What is Redshift?

What does redshift mean?

It’s often compared to the high-pitched whine of an ambulance siren that gets lower as the ambulance moves past you and then away from you. The Doppler effect is what causes the sound of an ambulance to change in this way. It’s a good comparison because both sound and light move through air and space in waves, which change as they move.

Sound can only travel at a certain speed through the air, about 750 miles (1,200 kilometers) per hour. As an ambulance moves forward and its siren goes off, the sound waves in front of the ambulance get squashed together. In the meantime, the sound waves behind the ambulance spread out. Said another way, the frequency of the sound waves is higher in front of the ambulance and lower behind it. This means that more sound waves will hit a listener’s ear in a set amount of time if they are in front of the ambulance, and fewer sound waves will hit a listener’s ear in the same amount of time if they are behind the ambulance. Changes in the frequency of sound waves are heard as changes in pitch by our brains.

Light, like sound, is a wave that moves at a constant speed: 186,000 miles (300,000 km) per second, which is about one billion kilometers per hour. So, light plays by the same rules as sound.

But when it comes to light, changes in the frequency of the waves show up as changes in color, not in pitch. For example, if a lightbulb moved quickly through space, the light would look blue as it came toward you and turn red as it went by. This is similar to the ambulance situation because as the light is moving away from us, the light waves are getting more spread out thus decreasing their frequency. This decrease in frequency causes the light to look redder in color.

How is redshift useful to us?

One of the most important discoveries of the 20th century was made because of redshifts. In the 1910s, astronomers at Lowell Observatory and elsewhere noticed that the light from almost every galaxy was redshifted relative to Earth. That meant that most galaxies were moving away from us. A Belgian scientist named Georges Lemaitre noticed that the speed at which galaxies were moving away from each other could be explained by a shocking fact, the universe is growing!

In 1929, the American astronomer Edwin Hubble compared measurements of redshift to estimates of how far away galaxies were. He found something amazing. The farther away a galaxy is, the faster it’s moving away from Earth. The combination of their discoveries was named the Hubble–Lemaitre law by the International Astronomical Union in 2018.

What became known as “cosmological redshift” became the first part of the Big Bang theory, an explanation of how our universe started.

As astronomers find more and more redshifted objects on the edge of what can be seen, the list of the most distant objects is always changing. Galaxies travel across the universe for billions of years, sending out their faint red light and revealing a little more about how the universe works.

Glossary

Frequency: The number of waves that pass a fixed place in a given amount of time

Pitch: The quality of a sound that is based on how fast its vibrations are.

Big Bang Theory: An explanation of how the universe began. Simply put, it says that the universe as we know it began as a single point that was infinitely hot and dense which then grew and expanded.

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