## Is the universe really like an expanding balloon? (Intermediate)

I've heard that the universe is expanding similar to a balloon being blown up with all reference points growing further apart. If this is true, then all movement is perpendicular to the point of origin (the "center of the balloon").

An observer on the balloon (Earth) would see lateral movement of all stars in all directions except ahead and behind. The quantity of material moving laterally could be evaluated to determine our target direction.

1. Do we see lateral movement of the stars in all but the two 180 degree opposed directions?

2. Where is the line of the two non-lateral shifts relative to our solar system?

Your question is the result of a misunderstanding, but the misunderstanding isn't your fault; rather, it's the fault of the famous (or more accurately, infamous) "balloon analogy" for the universe's expansion, which, in my humble opinion, should be banished forever into the dustbin of history because it's the source of so much confusion!

The problem with the balloon analogy is that it's a two-dimensional analogy for a three-dimensional situation. The way you're supposed to think about the balloon analogy is that everything which happens in two dimensions on the balloon's surface actually happens in three dimensions in the universe. For example, the balloon's surface "stretches" proportionally in TWO directions as the balloon gets blown up, but our universe stretches proportionally in THREE directions. The third dimension in the balloon analogy (i.e. the direction which is perpendicular to the balloon's surface and which allows us to see the balloon's curvature) is the equivalent of the FOURTH dimension in our universe.

Naturally, of course, no normal-thinking human being is going to pick up on this subtle aspect of the analogy without having it pounded into their head when the analogy is first told to them. So I typically hear all sorts of questions like whether or not we can measure the motion of galaxies or the thickness of the universe in the direction perpendicular to the balloon's surface, when in reality these questions make no sense because they actually refer to the "fourth dimension," which we have no way to observe (if it exists at all).

I suppose the reason the balloon analogy got started in the first place was that people used to believe the universe had a significant amount of "curvature" (and by that I mean curvature in a hypothetical FOURTH dimension, i.e. something which we can't see directly but whose geometrical effects on our three-dimensional world might theoretically be detectable). The balloon analogy does sort of give us a way to picture that curvature, but I don't think it's worth the trouble. Furthermore, recent observational evidence strongly suggests that the universe's curvature is extremely small, if it exists at all, thereby making the balloon analogy even more needlessly complex.

The analogy for the universe's expansion which I prefer is the "dough and raisins" analogy (which has been around at least since Martin Gardner's 1962 book Relativity for the Million, if not earlier). In this analogy, we picture the universe as a gigantic blob of dough which is placed in an oven and begins to expand. Embedded throughout the dough are a bunch of raisins, each of which represents a galaxy (including one for our galaxy, the Milky Way). As the dough expands, the distances within it all stretch proportionally, and the raisins move away from each other IN ALL THREE DIRECTIONS.

It turns out that this analogy corresponds to what we see when we observe the motion of faraway galaxies in our universe. They appear to be moving away from us in all directions equally, with no preferred direction for the expansion.

#### Dave Rothstein

Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of the development for the former version of the site.

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