The difference between redshift and blueshift

How does this weird phenomenon help us study the distant universe?

The Andromeda Galaxy is moving towards us, so its light is blueshifted (Image credit: Adam Evans)

If you’ve ever heard a police car drive by with its sirens blaring, you’ll be able to understand redshift and blueshift. As the car went past you probably noticed that it sounded higher pitched as it approached you and lower pitched as it drove away. This is known as the Doppler effect, and it’s caused by sound waves being pushed closer or further away from each other.

The same thing happens with light. It turns out that as a light source moves towards or away from us on a large scale, the light also gets shifted – but in this case its wavelength on the electromagnetic spectrum gets shorter or longer. Wavelength is basically an energy pattern in light that determines what colour it is. Longer wavelengths correspond to red, while shorter wavelengths correspond to blue or violet.

When we observe a galaxy in the universe, we find that its light is generally either redshifted or blueshifted. The former is more common, as the universe is expanding and everything is moving away from everything else. The more distant a galaxy is – and thus the faster it is moving away from us – the higher its redshift is. A few galaxies, like the Andromeda Galaxy, are moving towards us however and are on a collision course with our Milky Way. Andromeda’s light is blueshifted. Galaxies that are spinning can also exhibit a slight blue or redshift, as one side of the galaxy moves towards us while the other moves away from us.

Pushed and pulled

How can a moving object change both its sound and light?

Click to enlarge image (Image credit: Future PLC)

The most distant galaxy

We can use redshift to measure how far away the most distant galaxies we can see are. As a galaxy increases in speed and thus gets further away, its redshift increases. Currently, the most distant galaxy we’ve seen in the universe is GN-z11, which has a redshift value of 11.09. This corresponds to a distance of 13.4 billion lightyears in terms of how far the light has travelled to reach us across the universe. This also means we are looking far back in time, to just 400 million years after the Big Bang.

Astronomers are now hoping to look even further back towards the Big Bang itself to find some of the first galaxies that formed in the universe.


This article was originally published in How It Works issue 120, written by Jonny O’Callaghan


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