Question: How does redshift work?

Arttu Rajantie answered on 20 Jun 2011:

Redshift is essentially just the Doppler effect for light. For sound, the Doppler effect means that when the source of the sound is moving towards you, you hear a higher pitch, and when it moves away from you, the pitch is lower. You can notice this clearly when an ambulance passes you.

The reason for the effect is that sound is a wave, so it consists of peaks and troughs. If the source is moving towards you, it is a bit closer to you every time it send a new peak. Therefore the interval between the peaks is shorter than it would be if it was not moving. This means higher frequency, i.e., higher pitch.

Light is a wave, too, and therefore the same thing happens to it. Red light has lower frequency, so if the light source is moving away from you, its light will be slightly redder, and if it moves towards you, it is bluer.

This redshift is very useful in cosmology because we determine how fast distant objects are moving by measuring their light. This works well because each chemical element produces light at a handful of specific frequencies known as spectral lines. So, we need to take the spectrum of the light, identify the lines produced by, say, hydrogen, and measure their exact positions. The difference between this and the position measured in the laboratory tells very accurately the speed of the light source.