum or empty space isn't nothing, it has physical properties that can be measured. It has laws of physics that must be obeyed. Nothing has none of this and is something I doubt any human would ever witness.

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I must admit this is a fascinating question. I think of it as space is expanding into nothing. The problem is the concept of nothing is a difficult one to try and understand let alone explain. A vacuum or empty space isn't nothing, it has physical properties that can be measured. It has laws of physics that must be obeyed. Nothing has none of this and is something I doubt any human would ever witness.

There is no such thing as nothing in physics. There's a minimum quantum energy level and particles pop into and out of existence quite blithely. So far as I know there is no change in this minimum energy level in the 'new' space that is created so it isn't 'thinning out' in any way. Stretching would, therefore, be a bad analogy.

For a very accessible, informative and amusing discussion of 'nothing' (in all its forms ) it would be impossible to beat John D Barrow's The Book of Nothing - and you can have some confidence in his qualifications as well.

Olly

Edited by ollypenrice
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This is allways a facinating subject the part i always have problems with is how the "nothing" space between particles came into existance and if was instant or a gradual change from what was there before which could have been a medium that supported faster than light propogation so no need for inflation.

Alan

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This is allways a facinating subject the part i always have problems with is how the "nothing" space between particles came into existance and if was instant or a gradual change from what was there before which could have been a medium that supported faster than light propogation so no need for inflation.

Alan

The question which troubles many thinkers is the simple one; Why is there anything? Why is there not nothing? It's more metaphysical than physical, I guess, but it's a good 'un!

I'm always wary of questions beginning with 'why' because they often imply that there has to be a reason for things (in the sense of an intention). I see no intentions in nature. We have intentions but nature doesn't.  I see only consequences.  So I'm happy with 'why' questions if they are simply asking, 'What previous condition led to the present one and by what means?' but if they are asking for intention or motivation then I think they are invalid questions and cannot be answered.

Was there ever nothing? I have to say that I doubt it, but that is just me imposing on nature my own prejudices born of local experience, perhaps.

Olly

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We have intentions but nature doesn't.  I see only consequences.

Olly

I've read somewhere that not all of what we consider to be free will is under our control.

Our consciousness is, to some degree, a dumb onlooker.

The cerebral processes that we perceive as "thought" can still happen without our conscience looking on.

I'm not talking about lower functions and reflexes. They obviously don't need our supervision.

When I look inside "me" I'm sure I do see signs that "I" am just an observer....

Edited by Paul M
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I always found it easier when they just put Two dots next to each other on a balloon and then blew it up showing how two close dots then move apart, Then I realised, when you blow up a balloon, it takes up space around it, it does not stay within a boundary, hence I can see the issue of a flat universe. But I am utterly baffled and thanks all for such an interesting read.

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As an astronomy provider I'm often asked to discuss the expansion of the universe. 'But what does space expand into?' is the common objection and I usually reply that it doesn't expand into anything. I think the reason that this question comes up is that newcomers wrongly think of the universe as a sphere with an edge that must move outwards into 'something' as it expands. If this idea is deep rooted, as it usually is, it is hard to dislodge.

No-one knows what the 'Universe expands into' as we can't yet see beyond the CMB radiation. If cosmologists were able to hit upon the 100% correct model of the universe the prediction of 'what the universe expands into' would sadly be unverifiable by observation.

So lying in bed this morning I had a different idea. Next time I'm asked I'm going to try saying, 'It expands into the new space it has just created.' I can then go on to say that this new space is not created outside the universe (which has no outside) but inside it. Each cubic metre of space creates a new bit of space. Space grows.

Olly

Not only does space seemingly grow, it also changes the manner in which it accommodates the passage of photons (cosmological redshift). It seems more natural to say that space 'stretches'.

Note that this post is not about cosmology, it is only about explaining cosmology.

Olly

Prof. Bernard Schutz's 'Gravity from the ground up' would interest you I am sure. Schutz is a first-rate teacher. http://www.gravityfromthegroundup.org/

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Well, this thread has dragged on!

I just read the August issue of BBC Focus magazine and in the Q&A section a reader asks "how can the universe be infinite if it's expanding?"

It took the respondent just 53 words to nail it with the coins stuck to a balloon scenario.

Simples...

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There once was a Dutch physics professor who for some reason decided to take a posting at a small rural school in New Zealand. Unluckily for him in 2004 I rocked up in his class, one of three students, and we hit on the dots on a balloon example. I was one of those that just couldn't get my head around that example and was fixated on the fact that the balloon is expanding into the room around it and the internal volume of the balloon is what's really expanding.

The classic alternative was offered, the humble raisin cake baking away in the oven - uh uh - still no comprendo, same problem as before. I don't think Mr Ver Maat and I ever succeeded here and perhaps just chose to move on at some point.

I have to admit that I still don't understand it. It seems that so many models are plausible. Earlier in the thread, somebody suggested that perhaps the exact opposite is happening, the universe is in fact contracting locally giving the illusion of expansion - I suppose that is possible, can we disprove that?

I think Olly is on the money with language being the limitation. As soon as we say, something is moving we imply three dimensional space and distance is only meaningful if it's relative to something.

The thing that really makes my brain squirm is when they discuss the shape of the universe. Is it a sphere, flat or a saddle shape? Oh dear lord - how can something have a shape if it's everything? And surely it can't be flat because that's a 2 dimensional shape? It at least needs to be cube, ie. a sphere with corners.0

Hehe it's been a good learning session reading this thread. I hope to maybe get my head around this one day. A geeky item for the bucket list!

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Was there ever nothing? I have to say that I doubt it, but that is just me imposing on nature my own prejudices born of local experience, perhaps.

I think that kind of highlights a bit of a paradox, with how we think about it all anyway.

Of cause, all this talk about the big bang et all assumes we are in some way on the right track in the first place as far as there being a big bang as we currently view it. Although I realise this is in no way helpful with the original post

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..yep, the universe is not expanding, it's just that the amount of space between things is getting bigger. Due to conservation laws this has to come from somewhere, and that explains why visitors can never find anywhere to park in my street these days...

P

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