# "Inside" the balloon analogy

1. Apr 15, 2015

### PWiz

Sorry to bother you all with another-of-its-kind (most probably unsatisfactory) analogy that I came up with myself (error detection sirens should be blaring now) to better understand "energy" in cosmological expansion, but I just can't rest until I find out how (in)accurate it is from you all. I mean seriously, no personal theory here, just an analogy to simplify certain concepts. Here it is:

Think of what happens inside a balloon as you heat it. That's right, just like why we can't say what caused the balloon to start expanding in the balloon analogy ( a poor guy blowing his lungs out from 13.8 billion years for no apparent reason), I'll be leaving the cause of "heating" aside for the moment. The number of air molecules inside the balloon stays constant. Let's attribute this to the cosmological constant. As the balloon heats up, it increases in volume, and the particles move away from each other more and more. Let the volume of the balloon represent energy content, and the volume between the particles denote dark energy. When the balloon is relatively small in volume (I mean really small), the total volume mostly comprises of the volume of the particles themselves, but as the balloon grows in size, the interspaces between the particles become bigger contributers to the total volume, and their ownership rights to the balloon volume increase continually with time.

What this incoherent analogy attempted to explain in a simpler way:
-The invariance of the cosmological constant
-The increase in energy content of the universe
- The increase in dark energy content of the universe

What this analogy does not wish to address (no, it just won't ):
-Any link with the actual volume of the universe
-Reference to any geometric center of the universe
- The shape or boundary of the universe
- What's outside the balloon
- Whether the guy blowing the balloon will run out of gas

I'm positively sure you'll be sighing with pity by the time you start reading this thinking how wrong I am, so if I didn't give your eyes an unsightly mess, I hope that I gave you a good laugh :)

EDIT: This analogy is not a direct comparison of quantities. Temperature, pressure, etc and other quantities of the balloon on which no comparisons have been made (to the real world) are NOT being represented by their respective counterparts in the universe, so they have no significance here. Please don't extrapolate this analogy to explain things not mentioned here.

Last edited: Apr 15, 2015
2. Apr 15, 2015

### wabbit

This seems misleading to me in two ways :
- introducing the "interior" of the ballon (which doesn't have any counterpart in our universe) leads the anology right toward one way it breaks down
- the universe actually cools down as it expands. I wouldn't be looking forward to explaining that starting from the heated balloon picture:)

I think to make the ballon analogy work as a proxy of an expanding universe, one of the key mental steps / difficulty one must overcome, is to be able to think that it has no interior, so I view any element going against that, such as heating the inside, as adding confusion instead of clarifying things.

Last edited: Apr 15, 2015
3. Apr 15, 2015

### PWiz

Please don't attribute temperature to the analogy! I'm not saying that the inside of the balloon represents the universe (I'm not mapping other quantities exactly onto things we observe in the universe), nor that the kinetic energy of the particles inside the balloon means anything. Just the things I've compared, nothing more. (yet :P)

4. Apr 15, 2015

### wabbit

I realize you're not saying that, but there is no motivation for introducing anything outside the surface of the balloon, and even if that did provide an "explanation" for something, I think this explanation would at best express a coincidence and would not add insight. Sorry but I can't bring myself to analysing what conclusions might be drawn about the CC from your analogy, because I see no relation between that analogy and something in the universe.

The surface of the ballon is an expanding 2d sphere, like a finite universe is an expanding 3d sphere: this is what motivates the balloon analogy and makes it work, and why you can actually carry it quite far if first you accept that there is no interior, and then also realize that there is no surface, only points on the surface getting farther away from each other (did you try that last step?)

Last edited: Apr 15, 2015
5. Apr 15, 2015

### PWiz

Okay, thanks for your input! (See? I was correct about something like this happening! Hahaha...ha...ha.......)

6. Apr 16, 2015

### Staff: Mentor

I don't see how the analogy actually explains anything. Perhaps you should look at an actual (heuristic) explanation of how the cosmological constant works:

http://www.astro.ucla.edu/~wright/cosmo_constant.html

7. Apr 17, 2015

### Staff: Mentor

This thread is closed since it is verging on personal theory. If there are questions about how our current mainstream cosmological models deal with "energy of the universe", please start a separate thread focused on that specific area.