What is Redshift?

“Redshift” is an essential term that astronomers use regularly when discussing the early Universe and expansion of space. Redshift refers to a literal shift in the wavelength of light towards the red end of the electromagnetic spectrum (the spectrum on which visible light and other wavelengths, such as x-ray, ultraviolet, infrared, and microwaves exist).

Redshift can be observed when an object in space is moving away from us. As it moves away, it takes longer and longer for wavelengths of light from the object to reach us. Because red light wavelengths are longer than other forms of light, the object appears redder than we would expect.

The electromagnetic spectrum. Notice that red wavelengths are longer than blue. Credit: SERC

A similar process, called the Doppler Effect, occurs with sound waves. You can observe the Doppler Effect when you hear something like a police siren whizzing past you. The change in pitch that you hear is a direct result of the Doppler Effect, where the sound waves are shortened and thus sound higher pitched as it approaches you, and lengthened as it speeds away, making a lower pitch.

Redshift works in a similar way to the Doppler Effect but is much less noticeable on smaller scales. Light moves incredibly fast (a million times faster than sound), so redshift is simply not noticeable in everyday phenomena. When you look at cosmic scales, however, redshift becomes much more apparent.