The Balloon Model of the Universe
(Why there is no "center" to 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
- 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.
- 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).
- Now blow up the universe to size "large." Repeat the distance
measurements and record them.
- 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?
- 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