How does the Doppler effect relate to universal expansion?

The Doppler effect is a phenomenon observed when the source of a wave moves relative to an observer, resulting in a change in frequency and wavelength of the wave. In the context of universal expansion, this effect is particularly relevant when considering the light emitted by celestial objects.

When we observe light from distant galaxies, we note that many of them exhibit a redshift. This redshift occurs because these galaxies are moving away from us; as they recede, the wavelengths of the light they emit stretch, shifting toward the red end of the spectrum. This phenomenon serves as crucial evidence for the expansion of the universe, as it indicates that the further away a galaxy is, the faster it seems to be moving away from us. This relationship was one of the key observations that led to the formulation of the Big Bang theory and our understanding of how the universe has been expanding over time.

In contrast to the redshift observed in receding galaxies, blue shifts would indicate that an object is moving toward us, which does occur under certain circumstances but is not the primary evidence for universal expansion. The other options pertain to different phenomena unrelated to the expansion aspect, such as the gravitational effects of black holes or the orbital mechanics of planets around stars, rather than directly illustrating