Redshift
Redshift is the phenomenon where light from distant galaxies is stretched to longer, redder wavelengths due to the expansion of the universe. It serves as a key observational evidence for the universe's ongoing expansion.
Source: science.nasa.gov
APODs including "Redshift"
Spiral Galaxy M90
16/03/1996
Spiral galaxy M90 is near the center of the Virgo Cluster of Galaxies - the closest cluster of galaxies to the our own Milky Way Galaxy. Also dubbed NGC 4569, this galaxy has a very compact and bright nucleus. Because of M90's proximity and motion inside the Virgo Cluster, M90 actually shows a blueshift - indicating that it is moving toward us rather than away. Most galaxies show a redshift which indicates that they move away from us. Calibrating exactly how redshift relates to distance would indicate a scale for our universe - a topic of much debate recently. Information: The Scale of the Universe Debate in April 1996