Final answer:
The Doppler effect demonstrates that galaxies are moving away from us, with a redshift indicating their recessional velocity. This effect provides evidence for the universe's expansion and the presence of dark matter. The relativistic Doppler effect complements these findings by accounting for high-velocity cosmic objects.
Step-by-step explanation:
The Doppler effect serves as evidence for the relative motion of galaxies. When we observe a red shift in the spectral lines of a galaxy, it suggests that the galaxy is moving away from us. This phenomenon is consistent with Hubble's observation that galaxies exhibit a redshift proportional to their distance from us, which implies that the universe is expanding. This expansion means that galaxies are receding from each other with recessional velocities that can be measured using the Doppler shift. According to Zwicky and Rubin's research, the motion and mass distribution of galaxies, as well as the redshift observed, indicate the presence of dark matter that does not emit light but has a gravitational effect on the motion of stars and galaxies.
The relativistic Doppler effect is an enhancement of the classical Doppler effect, accounting for the fact that the universe's expansion can reach speeds close to the speed of light, necessitating the use of relativity to accurately measure distances and velocities.