Expansion of the Universe
Earth and Space

# Elastic band universe

Practical Activity for 14-16 16-19

In this activity students build a model universe using washers and elastic bands. You can use it to introduce Hubble’s law.

## Preparation

The student worksheet below includes information on how to make a model universe. Alternatively, to save time you may want to make these for each group before the lesson.

## Equipment

Each student will need:

• 6 assorted washers (or paper clips)
• 5 elastic bands of the same thickness (and ideally of different lengths)
• Small sticker to indicate ‘home’
• Ruler or tape measure
• Graph paper (or laptop with Microsoft Excel or similar)
• Sticky tape

## Procedure

1. Choose one washer to be the home galaxy and label it with a sticker. Label the other galaxies with letters A to E.
2. Measure the distance from the home galaxy to galaxy A. Repeat for the other galaxies.
3. Expand the universe until it is twice its original length and then tape down the ends to a table or the floor to hold it in place.
4. Measure the new distance from the home to the other galaxies.
5. Calculate the change in distance for each of the galaxies.
6. Plot a graph of “change in distance” against “initial distance” and draw a line of best fit.

## Discussion prompts

• How does the change in distance depend on how far away the galaxy is?
• Does it matter which galaxy we label home?

## Teaching notes

When students plot a change in distance against distance graph they should find that it is a straight line. The galaxies move away from them us at a speed that is proportional to their distance from our galaxy. This is known as Hubble’s law.

 Model Universe The washers do not expand Galaxies do not expand (they are gravitationally bound) The elastic bands expand, carrying washers with them Space between the galaxies expands, carrying galaxies with it

As an extension students can follow the instructions on the worksheet (below) to explore the viewpoint from other galaxies. The gradient of the graph is the same irrespective of which washer they consider to be ‘home’. Like real galaxies, the galaxies in the model seem to move away from home, but home is not the centre of the expansion.

## Learning outcome

Students explain why galaxies move away from our galaxy with a speed that is proportional to their distance.

With thanks to the Perimeter Institute of Theoretical Physics for permission to adapt their activity