Exploring cosmology: a simple experiment to illustrate the cosmological principle and the Hubble–Lemaître Law

Abstract We present an activity to illustrate the Hubble–Lemaître law and the cosmological principle. Using an elastic band to represent the expanding Universe and dots on the band to represent galaxies, we use Tracker software to measure recession velocities as a function of distance and analyse their relationship in different reference frames. Unlike previous work, which has focused on changes in position, this approach allows direct and quick measurement of velocities for different reference frames and different expansion rates. The activity is simple and can be incorporated into different learning cycles or used as homework.