Final answer:
Cosmological redshift occurs when the expansion of space stretches the light waves from galaxies moving away from us, making them appear redder, and is indicative of the universe's expansion since the Big Bang.
Step-by-step explanation:
When galaxies appear to move away from us, the light they emit experiences a phenomenon known as cosmological redshift. This redshift occurs because as space itself expands, wavelengths of light stretch out, making the light appear redder, thus lowering the observed energy. This is different from the Doppler shift, which is a result of an object moving through space rather than space expanding. When a galaxy is moving towards us, the opposite effect called blueshift is observed, where light wavelengths appear shorter, making the light seem bluer and higher in energy. The study of these redshifts has helped astronomers understand the universal expansion following the Big Bang and determine the distances to far-off galaxies.
Understanding cosmic redshift not only informs us about the speed and direction of a galaxy's movement, but also offers insight into the mass and gravitational forces within galaxies due to the related broadening of spectral lines caused by the varying speeds of its constituent stars.