Red shift in astronomy
Red shift is the lengthening of light's wavelength from distant sources. It reveals motion, cosmic expansion, and gravity effects and is measured by comparing shifted spectral lines via spectroscopy.
Overview
Red shift describes the change in the wavelength of light from a source so that it appears longer—toward the red end of the visible spectrum. Astronomers astronomers use red shift to determine whether an object in the Universe is moving away from us and, under appropriate conditions, to estimate its speed or distance. Conceptually it is a manifestation of the Doppler effect, the same phenomenon that makes a passing vehicle's siren change in pitch.
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How it works
In everyday terms the Doppler effect can be heard when a train passes: the sound is higher in pitch as it approaches and lower as it recedes because the frequency of the waves is compressed or stretched. Light behaves similarly: when a luminous object such as a star or galaxy moves toward an observer its emitted light shifts to shorter wavelengths (blue shift, blue shift); when it moves away the light shifts to longer wavelengths (red shift).
Measurement and interpretation
Practical measurement relies on spectroscopy spectroscopy. Chemical substances like chemical elements produce characteristic spectral features—lines at well-defined wavelengths (for example, those of hydrogen or oxygen). By comparing observed spectral lines with laboratory wavelengths one computes the red shift z = (λ_obs − λ_emit)/λ_emit. A positive z usually indicates recession; the larger the z, the greater the fractional wavelength change and, often, the greater the recessional speed.
Types and causes
- Velocity (Doppler) red shift: produced by relative motion between source and observer, analogous to the moving-train example.
- Cosmological red shift: produced by the expansion of space itself, stretching light as it travels across the expanding Universe and commonly used in extragalactic astronomy.
- Gravitational red shift: predicted by general relativity, where light climbing out of a strong gravitational field loses energy and shifts to longer wavelengths.
Historical and practical importance
Measurements of red shift in the early 20th century revealed that most galaxies have red-shifted spectra, implying they are receding from the Milky Way. This observation, when combined with independent distance estimates, led to the concept that the cosmos is expanding. Today red shift is a primary tool for mapping large-scale structure, estimating distances (via Hubble's law in the appropriate regime), identifying distant objects such as quasars, and studying the early Universe through strongly red-shifted radiation like the cosmic microwave background.
Useful examples and special notes
Not every object shows red shift: some nearby objects, for example the Andromeda galaxy, have a measurable blue shift because they are moving toward us in local motion within the galaxy group. Interpreting red shift also requires care: at high velocities relativistic formulas replace the simple Doppler approximation; cosmological red shift is not strictly a velocity in the Newtonian sense but a result of metric expansion; and local motions (peculiar velocities) can add or subtract from a cosmological signal. For more background or technical details see material aimed at both general and specialist readers (sound analogy, frequency, and descriptive introductions for spectroscopy).
Red shift remains a central observable in modern astronomy: it links laboratory physics (elemental spectral fingerprints) to large-scale cosmic dynamics and to tests of fundamental physics.
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 
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
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AlegsaOnline.com Red shift in astronomy Leandro Alegsa
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