What is Red Shift?

Redshift Explained

Visible light is an electromagnetic wave composed of many frequencies. The varying frequencies of visible light are experienced as the varying colors of the rainbow. The following colors are in the order from lowest frequency to highest frequency of electromagnetic waves: red, orange, yellow, green, blue, indigo, violet. Again, redshift occurs when the source of light is moving away from an observer; this causes the frequency of the light to lower and be shifted toward the color red. But exactly what are electromagnetic waves and visible light? How does a source of light moving change the color of the light? First, the redshift meaning must be understood through redshift science or "the science of waves."

What is a Wave?

Within physics, a wave is a propagating dynamic deformation of a medium. Sound waves are waves of energy that propagate through the deformation of particles and molecules of air. Electromagnetic waves such as visible light are waves of energy that propagate through electric and magnetic fields. Light waves can be imagined as simple transverse waves.

Wavelength is labeled in this image. Redshift explained as increasing the wavelength of light waves.

The wavelength of a wave is the distance between common points on two waves, such as the crests or tops of two waves. The ''frequency'' of a wave is the number of waves that pass a given point per second. Waves that have shorter wavelengths have higher frequencies because the waves are closer together and therefore more waves can pass a given point per second. Whereas longer wavelengths create lower frequencies because it takes longer for a complete wave cycle to pass a given point. When a source of a continuous stream of sound waves or light waves is moving away from an observer, the wavelengths experienced by the observer are elongated. Because red has the longest wavelengths of the colors of light, light experienced by the observer are shifted toward the color red (or shifted toward longer wavelengths). But does this actually change the color of light? This depends on the different types of redshifted light.

Types of Redshift Light

There are three main ways for light to be redshifted:

• Redshift through the expansion of the universe is the most effective at elongating wavelengths of light because the expansion of the universe at large scales can be happening at rates faster than the speed of light. This phenomenon was discovered by Edwin Hubble when he observed light from distant galaxies being red-shifted. Hubble discovered that the further away a galaxy is the faster it is moving away because the most distant galaxies emit light that is the most red-shifted. This meant that the universe is expanding like a balloon, with the distances between galaxies getting larger not because the galaxies are moving away from each other, but because the space between them is literally expanding. This is the only kind of redshift that actually results in the images of the galaxies appearing reddish, at least in the case of the most distant of galaxies. The light of very distant galaxies passes through the vastness of space which because of its expansion causes the wavelengths of the light to get stretched out toward the frequencies and wavelengths of red. This is also called cosmological redshift. However, other types of red-shifted light are undetectable by the human eye. For example, light-emitting from a car is not going to appear redder if it is driving away from an observer.

Some of the galaxies in this picture from the Hubble space telescope are more obviously red-shifted than others.

• In addition to expanding space pushing galaxies further apart, light emitted by galaxies is also red-shifted due to the movement of the galaxies themselves or the stars within them. Galaxies have rotation, therefore one side of a galaxy can be rotating toward an observer and therefore be blue-shifted, while the other side of the galaxy's center would be rotating away from the observer and be red-shifted. The light emitted by the individual stars within the galaxy can be red-shifted or blue-shifted depending on their movement in relation to an observer. The stars that are moving parallel to the observer would neither be red-shifted nor blue-shifted. This is also called relativistic redshift because it occurs due to the movement of sources of light relative to an observer. This type of redshift like most is not detectable by the human eye.
• Visible light can also have its wavelengths physically stretched by gravity. Gravity can be thought of as the deformation of spacetime, through stretching like a fabric. Gravitational redshift can occur when the light emitted by a source moves toward an object that has weaker gravity and therefore has less strongly curved spacetime.

Redshift, especially relativistic redshift is a type of Doppler effect.

The Doppler Effect

Like sound waves, waves of visible light travel at a finite speed. When a source of a continuous propagation of sound or light is moving in reference to an observer, the Doppler effect is demonstrated by the waves of light or sound. In the case of sound waves, a common example of the Doppler effect is a car horn or a police siren passing by an observer. When a car is driving by an observer while honking its horn, the sound emitted by the horn changes pitch from the perspective of the observer. When the car is driving toward the observer, the frequency of the sound waves is experienced at a higher rate. Because the frequency of sound waves is experienced as pitch, the car horn sounds higher pitched while the car is driving toward the observer. Once the car passes the observer, the sound wave frequency is getting elongated due to the car's movement away from the observer. This lowers the frequency, and the observer experiences the car horn as having a lower pitch.

This illustration shows the effect of the Doppler effect on the wavelengths of sound waves.

The frequency for sound waves is experienced as pitch, but the frequency for light waves is experienced instead of color. Light can become red-shifted or blue-shifted when the source of the light relative to an observer is moving toward or away from the observer. When the source of light is moving toward the observer, the frequency of the light waves increases because the wavelengths of the light waves get shorter or more scrunched up. Blue and violet have the shortest wavelengths and highest frequencies. Therefore, if a source of light is moving toward an observer, the observer will experience the light at a higher frequency and it will therefore be blue-shifted. If the source of light is moving away from the observer, its wavelengths get elongated and the frequency of the light experienced by the observer will be lower and therefore will be red-shifted.

This diagram shows the nature of redshift at position 1 and blueshift at position 4.

Redshift Uses in Science

The major implication for red-shifted light is what Edwin Hubble discovered by observing very distant galaxies. As previously mentioned, Hubble discovered that the further away a galaxy is, the more red-shifted its light. Therefore, the further away a galaxy is, the faster it is moving away from Earth. However, this means that galaxies at a certain distance would therefore be moving away from Earth faster than the speed of light. This meant that the galaxies themselves could not be moving, but the space between them must be getting larger. If almost all galaxies are red-shifted and expanding away from each other, then they must have been closer together in the distant past. This is one of the major pieces of evidence for the Big Bang Theory as the best possible explanation for the origin of the universe. The Big Bang Theory explains how and why almost all galaxies are moving away from each other besides the ones that are gravitationally bound in local groups. This discovery made by Edwin Hubble shifted the paradigm in cosmology, astronomy, and astrophysics.

Lesson Summary

Overall, redshift is a type of Doppler effect that occurs with visible light. The Doppler effect is the increase or decrease of wavelength when the source of propagating waves is moving away from or toward an observer. Wavelength is the distance between common points on two waves such as the distance between two wave crests or peaks of waves.

Redshift occurs when a light source moves away from an observer and therefore increases and elongates its wavelengths of light, resulting in the light having redder frequencies. Light can also be blue-shifted when the source of the light is moving toward an observer. Scientists use blueshift and redshift to study the universe by studying how redshift occurs with light from distant galaxies and stars which is therefore supporting evidence for the universe's expansion.