AST 111 Astronomy of the Universe Lab

​ Classical vs. Cosmological Redshift Background

In order to understand the difference between classical and cosmological redshift, there is a good bit of background material that must be covered for completeness. The first step will be to recognize the two different types of waves. The nerdy definition of a wave is an oscillation that carries with it a transfer of energy. There are three general classifications for wave type:

1) Transverse waves: Waves where the oscillation (sometimes referred to as the distortion) is perpendicular to the direction of travel.

2) Longitudinal waves: Waves where the oscillation is parallel to the direction of travel.

3) Surface waves: Surface waves travel along an interface between two media, and contain a circular motion as they propagate outward. A consequence of this type of motion is that the size of the wave diminishes with distance traveled.

As the attraction between nearby molecules is relatively weak in fluids, below the surface, fluids typically transmit longitudinal waves. At the surface of a lake, ocean, or large body of water, water molecules follow a path that is partly longitudinal and partly transverse. The molecules themselves do not move along with the wave. They complete a circle each time the wave passes.

Ocean waves are surface waves, sound is a longitudinal wave, and light is a transverse wave. For the remainder of this lab we will only concern ourselves with light waves, which are also referred to as electromagnetic waves.

The next background topic is the Doppler shift. This is a distortion that arises when the person or object emitting a wave is in motion with respect to the person or object receiving the wave. Note that the emitter, the receiver, or both can be moving as long as they are not at rest with respect to one another. Here is a diagram showing how the Doppler shift works for sound waves:

Fig. 2: Sounds emitted by a source moving to the right spread out from the points at which they were emitted. The wavelength is reduced and, consequently, the frequency is increased in the direction of motion, so that the observer on the right hears a higher-pitch sound. The opposite is true for the observer on the left, where the wavelength is increased and the frequency is reduced.

Following is a similar diagram for light waves. Notice the similarities between the two diagrams. It doesn’t matter if the wave is transverse or longitudinal. It doesn’t matter if the wave needs a medium to travel through (sound) or not (light). The Doppler effect arises simply because the emitter and receiver are in motion with respect to one another.

 Fig. 3: When a celestial body, such as a star, moves away from us, it moves away from the waves it creates, and the waves appear to expand from the perspective of the observer.  When it moves toward us, it moves toward from the waves it creates, and the waves appear to contract from the perspective of the observer.  (Image credit: modification of work by NASA/SDO)

Now it is time to talk about redshift. Note that the term redshift is a generic term that is used in place of Doppler shift. I believe this has come about because our observations suggest that the Universe is expanding, and all of the galaxies that we see are redshifted (getting further away from us) except for our nearest neighbor M31 (also called the Andromeda galaxy).

There are two types of redshift: classical and cosmological. Put simply, classical redshift is motion through the environment while cosmological redshift is motion with the environment. A good example of this is a moving sidewalk.

If you walk along a regular sidewalk, you are moving through the environment. This means that your motion is classical. If you are standing on a moving sidewalk (such as you would find in an airport), the sidewalk carries you from one place to another. This is motion with the environment and as such is cosmological. Note that if you are on a moving sidewalk and walking along the sidewalk then your motion is a combination of classical and cosmological. This is what we observe in the Universe, and it is important to understand the relative contribution of both.