Redshift
In astronomy, redshift is the change in position of identified spectral lines in the emission and absorption spectra of astronomical objects in the direction of longer wavelengths. Redshift is defined as the ratio of the wavelength change to the original wavelength:
The name refers to the red light at the long-wave end of the visible spectrum. With infrared emission, the spectral lines shift accordingly in the direction of the even longer-wavelength terahertz radiation. A shift to shorter wavelengths is called a blue shift.
The redshift is determined by comparing known atomic and molecular spectra with the values measured by spectroscopy, i.e. after analysing the spectral lines of emissions or absorptions in starlight, for example of hydrogen.
The effect is also important in molecular spectroscopy, where photons of lower energy appear after elastic scattering with energy transfer.
Illustration of the redshift of the spectral lines for a distant supergalaxy cluster on the right compared to the Sun on the left.
Causes
Causes of redshift can be:
- A relative motion of source and observer (Doppler effect)
- Different gravitational potentials of source and observer (relativity)
- The expanding universe between source and observer (cosmology)
- Stokes shift in the transfer of discrete energy amounts between photons and molecules in Raman scattering
The first three of these causes are discussed in more detail below.
Red and blue shift due to relative motion
Redshift and blueshift are terms from spectroscopy, in which spectral lines of atomic nuclei, atoms and molecules are examined. These can occur in absorption or emission, depending on whether energy is absorbed or emitted. The energy is exchanged by electromagnetic radiation in the form of photons, so it is quantized. Where the spectral lines are located in the spectrum depends not only on the details of the quantum transition, but also on the state of motion of the radiation source relative to the observer (Doppler effect) and on the curvature of space-time.
If one is in the rest system of the emitter (relative velocity zero between emitter and observer), one measures the spectral line at its rest wavelength. Now, however, there can also be a relative motion between the radiation source and the detector. Only the velocity component pointing in the direction of the detector is significant. This component is called radial velocity. Its magnitude is the relative velocity between emitter and observer. Electromagnetic radiation travels at the speed of light during both emission and absorption, regardless of how fast the source and target are moving relative to each other.
If the radiation source moves away from the observer, the spectral line is shifted towards longer, red wavelengths. The wave is pulled apart, so to speak. This is called redshift. If the radiation source moves towards the observer, the spectral line is shifted towards smaller wavelengths. This is just the blue shift because the line is shifted to the blue part of the spectrum. Vividly, you can imagine how the electromagnetic wave is compressed.
The whole atomic and molecular world is in motion due to thermodynamics. At finite temperature, these radiators move slightly around a rest position. Spectral lines therefore have a natural width due to atomic motion and molecular motion because they are always moving back and forth a little relative to the detector. Physicists call this phenomenon thermal Doppler broadening. So the rest wavelength is not arbitrarily sharp. Nor can it be, because of the Heisenberg uncertainty of quantum theory.
Special relativity gives the following relation for the relation between radial velocity v and Doppler shift z (with the speed of light 
):
and vice versa
At low velocities ( 
), this relation can be approximated by 
Movement of a light source relative to the observer
Questions and Answers
Q: What is red shift?
A: Red shift is a way astronomers use to tell the speed of any object that is very far away in the Universe. It is an example of the Doppler effect, where light from an object moving towards us will look more blue (blue shift) and light from an object moving away from us will look more red (red shift).
Q: How can we experience the Doppler effect?
A: The easiest way to experience the Doppler effect is to listen to a moving train. As it moves towards a person, the sound it makes as it comes towards them sounds like it has a higher tone, since the frequency of the sound is squeezed together a little bit. As the train speeds away, the sound gets stretched out, and sounds lower in tone.
Q: How do astronomers measure red shift?
A: Astronomers use spectroscopy to analyse the light from an object (galaxy or star). Once they know that, they check to see how much difference there is between where its spectral lines are compared to where they normally are. From this information, they can tell whether it is moving toward us or away from us, and also how fast it is going. The faster it goes, the farther its spectral lines are shifted from their normal position in the spectrum.
Q: What causes blue shift?
A: Blue shift occurs when an object that emits light moves very fast towards us. This causes its light to appear more blue than usual due to compression of its frequency waves as it approaches our frame of reference.
Q: What elements do astronomers use for spectroscopy?
A: Astronomers use chemical elements such as hydrogen and oxygen for spectroscopy because these elements have unique fingerprints of light that no other element has.
Q: How does redshift get its name? A: Redshift gets its name because when an object moves away from us in our frame of reference, its light appears more red than usual due to stretching out of its frequency waves - thus shifting colors towards the red end of spectrum.
Q: What happens if an object moves faster? A: If an object moves faster then astronomers can tell by looking at how much further apart its spectral lines are compared with their normal positions in spectrum - indicating greater distance traveled by those waves due to increased speed
