Classroom Activity for 14-16
What the Activity is for
Here we're looking at how stretching space predicts the change in recession velocity of galaxies. If you stretch a piece of elastic uniformly, the farther away any point is from the centre the faster it moves from that centre.
What to Prepare
- a length of elastic cord of low stiffness, with galaxies attached
- a metre ruler
Safety note: Take care to release the tension in the elastic cord carefully – choose the anchoring students wisely.
What Happens During this Activity
This is a demonstration, and will need building up. The explanation for cosmic redshift is that space itself stretches: that is, the space between galaxies not the space within galaxies. Here the galaxies are spread out along an elastic thread. It is best to anchor the galaxies at both of their extremities to avoid difficulties with space stretching through the galaxy.
Start with the elastic cord loose – just, but only just, pulled tight between two students, one at each end. Choose a galaxy to be home. Mark this galaxy in some way, perhaps with a Post-it note. Choose a pair of other galaxies to measure, one of which is further away than the other. Mark their positions. Now have the students at the ends move apart a short distance to simulate the evolution of the universe, as space expands. Mark the new positions of the chosen galaxies. From a change in position you can calculate their recession velocity – how fast they are moving away from the home galaxy. From their average position you can work out how far away they are.
The recession velocity increases as the distance from the home galaxy increases, as a simple result of the uniform stretching of space. The advantage of using elastic cord is that the space is is obviously uniformly stretched. You might measure the change in length of different parts of the cord to show that this is so. You might make multiple measurements across several galaxies to show that the recession velocity is proportional to the distance from the chosen or home galaxy.
The important final step is to point out that this is one direction only. You might like to make the link with the homogeneous make-up of space, leading to the idea that the recession velocities will vary uniformly in any direction.