This demonstration uses a balloon with stickers or dots to represent galaxies and drawn waves to represent light. As the balloon inflates, the galaxies move farther apart and the waves stretch, modeling how the universe expands and how lightwaves are redshifted over cosmic distances.

1. Provide each group of students with a balloon, small stickers, and a marker.
2. Inflate the balloon slightly so that it just takes shape, and hold it closed without tying.
3. Place 5–10 stickers (or draw dots) on the surface to represent galaxies. Mark them so they can be replaced if they fall off.
4. Draw a wavy line between two stickers to represent a lightwave traveling across space.
5. Measure and record the distance between two galaxies and the wavelength of the drawn wave.
6. Inflate the balloon a little more, hold it closed, and repeat the measurements.
7. Continue inflating in steps until the balloon is nearly full, recording how distances and wave lengths change.
8. Compare observations to animations or diagrams showing constant versus accelerating expansion.

Modeling the expanding universe with a balloon and pen - Simon Lloyd:

Expanding Universe Balloons - Science Never Stops - U.S. Space & Rocket Center:

📄 Model the Expanding Universe - NASA Jet Propulsion Laboratory: https://www.jpl.nasa.gov/edu/resources/lesson-plan/model-the-expanding-universe/

• Use a balloon pump for steadier inflation.
• Try placing galaxies close together and far apart, and compare how the spacing changes.
• Color-code galaxies or draw different types of waves to track changes.
• Connect this demo to a math activity by graphing galaxy separation versus balloon size.

• Balloons can pop unexpectedly; have extras available.
• Keep balloons away from faces while inflating.
• Use a balloon pump if students are sensitive to latex or blowing balloons.

• What does each sticker represent? (Galaxies.)
• Why do all galaxies move farther apart as the balloon inflates? (The balloon’s surface expands everywhere, modeling universal expansion.)
• What happens to the drawn wave as the balloon stretches? (Its wavelength increases, showing redshift of light in an expanding universe.)
• Do the galaxies themselves expand? (No, only the space between them expands.)
• What are limitations of this model? (It represents only the balloon’s surface, not three dimensions, and expansion only occurs when inflated rather than continuously.)