As the spectral lines of distant stars have lower frequency than that of hydrogen, this effect can be used to show that the universe is expanding.
This phenomenon causes a quadratic term to appear in pulsar periods in the Shklovskii effect. Three unevenly spaced wavelengths were produced by using concave mirror to ignore one of its forms, confirming another form of this phenomenon caused by time dilation in the Ives-Stilwell experiment. Photons emitted from particles in thermal motion undergo spectral (*) broadening due to this effect. It was independently confirmed by Fizeau and Buys Ballot. As the spectral lines of distant stars have lower frequency than that of hydrogen, this effect can be used to show that the universe is expanding. For 10 points, redshift is a manifestation of what effect that causes the observed pitch of a siren to decrease as an ambulance passes by?
This phenomenon causes a quadratic term to appear in pulsar periods in the Shklovskii effect.
Doppler effect [accept transverse Doppler effect; prompt on redshift or blueshift]