The Balloon Model of the Universe

An easy way to visualize the expanding universe is to drop a spatial dimension and pretend that the observed universe is 2-dimensional. So we have a balloon for a model. The universe is the surface of the balloon ONLY. What we would call the inside and outside of the balloon is NOT part of the model, just the 2-d rubber part.

1. Blow up the balloon to a "small" size. Don't tie it off. Draw 5 "galaxies" randomly on the balloon and number them. You might find it easier in the next step if you put all 5 on the same hemisphere. You can draw more galaxies, if they are decorative.
2. With the universe at a "small" size, measure the distance from galaxy #1 to galaxies 2, 3, 4, and 5. Record these values. Now measure from galaxy #5 to galaxies 2, 3, and 4 (you already measured the #1-#5 distance).
3. Now blow up the universe to size "large." Repeat the distance measurements and record them.
4. Did ALL galaxies get farther away from galaxy #1? Did all galaxies get farther away from galaxy #5? Does the universe always look like it is expanding, no matter which galaxy one sits on?
5. Let's see if these galaxies follow a Hubble law. Compute the "large" minus "small" distance difference. This is like an expansion velocity. Plot this "velocity" against "large" distance (or "small" distance; should work either way). Do you get a straight-ish line? If so, your balloon expands according to the Hubble Law

---

The real universe occupies 3 space dimensions, perhaps curving in a 4th dimension like the surface of our balloon. But the essential points remain the same: no center to the universe, any spot in the universe observes the same expansion, the Hubble Law is valid anywhere in the universe.

Whether the universe will expand forever or not depends on how much matter (and therefore gravity) is in the universe compared to how fast it is expanding. If there is a lot of matter, the universe will at some point stop expanding and collapse again. If not, it will expand forever.

---