* Redshift: When light from distant galaxies is observed, it appears shifted towards the red end of the spectrum. This is known as redshift. The amount of redshift is directly proportional to the distance of the galaxy.
* Doppler Effect: This phenomenon is similar to the Doppler effect we experience with sound waves. As a source of sound moves away from us, the sound waves get stretched out, resulting in a lower pitch. Similarly, as a galaxy moves away from us, the light waves it emits get stretched, causing the light to appear redder.
* Hubble's Law: Edwin Hubble observed this redshift phenomenon and formulated Hubble's Law, which states that the recessional velocity of a galaxy is directly proportional to its distance from us. This means that galaxies farther away are moving away from us at a faster rate.
Therefore, the observed redshift of galaxies provides strong evidence that the universe is expanding, and that galaxies are moving further apart from each other.
