Expert: Courtney Seligman - 2/6/2011

Question
We know  that our universe is expanding. Into what kind of infinity (rhetorical)? What do you think of the possibility,that as you leave our universe in travelling through space, we would encounter another universe? It might be expanding toward ours or may be entirely different.Just as there are billions of solar systems in our universe, why couldn't there be one or more than one universe outside our own? This is one of the less stupid questions I have pondered for many years and I thank you for your time.

Answer
The most commonly used example of how the Universe works is to imagine the surface of a balloon as representing the entire Universe (the shape is wrong, because the Universe isn't "round"; but it's just for the purpose of visualization). As the balloon gets bigger, dots on its surface (specks of dust, say) get further apart, but they aren't going into some previously empty space. Instead, the empty space already between them is simply getting larger.

Of course, in this view, you could ask what about the region inside the balloon's surface, or outside it? That's the problem with analogies of this sort. Only the surface of the balloon represents the Universe. The "inside" and "outside" are there only so that we can visualize what is going on. They don't have any existence, real or imaginary.

So as the Universe gets "bigger", things get further apart, but they don't really go into any previously empty region.

Of course, you could ask if there are other "balloons" around, which ours might run into, particularly since models in which there is an infinity of Universes often show a bunch of bubbles scattered all over the place, and if that was an accurate representation, then the bubbles could run into each other if they got big enough.

For that reason, I like a variation on the "balloon" model in which the balloon stays the same size, but any objects (like yardsticks) which are on the surface of the balloon gradually get smaller. In this way of looking at things, the "Universe" is static and unchanging, but the distances between things still seem to get larger, because the yardsticks we use to measure distances get smaller. The viewpoint is different, but the results are the same.

In any event, to answer the last part of your question, it is generally believed that there is an infinity of Universes, but that each of them has a space-time which is completely separate from every other one, so that no matter how "big" the space-time of any given Universe gets, it will never intersect the space-time of any other Universe. So, at least per current theory, no matter how far you travel (or the Universe expands), you would never leave our Universe, or enter another one.