Does It Matter What the Universe Expands Into? a Natural

Does It Matter What the Universe Expands Into? A natural question of anyone who meets Hubble's Law for the first time is to ask what exactly the universe is expanding into. This is often further motivated by the analogy of an inflating balloon universe, where two- dimensional galaxies ride along on the surface of the balloon. Imagine that you live in one of these balloon- borne galaxies. Your universe is the surface of the balloon. You can move along the surface, but you can never exist above or below it. Now one of your two- dimensional friends asks what your (balloon)space is expanding into. Do you need to answer this question, or can the expansion you experience be completely described without reference to an unseen, third- dimension? Imagine someone argues that question "what is the universe expanding into?" does not even need an answer! Would you agree or disagree? Explain. If you disagree, would there be any way to gather scientific evidence to answer this question?