Redshift

Redshift is the stretching of light to longer, redder wavelengths. When a source of light moves away from us, or when the space between us and it expands, the light waves are stretched out and shifted toward the red end of the spectrum. The faster the recession, the greater the redshift. The opposite, light shifted toward the blue, happens when a source approaches.

Why it matters

Redshift is one of the most important tools in all of cosmology. In the 1920s Edwin Hubble found that almost every distant galaxy is redshifted, and that the more distant ones are redshifted more. The only sensible explanation was that the universe itself is expanding, carrying galaxies apart. This was the discovery that turned the Big Bang from speculation into the leading model: if everything is flying apart now, it was once packed together.

The connection to Hawking

The expanding universe revealed by redshift is the starting point of Hawking's cosmology. His singularity theorems take that expansion and run it backwards in time, showing that it must trace back to an initial singularity. Redshift also appears in a second guise in his work: light climbing out of the steep gravity near a black hole is redshifted too, and at the event horizon this gravitational redshift becomes infinite, which is part of why the horizon appears, from outside, as a frozen edge.