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

bapowell

We don't use GR because gravity travels at the speed of light; we use it because it is the modern theory of gravity. You have not demonstrated in any rigorous way (beyond just using words) that Gauss's law gives you the observed expansion of the universe. In know this can't work, because, as I've stated to you, the net gravitational field in a homogeneous universe is zero. This is not a matter of opinion -- it can be calculated -- there simply is no potential gradient across a uniform energy density.

Drakkith

Newtonian physics is already known to be incorrect (or inaccurate if you prefer). Why would you use it? It does not explain many things that GR does.

Perduta

Well none of the geniuses here answered the simple Newtonian question in respect of the centre of gravity and the consequent accelleration we should see towards it... so what's the point in alluding to general relativity and more complex models. I thought we were discussing the balloon analogy. My bad.

Drakkith

The balloon analogy has nothing to do with Newtonian physics, so of course people either aren't going to be able to answer it or won't have much reason in trying. In Newtonian physics the balloon cannot expand. It doesn't even make any sense because there is no such thing as spacetime geometry in that theory. Don't take the analogy and use it where it doesn't apply, as it will not work. And please, enough with the attitude. You came here and asked us, we did not seek you out.

Perduta

I think the replies were condescending, dismissive and confrontational as well as going off at a tangent from my original question which is WHAT physical entity are we to understand is "expanding" with respect to the way measure it.

Secondly, the Equivalence principle clearly states that acceleration is equivalent to gravity which made me think on the spur of the moment, where could that gravity be coming from. Gauss clearly says that the volume integral of mass is equal to the change of flux through the surface of said volume so instead of thinking about it we should just poo-poo the whole of Newtonian physics and allude to some vastly more complicated explanations that only "real" scientists can understand.

OK, I got the gist of these forums, which is what I thought they would be like.

DaveC426913

Well, all I can tell you is that I see three types of people on PF.
1] There are professionals, who do this for a living.
2] There are laypeople, with varying degrees of knowledge, who come here to ask questions and learn from 1] and 2],
3] There are laypeople, with varying degrees of knowledge, who come here to voice their own opinions, and aren't interested in learning.

You're a 2] but if you're not open to corrections then you risk being treated as if you are a 3].

The key is to put forth your ideas as questions, not assertions.

I'm a 2]. I hope you stick around. We need more 2]s.

Drakkith

I'm sorry you feel that way. I think the replies were entirely free of all of that and were simply explaining what was incorrect with your line of thinking. This is an internet forum, and nonverbal and verbal cues as to how each person is talking to you do not exist. Most of the time people are simply giving facts and trying to explain the correct concepts and are not trying to be condescending or anything else at all. If you are not used to being corrected then it can seem like peoples posts are far harsher than they really are.

That's fine, but the expansion of the universe is not an acceleration in local space. IE you would never feel an acceleration because from your frame of reference, you are not accelerating. A galaxy 5 billion light years away is also not accelerating in it's frame of reference either. However the distance between us and that galaxy is increasing.

I hope that gist is of a forum with people willing to help people understand the current mainstream theories of science. Otherwise there is a misunderstanding between us.

StandardsGuy

Show me the evidence for that.

No the equivalence principle would not.

StandardsGuy

According to current theory, the universe has no center, so there is no center of gravity. You came here with an agenda (you are preaching). This forum has rules against that. I have been warned, so I am warning you.

The balloon analogy is 2-dimensional (the surface of the balloon only). You apply imagination to make it 3-dimensional (or more). It gets bigger with time. That represents the expansion of space itself. Objects get farther apart like the dots on the balloon. It's just a theory, but it's the main-stream one. You can't propose another one here.

GeorgeDishman

What is expanding is the vacuum.

The equivalent to that is to draw a line round an arbitrary region on the surface of the balloon. As it inflates, the region grows while the mass contained within it remains the same (neglecting small amounts crossing due to local 'proper motion'). A region like that is called a "co-moving volume".

This particular thread was intended to produce a common explanation of a well-known model for newcomers which is somewhat different to other threads where various alternative ideas are often discussed. If you come with an expectation, you will probably only see what fits it.

marcus

I think you put the right spin on it, George.
This thread is essentially an adjunct to this short animated "movie" which I hope everyone has watched.
http://www.astro.ucla.edu/~wright/Balloon2.html
Watching it for a few minutes is a valuable exercise of geometrical imagination.

You see galaxies (white) remaining in the same place while distances between them increase.
You see photons (colored) traveling among the galaxies always at the same constant speed.

To learn from the balloon model you need to think of it as simply telling you about the changing geometry of space. It is not a physical analogy. It doesn't mean that space is a physical substance like rubber, it is about imagining geometric relationships changing over time, not explaining why they do.

To learn you need to imagine that there is no inside or outside to the balloon. All existence is concentrated on the surface and there is no 'anywhere else'. It is a 2D analog of our 3D space. There is no time-coordinate in the picture. Spacetime is not shown. You just see change happen as the movie plays.

The message of this 2D toy model of changing geometry is not that our real 3D space is LIKE that in the sense of being finite, or having a sphere topology. It might be infinite or finite--we don't know. It's just an exercise to help one learn to imagine how galaxies can stay in the same place (i.e. same latitude longitude on the balloon) while distances between them increase. In the 2D space of the balloon surface, and that is all the space there is, they are not moving. Meanwhile the photons of light are moving (across the surface) always at the same speed.

It's a simple message to absorb. Once a newcomer gets it into his or her imagination it is up to them to extend the visualization up a dimension from 2D toy analogy to real 3D. Then there's more, that's only a beginning.

I think your post captured this way of looking at things, so what I am doing is amplifying on what you said. It sounds like we are on the same page about this.

GeorgeDishman

I think you need to a little careful about that. Whether spacetime is like a substance or not is somewhat philosophical ("substantivalism"), IMHO it would be better to rephrase that in a more neutral style or it may sound like "preaching" which was mentioned in a previous post.

That is one of the problems I highlighted with the model, however you can visualise the radial distance as comoving time.

Mostly, I've already voiced my concerns about the model in an earlier post and I don't agree with your view regarding "only geometry", there is a view that the gravitational field described by the metric can have as much claim to an existence as the quantum field. I think it's better to leave that as an open question in philosophy, the interpretation is separate from the science.

marcus

George Jones made a point concisely in the Phinds' Balloon Critique thread in a quote stressing the purely geometric message of a pattern of "increasing distances between observers at rest with respect to" the CMB or words to that effect.

This reminds me of some things I've been wanting to say. Analogies are ideally used in a controlled way with a clear intent. In this thread I've tried (subject to limitations of time and energy) to make clear how I intend the analogy to be taken. Above all this thread is an adjunct to the short animated movie I've frequently linked to:
http://www.astro.ucla.edu/~wright/Balloon2.html
The most important thing is to watch the animation--essentially that's the "balloon analogy" topic of the thread. That and the takeaway geometry message.

A. The analogy is not intended to suggest the U is spatially finite. And it might or might not be. It might or might not be topologically a 3-sphere. We don't know about those things. The analogy is not about finite/nonfinite. I think that has been made explicit from time to time.

B. the main purpose is to illustrate a pattern of increasing distances between (approximately) stationary observers. In the animation one sees photons moving at constant speed among the observers, represented by little galaxies. Some distances between galaxies are obviously increasing more rapidly than the photons move. In a given amount of time the photons can obviously cover more distance with the help of expansion than they would in a static setting.
The individual galaxies stay at the same lati-longitude positions.

C. the message is geometrical, learning to visualize changing geometry. It is not meant to suggest anything about what space is made of or not made of If people start talking about whether space is a "substance" or not then AFAICS the analogy has gone out of control. It MAY OR MAY NOT be some kind of substance. I don't want to say anything about that with the balloon imagery. The analogy, as I think it is ideally presented, is geometrical not physical.

D. the idea is to think of all existence concentrated on the 2D surface. No inside or outside of the balloon. No "center" which would have to be in some higher-dimensional 3D space that the 2D creatures living on the surface do not know about. The analogy is not intended to say anything about whether there is a higher dimensional space in which ours is embedded. It is a sketch or diagram illustrating changing geometric relations among stationary observers and light, within a 2D toy model of our 3D space, so you then have to mentally extend your comprehension to the analogous 3D geometry.

hitchiker

the common balloon analogy and bread-raisin analogy about bigbang creates more doubts in the mind of people than they already had

1.if bigbang created space ,then where did Bigbang happen in the first place? if space is nothing how it can be created?is space finite or infinite?

there bigbang have no answer and are open questions in astronomy and philosophy


I think the best way is to make an animation starting with present view of movement of bodies in universe and then going backwards in time drawing galaxies coming together and then congregating in to denser and tinier space

"Bigbang created "spacetime" and bigbang itself are best description based on current understanding of universe which varies from time to time as we venture out farther in space but many people take these theories for granted without giving much intellectual thought




i dont really like this animation but something like this with smoother transition back to bigbang ..but what i love here is before BB there's still space but with no matter&energy but an empty space that has the potential for a bigbang
http://resources.schoolscience.co.uk/STFC/bang/bang.swf

unsolved problems
http://en.wikipedia.org/wiki/Unsolve...ems_in_physics

marcus

Here's an essay about the different meanings of the words "Big Bang" and some of the confusions in the mind of the public.
http://www.einstein-online.info/spotlights/big_bangs
It's part of the outreach program of a research institute in Europe.

Ordinary mainstream cosmology only kicks in after expansion has already started. It is not about scientists' guesses as to how the expansion began.

The balloon analogy is intended to help imagine the expansion process AFTER we already had matter and light, and expansion of distances was under way. It is not intended to help picture the "Very Beginning" of the process.

There certainly are a prominent minority of cosmologists who devote time to speculating about the Very Beginning. That involves guesswork and unverified assumptions---it is fine for their colleagues and the general public as well to regard conjectures about the start of expansion in a critical and skeptical light, if they pay any attention at all. Doubt is a good scientific attitude.

Most cosmologists spend their time gathering data and checking to see how well it fits the standard cosmic model (a model which does NOT go back to the very start of expansion.)

There is some confusion that arises because "Big Bang" is also used in a second sense: to mean the whole expansion process over the course of billions of years, as described by the standard cosmic model. That is supported by a lot of observational data and has been scrutinized and checked by a lot of people. It does not say anything about what happened at the very start. The confusion that comes from people using the words in two different senses is picked apart in that "Two Big Bangs" essay I linked to.

There's also a good article that corrects common misconceptions about the standard cosmic model, "Big Bang" in the second sense:
http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf
The first page of the PDF file is blank, so scroll down.

Since the nature of the very start of expansion has not yet been determined, one is free to choose how one imagines it. I personally picture it as a rebound from a prior contracting phase. It has not been proven impossible for there to have been space and matter fields existing for 100s of years before the very start of expansion. Perhaps thousands of years or even more--I won't assume or suggest an upper limit. It's one possibility and it doesn't oblige you to worry about tricky stuff like "nothing" and "fluctuations in nothing". It is one of the alternatives being seriously studied by people whose specialty is called quantum cosmology (a small branch of research which is different from ordinary standard cosmology.) If you want a toy model to picture (simply to imagine, not to believe! ) think of the balloon deflating until it is fairly small and quantum effects resist further shrinking, and then rebounding and starting to swell up again. But no surrounding 3D existence, in this toy model, only the 2D surface.

It has not been scientifically shown that the very start of expansion must have also been a beginning of space or time. So one can choose to believe that or not, as one wishes.

But these sorts of of "Big Bang" issues are really OFF TOPIC in this thread, which is about understanding the geometry of expansion (after it got started) using a simple 2D model, namely:
http://www.astro.ucla.edu/~wright/Balloon2.html
So if you would like to discuss these things some more I'd like to suggest that you start a regular discussion thread (about the start of expansion and such matters) for that purpose.

GeorgeDishman

Of course I agree that the 2D surface has to represent 3D space and there is no implication of embedding in a higher dimensionality. The radial coordinate in the balloon analogy is strictly plotted as the scale factor which, for a matter-dominated universe, goes as

a(t) = (t/t₀)^2/3

or equivalently

t = t₀a^3/2

It is therefore reasonable to identify the radial coordinate with cosmological time albeit on a non-linear scale. A section through the centre then can look like this:

http://www.astro.ucla.edu/~wright/omega_2.gif

though the light cones would not be exactly at 45 degrees to the radials. Conversely, they can be exact if distances on the surface of the ballon are considered scaled relative to proper distance by a factor dependent on radius.

http://en.wikipedia.org/wiki/Comovin...roper_distance

This has the advantage that a small patch on the above diagram can be seen to be an approximation of the usual Minkowski spacetime diagram again bearing in mind some scaling issues.

hitchiker

thanks for saying it as it is .. you are a real science adviser