the balloon analogy (please critique)

bapowell

Is not a closed universe a "real model"?

phinds

Thanks for all the feedback. I've been meaning to get back to that page and update it with some of the comments above, and I'll take yours into consideration as well.

Naty1

If you haven't already, consider searching these forums under 'balloon analogy'..There are several dozen threads and I'll bet some offer insights that will be of interest, even if only perfecting an explanation you have already considered.

You mention under Doesn't work, #2...forget curved shape you note..but you could mention dimples in the surface as gravitational potential [space]wells...like the rubber sheet analogy which you do reference...

Also, someone posted, and I'd suggest you PM pervect because I think it was him, but may not have been recent, an oustanding list of balloon analogy strengths and weakness....
I will search more to see if I can locate it...

Under DOESN'T WORK: SOURCE OF CMBR might be worth considering ....all observers see a huge spherical source, the surface of last scattering.....and also WE ARE AT THE OLDEST POINT IN THE UNIVERSE ....since everybody else is in our past....[and we in theirs!]...Actually, every other observer in the universe sees a slightly warmer CMB than do we - for the simple reason they are observing it in our past.

edit: found this description from Marcus:
Marcus:
One thing the balloon analogy teaches is what it means to be not moving with respect to CMB. The balloon is a spherical surface and as it gradually expands a point that always stays at the same longitude and latitude is stationary with respect to CMB. Distances between such stationary points do increase as the universe expands. They increase at a regular percentage rate (larger distances increase more). In our 3D reality this is called Hubble Law. It is about distances between points which are at rest wrt CMB.

Don't know this source but I liked the description:

What passes for the “LOCATION OF THE CMB ORIGIN” is a large spherical surface called the surface of last scattering where the stuff is that emitted the light we are now getting. In the past we were getting CMB light from other stuff that is nearer, but that light has already gone by us. In the future we will be getting CMB light from other stuff that is out beyond our current surface of last scattering---but that light is still on its way and has not reached us.

"

Naty1

Also for cosideration:
[Source unknown]

Expansion: [maybe pervect??]

and related "Causes of redshift [over the baloon surface] in relativity. The expansion of wavelength correlates pretty much exactly with expansion of distance (that occurred while light was in transit.) It refers to the standard FRW metric and standard cosmological model. cosmo..... as the light ray falls into a galaxy's potential well, it gains a blue shift due to the added energy. When it climbs back out, it loses that energy. As long as the galaxy's gravitational potential stays the same during the light ray's passage, then the two effects perfectly cancel and there is no change in the light ray's energy (i.e. no redshift or blueshift).

Another explanation: [analogy to seeing over the balloon horizon??]
"Based on generally accepted cosmological models, it is very UNLIKELY there would be major distinctions in OUR universe just beyond our cosmological horizon. There is no widely accepted model that predicts such distinct variation in cosmological characteristics. In fact, as far as I know all models predict the SAME characteristics throughout our universe.

In other words, the light we observe each succeeding day reveals a bit more of our own cosmos, since more distant light reaches us, and the cosmic background radiation appears to reflect rather uniform, consistent, expected characteristics. Nobody has said "WOW, LOOK AT THAT!! ALL OF A SUDDEN WE ARE GETTING AN ENTIRELY DIFFERENT KIND OF CMBR"


Chalnoth: "We are at the temporal edge of the universe because nothing in the observable universe is older than us relative to the BB; it also appears we are at the center of the observable universe because we can see equally 'distant' in every direction. It is obviously illogical to be both at the center and edge of any geometrical shape aside from a point. The only logical alternative is the universe has no edge or center.

Marcus: What you see in ordinary mainstream cosmology (something like 99% of the published papers) is a model of the universe as something which is spatially without boundary.

That is, you could say, how the concept of universe is defined: spatially speaking it is the thing that has no boundary. This means that expansion can only be pictured/experienced from the inside. One experiences and measures expansion as the gradual increase of distances between stationary objects.

phinds

Likely what I'll do is split the page into two parts, BASIC balloon analogy facts and then EXPANDED balloon analogy facts.

That way I can maintain my goal of an initially simple discussion but still bring in some of the more detailed info many of you have presented.

Thanks to all. I'll post a note when I get it updated. Probably this weekend.

Paul

Naty1

Here is a really tricky one:

phinds

I don't get his statement that the recession of galaxies causes space to expand. Isn't that backwards?

I mean, if the galaxies' expansion CAUSES dark energy, what causes the galaxies to expand in the first place?

Doesn't make sense to me.

Calimero

Well, he is saying what i was trying to say in number of threads, but people here are persistent in lack of their pedagogical skills. He is saying that behavior of a test particle left at rest w.r. to us at cosmological distance is determined with acceleration of universe. In a universe in which space is still expanding but expansion is decelerating, particle will move towards us, fly past us, and then join Hubble flow at the other end of the sky. Not the kind of behavior you would expect if you take "expanding space" literally.

No expansion is not causing dark energy.

GeorgeDishman

What I meant is that in current realistic models, infinite and finite are equally plausible. AFAIK, observations don't currently select one over the other either but the balloon analogy strongly favours visualisation of a finite model hence in that sense it can be misleading IMHO.

I think the WMAP page covers this well:

http://map.gsfc.nasa.gov/universe/uni_shape.html

Naty1

phinds :
You will find this discussion interesting I think; starts in 2007, and it is long but worth the read thru 2007 at least;

Wallace has some interesting insights...he IS a cosmologist...:

Does space expand

http://www.physicsforums.com/showthr...nt+flow&page=2
Wallace:
"What do you think? When Cosmologist talk about the expansion of the universe, it is often phrased as space itself expanding. For instance, interpreting cosmological redshifts as due to the photons being 'stretched' as they pass through expanding space, rather than being due to a doppler shift (since for instance at cosmological distances galaxies can be receding at greater than c and hence the doppler formula breaks down).

People use analogies to dots on a balloon or raisins in bread but this seems to imply that the expansion of space (the rubber or the bread) is what carries the galaxies (the dots or raisins) apart.

The idea the space expands has been attacked by various people, including the well respected John Peacock. See here, click on the link 'Expanding Space'

Do people agree with this? Is Expanding Space a 'dangerous idea' or a necessary interpretation of the GR equations for FRW universes? The maths is not in dispute, but the interpretation seems to be...."

A few post examples:

""Expansion of space" is a completely wrong terminology.
It implies that space is some sort of a substance that can expand and contract.

That an observer measures a change in distance is perfectly valid in relativity but it has nothing to do with an expansion or contraction of space.

"Photon's being streched by exanding space" is another one these absurd phrases.

That an emitter and an absorber of a photon measures a different frequency is perfectly valid in relativity but it has nothing to do with a change in the state of the photon."


Marcus:
"Mainstream professional cosmologists (Wallace is one, SpaceTiger also) use a particular model (associated with names Friedman Lemaitre Roberson Walker and abbreviated FRW sometimes) into which you can plug various parameters ---and it gives you nice simple solutions to the main (Einstein) equation that you can try to fit to observational data.

And this FRW model has an idea of universal time..."


Pervect:
"While it may not make sense to use a pure FRW metric to describe a bound system, there are some papers that take the approach of using for instance a Schwarzschild De-sitter metric..."

Marcus
"...there is NO problem of "where extra space comes from" because space is not a material substance-----it is just the distances between things a web of geometric relations----you DONT HAVE TO MAKE MORE..."

Depending on whether you want to make your balloon analogy discussion a career or not, perhaps a 'basic' and advanced section would help clarify things for different readers....

Naty1

phinds...
maybe you can start a 'movement' here to ban the term 'expansion of space'.....??

maybe use something like 'increases in distance' or 'increase in observed distance'.....[which makes the connection to different frames more apparent] and relate such a terminology to changes in the acceleration of expansion rather than changes in the 'velocity' of expansion....just a thought. In such a sense, your balloon analogy is a reasonable fascimile since there is no 'new' balloon surface being created...but distances ARE increasing....!!

phinds

Yes, I have been thinking along exactly those lines for the update, and I appreciate your input in the previous post as well. Thanks.

George Jones

From the Conclusion of

http://arxiv.org/abs/0707.0380:

GeorgeDishman

This is a bit older and covers some different points but may also be of interest:

http://arxiv.org/abs/astro-ph/0310808

Ich

Ok, as there is evidence that the page doesn't get across an important point, let me stress once more:
Expansion doesn't drag things along.
But a balloon drags things along that are glued on it. So this is misleading. Better let the pennies float freely.
FWIW, here are some calculations I've done a while ago. There's also a paper out there examining the slippery balloon analogy.

sbrothy

...ONLY the fact that all the balloons move away from each other and that ones farther away move away faster.

Surely, you mustt mean that the pennies move away from each other or the analogy is getting really silly. Heh.

Naty1

phinds

I've been plowing through the old thread #162727.

There is a wonderful post by pervect, #90, that further explains an earlier insight by Wallace about different models providing superluminal or sub luminal speeds, and clarifies for 'non experts' a difficulty of interpreting 'distance' in cosmology:

and the following post cements the idea:

So for me, three key concepts from this thread which are not captured by the balloon analogy are that 'expanding space', ballon stretching, is misleading, distance increases are dependent on acceleration, not speed, characteristics, and distances are both model and coordinate dependent meaning, observer dependent. These issues are well illustrated by Wallace's example about deceleration.

edit: Depending on how far you wish to take all this, a short explanation of FLRW measures,conventions, assumptions, and how they compare with the balloon perspective could be helpful.

Here is a first draft list [in no particular order] : FLRW is the standard [cosmological] model; FLRW metric [distance measure] is an exact solution to the EFE but only approximates our universe because it assumes the universe is homogeneous and isotropic; how you measure things, your choice of coordinates, the model chosen, affects you answers. ..., being at rest with respect to Hubble flow and that is what defines the universal cosmological time parameter utilized; superluminal expansion distances are are result of the FLRW model metric; those FLRW distances are NOT great circles nor geodesics on the balloon, the FLRW metric starts after the initial inflationary epoch, the LCDM is a 'fine-tuned version' of the general FLRW where the parameters are chosen to get the best possible fit to our universe, the most common distance measure ,comoving distance, defines the chosen connecting curve to be a curve of constant cosmological time and operationally, comoving distances cannot be directly measured by a single Earth-bound observer, etc... took me a long time to gather those insights, most from expert posts here....!!!

edit; Does the balloon analogy model the Hubble parameter accurately??: wiki mentions:

"..the Hubble parameter seems to be decreasing with time, meaning that if we were to look at some fixed distance d and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones..."




I still have not found the excellent description of balloon analogy pros and cons previously posted!