The Expanding Universe:

The Doppler shift is the effect of sound increasing or decreasing in pitch as an object moves towards or away from you. If an object moves away, the pitch is lowered, and if moving towards you, the pitch is raised. Light works the same way, except light is shifted to redder colours when an object is moving away, and to blue colours when an object is travelling towards us.

In 1916 Vesto Slipher (to whose family I am indebted for helping fund my undergraduate education through a scholarship set up at the University of Arizona in his honour) observed about 50 nearby galaxies, spreading their light out using a prism, and recording the results onto film. The results confounded him and the other astronomers of the day. Almost every object he observed had its light stretched to redder colours, indicating essentially everything in the Universe was moving away from us.

Here we show the spectrum of a galaxy as Slipher would have seen it. The light is stretched in the bottom spectrum, so that the dark lines (the colours where elements such as Sodium absorb light), are stretched to redder colours.