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The Question

(Submitted July 7, 1998)

If the universe is expanding like a balloon and we are on that balloon, what is inside the balloon, or outside?

Is this balloon hallow? If it is not, are galaxies traveling outward (in radial direction) at the same speed with respect the "ground zero" (Big Bang)?

If it is hollow, should we see nothing if we look inward or outward?

Thank you for your time.

The Answer

The balloon analogy is very confusing. In fact, there is no direction in the universe which corresponds to inward or outward on the balloon. Thus the universe is not like a hollow balloon because there is no "inside" to be hollow.

Other books use the analogy of rising raisin bread dough (where the raisins represent galaxies). This has different problems (e.g., the Universe doesn't have anything analogous to the crust), but it doesn't raise questions of inside and outside.

All analogies break down eventually. The balloon analogy breaks down sooner than most.

David Palmer
for "Ask an Astrophysicist"

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