10 We know that one way of making frequencies appear lower than they should be is by having the source move away from you - think of a police siren sounding higher than usual as it approaches you and lower as it goes away. So we deduce (perhaps incorrectly: no-one knows for sure) that distant galaxies are moving away from us, and that the most distant galaxies are moving fastest.

This idea gives rise to the notion of an expanding Universe.