What is the Doppler effect in the context of distant galaxies?

The Doppler effect in the context of distant galaxies is the shift in wavelength of light due to their motion.

When light from a distant galaxy reaches us, its wavelength can be shifted towards the red end of the spectrum (redshift) or towards the blue end (blueshift). This shift is caused by the motion of the galaxy relative to us. If the galaxy is moving away from us, the light waves are stretched out and the wavelength is increased, resulting in a redshift. Conversely, if the galaxy is moving towards us, the light waves are compressed and the wavelength is decreased, resulting in a blueshift.

The Doppler effect can be used to determine the speed and direction of a galaxy's motion. By analysing the redshift or blueshift of its light, astronomers can calculate the galaxy's velocity relative to us. This information can be used to study the large-scale structure of the universe and the distribution of galaxies within it.

The Doppler effect also provides evidence for the expansion of the universe. The observation that almost all distant galaxies exhibit a redshift suggests that they are moving away from us, and from each other, at an accelerating rate. This supports the theory of cosmic inflation, which proposes that the universe underwent a period of rapid expansion shortly after the Big Bang.