 Quote by geronimo
Having ditched the balloon analogy as being too simplistic, I am visualising the mechanism as more like expanding gap-filling foam...
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No special comment needed, I think. Neither balloon nor foam represent a mechanism.
The balloon image is intended to aid visualizing how distances between stationary points increase. And how they increase at a percentage rate, so that longer distances increase more. Meanwhile (if you recall Ned Wright's animations) wriggles of light slowly travel from one stationary point to another. So this says nothing about how the universe works, it is an key exercise in picturing changing distance relations---in visualizing Hubble law. If foam helps you assimilate Hubble law better than balloon, go with it! Of course neither provide a physical analog to the Friedmann equations, so neither teaches you any understanding of how geometry and matter actually
work. Once you can visualize the pattern, if you want to explore the mechanism one way is to experiment with the online calculators which embody the Friedmann equations. I don't know any physical analog (like a balloon or whatnot) but the calculators are fun to play around with.
Quote by Chilli
On the balloon analogy and the Cosmic Microwave Background radiation ...
If a particle radiates from location A in the direction of location B, once it leaves location A it is no longer there, although A may remain the particle's apparent location from any number of viewing perspectives over time. What I don't understand in the balloon analogy is where are the "A" locations that are null of radiation? Does the CMB radiation just continuously criss-cross itself? If yes, why is the radiation still uniform? If no, where are CMB radiation source locations in the model?
Kind regards
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Chilli, think of it this way: Everybody in the universe is currently receiving CMB radiation which was emitted by matter which is currently at a distance of 46 billion lightyears from them. And that matter has gone thru a lot of changes since it emitted the light that's now arriving.
In line with your example pick spots A and B on the balloon surface.
At a certain time (380,000 y) space is more or less uniformly filled with hot glowing stuff and it is turning transparent for the first time, as it cools below 3000 kelvin.
The balloon is small and A and B are close together (only 42 million ly)
All points including A and B send out light uniformly in all directions. Some of A's light heads towards B, some of B's light heads towards A.
The light doesn't get there right away, or any time soon, because of expansion of distances. The original distance of 42 million ly increases a thousand-fold while the light is traveling. More exactly by a factor of 1090. So today the distance between A and B is 46 billion ly, and this light has traveled 13.7 billion y and is just now arriving.
The balloon is 1090 times bigger now than it was. Some of A's light is arriving at B and some of B's (that didn't go in other directions) is arriving at A.
By now both A and B have matured in the sense that they are no longer hot glowing gas---the gas has condensed into stars and galaxies and some stars have planets and some planets may have life and so on. So each of A and B could have creatures that construct antennas and receive the light----whose wavelengths are now longer by a factor of 1090.
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Does the CMB radiation just continuously criss-cross itself? If yes, why is the radiation still uniform?
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I'm not sure what you have in mind by continuously criss-crossing, but I think yes it does because there is uniform radiation going in all directions at every spot at all times. It is almost perfectly uniform because the whole shebang that emitted it was approximately uniform---all space filled about evenly with hot partly ionized hydrogen etc. all at about the same temperature and all turning transparent at the same time. There is no way that a lot of non-uniformity could arise. Some perhaps, but not a lot.
Remember that in the balloon analogy, all existence is concentrated in the 2D surface of the balloon and there are no directions off the surface. So if radiation starts out uniform it will always remain so.
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BTW Chilli is an excellent choice of name---reminds me of a favorite comic gangster movie. Looks like the above was your first post: welcome to the forums!