Scientists in general are pretty smart dudes and dudettes. Especially astronomers: because they study stuff that they can’t and never will be able to look at up close, their work is that much harder and, therefore, they themselves must be that much smarter. (Flawless logic, that.)
So, when astronomers say that the universe is constantly expanding, I’m inclined to believe them. But, just how do they know that? And how can one effectively demonstrate such to students?
Ink & Rubber Analog
For this Demo Science science demo, all you’ll need is a balloon and a differently colored permanent marker. To make it especially outer space-like, I recommend a black balloon with a gold- or silver-ink marker, but any two at least moderately contrasting colors will work.
Gather your students ‘round and tell ‘em to shut up. Inflate the balloon to about the size of a softball, and pinch off the end so it doesn’t deflate. Then, use your marker to add fifteen to twenty dots, randomly spaced on the surface of the balloon.
Have the smelly little goobers pay special attention to this next part. Continue to inflate the balloon, blowing it up as large as you think you can. It’s best if it doesn’t pop, but the demonstration will still be valid if it does. Use your own judgement, is what I’m saying, I guess.
LITERALLY Exactly Like the Actual Universe
As you inflate the balloon, the dots will, of course, move away from each other. Observant students will note that some of the dots move farther apart than others, depending on their placement on the balloon’s surface; even your less impressive students (lookin’ at you, Kevin) will notice that none of the dots get closer together.
The moving dots are analogous to galaxies: they’re always moving away from each other (as astronomers have noted via the Hubble and other high-powered telescopes), never closer. Because these countless, cosmically-huge galaxies are constantly spreading out, it implies that the universe itself must be expanding.
Regarding the fact that some galaxies/dots move farther apart than others as the universe expands/you inflate your balloon: the farther away a galaxy is from Earth, the faster it’s moving away from us, and we from it.