How Redshift Shows the Universe is Expanding


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When stargazers look up at the night sky, they see light. It's an essential part of the universe that has traveled across great distances. That light, formally called "electromagnetic radiation", contains a treasury of information about the object it came from, ranging from its temperature to its motions.

Astronomers study light in a technique called "spectroscopy". It allows them 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 objects moving away from each other 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 anyone can "see." It's an effect that astronomers measure in light by studying its wavelengths. 

How Redshift Works

An object (usually called "the source") emits or absorbs electromagnetic radiation of a specific wavelength or set of wavelengths. Most stars give off 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 "stretch out" or increase. Each peak is emitted farther away from the previous peak as the object gets 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. People on Earth are familiar with Doppler shift in pretty practical ways. For example, some of the most common applications of the doppler effect (both redshift and blueshift) are police radar guns. They bounce signals off of a vehicle and the amount of redshift or blueshift tells an officer how fast it's going. Doppler weather radar tells forecasters how fast a storm system is moving. The use of Doppler techniques in astronomy follows the same principles, but instead of ticketing galaxies, astronomers use it to learn about their motions. 

The way astronomers determine redshift (and blueshift) is to use an instrument called a spectrograph (or spectrometer) to look at the light emitted by an object. Tiny differences in the spectral lines show a shift toward the red (for redshift) or the blue (for blueshift). If the differences show a redshift, it means the object is receding away. If they're blue, then the object is approaching.

The Expansion of the Universe

In the early 1900s, astronomers thought that the entire universe was encased inside our own galaxy, the Milky Way. However, measurements made of other galaxies, which were thought to be simply nebulae inside our own, showed they were really outside of the Milky Way. This discovery was made by astronomer Edwin P. Hubble, based on measurements of variable stars by another astronomer named 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. It helps astronomers define the expansion of the universe.  It also shows 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".)  For the most part, objects in the universe are receding away from each other and that motion can be measured by analyzing their redshifts.

Other Uses of Redshift in Astronomy

Astronomers can use redshift to determine the motion of the Milky Way. They do that by measuring the Doppler shift of objects in our galaxy. That information reveals how other stars and nebulae are moving in relation to Earth. They can also measure the motion of very distant galaxies — called "high redshift galaxies".  This is a rapidly growing field of astronomy. 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. Astronomers assign the letter z to redshift. That explains why sometimes a story will come out that says a galaxy has a redshift of z=1 or something like that. The earliest epochs of the universe lie at a z of about 100.  So, redshift also gives astronomers a way to understand how far away things are in addition to how fast they are moving. 

The study of distant objects also gives astronomers a snapshot of the state of the universe some 13.7 billion years ago. That's when cosmic history began with the Big Bang. The universe not only appears to be expanding since that time, but its expansion is also accelerating. The source of this effect is dark energya not-well-understood part of the universe. Astronomers using redshift to measure cosmological (large) distances​ find that the acceleration has not always been the same throughout cosmic history. 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. "How Redshift Shows the Universe is Expanding." ThoughtCo, Aug. 28, 2020, Millis, John P., Ph.D. (2020, August 28). How Redshift Shows the Universe is Expanding. Retrieved from Millis, John P., Ph.D. "How Redshift Shows the Universe is Expanding." ThoughtCo. (accessed January 30, 2023).