What is Redshift?

Astronomers use redshifted light to understand the motions of distant objects in space. NASA, ESA, G. Illingworth (UCO/Lick Observatory and UC Santa Cruz) and the HUDF09 Team.

When you go outside at night to look up at the stars, you're seeing light that has traveled across great distances to reach your eyes. That light contains a treasury of information about the object that emitted it. Astronomers can study light in a technique called "spectroscopy" to dissect it down to its wavelengths to create what's called a "spectrum". Among other things, they can tell if an object is moving away from us.

They use a property called a "redshift" to describe the motion of an object moving away from another one in space.

Redshift occurs when an object emitting electromagnetic radiation recedes from an observer. The light detected appears "redder" than it should be because it is shifted toward the "red" end of the spectrum. Redshift is not something you "see" with your eye. It is an effect that we measure on light by studying its wavelengths.

How Redshift Works

An object (often referred to as the source) will emit electromagnetic radiation of a specific wavelength or set of wavelengths. Most stars emit a wide range of light, from visible to infrared, ultraviolet, x-ray, and so on.

As the source moves away from the observer, the wavelength appears to increase (since each peak is emitted further away from the previous peak as the object recedes). Similarly, while the wavelength increases (gets redder) the frequency, and therefore the energy, decreases.

The faster the object recedes, the greater its redshift. This phenomenon is due to the doppler effect. Some of the most common applications of the doppler effect (both redshift and blueshift) are police radar guns, as well as doppler weather radar. Its use in astronomy follows the same principles, but instead of ticketing galaxies, astronomers use it to learn about motions of objects in the cosmos.


The way astronomers determine redshift (and blueshift) is to look at the light emitted by an object through an instrument called a spectrograph (or a spectrometer). Tiny differences in the spectral lines show a shift toward the red (for redshift) or the blue (for blueshift). 

The Expansion of the Universe

In the early 1900s astronomers thought that the entire universe was encased inside our own galaxy, the Milky Way. But measurements made of other galaxies, which were thought to be simply nebulae inside our own galaxy, showed that these objects were, in fact, outside of the Milky Way. This discovery was made by astronomer Edwin P. Hubble, based on measurements of variable stars by Henrietta Leavitt. 

Furthermore, redshifts (and in some cases blueshifts) were measured for these galaxies, as well as their distances. Hubble made the startling discovery that the farther away a galaxy is, the greater its redshift appears to us.

This correlation is now known as Hubble's Law, and it helps astronomers define the expansion of the universe. Specifically, that the farther away objects are from us, the faster they are receding. (This is true in the broad sense, there are local galaxies, for instance, that are moving towards us due to the motion of our "local group".)  Thus, it appears that all objects in the universe (particularly the galaxies) are expanding outward.

Other Uses of Redshift in Astronomy

Astronomers can tell the motion of the Milky Way by measuring the doppler shift of objects in our galaxy, they can determine how they are moving in relation to us on Earth. They can also measure the motion of very distant galaxies — called "high redshift galaxies".  This is a rapidly growing field of astrono,y. It focuses not just on galaxies, but also on other other objects, such as the sources of gamma-ray bursts.

These objects have a very high redshift, which means they are moving away from us at tremendously high velocities. This study of distant objects also gives us a snapshot of the state of the universe some 13.7 billion years ago, when cosmic history began in the Big Bang. At these distances, Hubble's Law breaks down, as the universe not only appears to be expanding, but accelerating.

The source of this effect is dark energya not well-understood part of the universe that is driving this acceleration at large astronomical distances. Astronomers have also found that the acceleration has not always been the same throughout the history of the universe. The reason for that change is still not known and this effect of dark energy remains an intriguing area of study in cosmology (the study of the origin and evolution of the universe.)


Edited by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "What is Redshift?" ThoughtCo, Mar. 2, 2017, thoughtco.com/what-is-redshift-3072290. Millis, John P., Ph.D. (2017, March 2). What is Redshift? Retrieved from https://www.thoughtco.com/what-is-redshift-3072290 Millis, John P., Ph.D. "What is Redshift?" ThoughtCo. https://www.thoughtco.com/what-is-redshift-3072290 (accessed February 23, 2018).