Redshift

The tone of the sound of the whistle drops as an approaching train passes by us and and races on into the distance. The approaching train had shortened the sound wave while the receding train had lengthened the waves - a phenomenon known as the Doppler effect. Astronomers have applied this principle as they examine the light of distant galaxies. They found that the light was shifted toward the red end of the spectrum i.e. the longer wave lengths. The farther away the galaxies, the greater the redshift - thus their conclusion that our universe is ever expanding.