To measure the expansion rate of the Universe and its dark matter and dark energy content, however, it is necessary to measure not just the distance but also the cosmological redshift of the source, namely how quickly distant galaxies are receding from us. Redshift is the name given to the phenomenon whereby electromagnetic radiation is increased in wavelength with distance. A commonly-known example of redshift is the apparent change in the pitch of an emergency siren as police cars or ambulances move closer and further away from an observer.