Astronomy and Astrophysics 101: What is a “redshift?”

(Click the image to see the full infographic.) The universe is expanding, and this expansion stretches the light that passes through space, in a phenomenon known as cosmological redshift. The greater the redshift, the greater the distance the light traveled. As a result, telescopes with infrared detectors are needed to see the light of the first, most distant galaxies. Author: NASA, ESA & L. Gustak (STSci)

The “redshift” is a key concept for astronomers. The term can be taken literally – the wavelength of light is stretched, so light is seen as a “shift” toward the red part of the spectrum.

Something similar happens with sound waves when a sound source moves relative to an observer. This effect is called the “Doppler effect” in honor of Christian Andreas Doppler, an Austrian mathematician who discovered that the frequency of sound waves changes when the sound source and the observer move relative to each other.

As the two approach, the frequency the observer hears is higher; when they move away from each other, the audible frequency is lower.

There are many everyday examples of the Doppler effect – changing the heights of police and ambulance signals, or the whistles of trains and racing car engines as they pass by. In each case, as the source approaches and then passes by the observer, a change in altitude is audible.


Absorption lines in the visible spectrum of supercollection of distant galaxies (right) compared to absorption lines in the visible spectrum of the Sun (left). Arrows indicate redshift. The wavelength increases to red and further (frequency decreases). Credit: Georg Vior (Dr. Shores) CC BY-SA 3.0

Everyone has heard the height of the approaching police siren increase and decrease sharply as the siren passes by and recedes. The effect occurs because sound waves come to the listener’s ear closer to each other as the source approaches, and farther as it recedes.

Light behaves like a wave, so light from a luminous object undergoes a Doppler shift when the source moves relative to us. Since 1929, when Edwin Hubble discovered that the universe was expanding, we knew that most other galaxies were moving away from us. The light from these galaxies shifts to longer (that is, more red) wavelengths – in other words, it is a “shift in red.”

Because light moves at such a high speed compared to everyday phenomena (a million times faster than sound), we do not experience this redshift in our daily lives.

The redshift of a distant galaxy or quasar is easy to measure by comparing its spectrum with that of a reference laboratory. Atomic radiation and absorption lines occur at well-known wavelengths. By measuring the location of these lines in astronomical spectra, astronomers can determine the redshift of receding sources.

However, to be precise, the redshifts observed in distant objects are not entirely due to the Doppler phenomenon, but are rather the result of the expansion of the universe.

Doppler shifts result from the relative motion of the source and the observer in space, while astronomical redshifts are “redshifts of expansion” due to the expansion of space itself.

Two objects can actually be motionless in space and still experience a redshift when space itself expands.

A convenient analogy for the expansion of the universe is a loaf of unbaked raisin bread. Raisins are at rest relative to each other in the dough before putting it in the oven. When the bread rises, it also expands, thus increasing the space between the raisins.

If the raisins could see, they would notice that all the other raisins are moving away from them, even though they themselves are motionless in the loaf. Only the dough – their “Universe” – expands. Astronomy and Astrophysics 101: What is a “redshift?”

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