# Is a soap bubble a valid analogy for the expansion of space?

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Is a soap bubble a valid analogy for the expansion of space?

Most science-related discussions at home currently starts with questions from the kids meaning I need to use simple and concrete examples*. I find the the analogy with pennies on a balloon** useful when thinking about basic aspects of big bang and expansion of space. How about a modified balloon analogy with ”observers” that are more part of the analogy than us outside watching the balloon growing in 3D space? The goal is to have an equally valid analogy and replace the balloon surface with something where it, hopefully, would be easier to imagine an observer of the expansion.

Imagine some very small creatures living in a very shallow puddle of soap***. The creatures stay all their lives in soap and they don’t go near the surface or the bottom of the puddle. The creatures don’t swim up or down and they have no knowledge of or interest in the possibility of a surface or a bottom of the soap or that there might be something outside/beyond soap.

Now imagine that the soap from one of the puddles, with creatures in it, is used to blow a soap bubble. The creatures live on as usual in a thin layer of soap and swim around, ignorant of anything beyond or outside soap. Blow more air into the bubble. Now the creatures will become more and more separated from each other. From their point of view there is not a center, all of them believes all other creatures are moving away****. The more separated they are the faster they will seem to move away from each other.

Question: is the soap bubble analogy as good/bad as a balloon analogy?

If the analogy works I’ve some ideas about expanding upon it for other purposes.

/Gideon

(*) Requirement: Simple examples and analogies that also correctly describe some, possibly limited, aspect of mainstream science. If possible, easy to identify with and exiting enough to spark or maintain interest. Mathematical models are not yet an option but hopefully will be an option later on.

(**) There may be more than one analogy using a balloon, for this post I refer to one from physicsforums, https://www.physicsforums.com/insights/balloon-analogy-good-bad-ugly/. Short version of some of the text:
the Balloon Analogy is intended to describe:
(1) The universe is expanding outside of systems that are gravitationally bound
(2) The expansion has no center and everything is moving away from everything else.
The Analogy: Think of each gravitationally bound system as a penny, and glue a bunch of these pennies onto a balloon that is only slightly blown up. Now blow up the balloon more. All of the pennies move away from each other uniformly, and those that are farther away from each other move away from each other faster than those that are closer together. No penny is the center of the expansion.

(***) I haven’t found a soap bubble version of the analogy when searching so no source is given, plagiarism is not intended. Description is a bit childish but that’s intended in this case.

(****) In reality there would of course be lots of things that may happen; the soap is fluid so creatures would drift around, nothing prevents the creatures from going to the soap surface etc.

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