Redshift

Redshift refers to light being 'shifted' towards the redder end of the spectrum — longer wavelengths — as objects move away from each other. If a light source is moving towards us, then the light is blueshifted, shifting towards the bluer end of the spectrum, which corresponds to shorter wavelengths.

Imagine waving a spring back and forth to create a wave and then starting to run away – the waves would be stretched longer.

Because the universe is expanding 🤯, distant galaxies are moving away from us faster than nearer ones. Picture some chocolate chip cookies baking in an oven. As the cookies bake and grow, the chocolate chips move apart from each other.

By comparing the redshift of light seen from distant galaxies with what we would expect to see, it's possible to use redshift to determine how far they are away.

I once heard :

Roses are red, roses are blue,

depending on their velocity relative to you.

Redshift is an example of the Doppler effect, or Doppler shift, in action. It's more commonly known from the stretching or compressing of soundwaves, as, say, an ambulance moves towards or away from you, or how the sound in front of a moving aircraft eventually can produce a sonic boom.