What is Redshift: Definition and 384 Discussions

In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.
In astronomy and cosmology, the three main causes of electromagnetic redshift are

The radiation travels between objects which are moving apart ("relativistic" redshift, an example of the relativistic Doppler effect)
The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift)
The radiation travels through expanding space (cosmological redshift). The observation that all sufficiently distant light sources show redshift corresponding to their distance from Earth is known as Hubble's law.Relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.
Examples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns.
Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer).
The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is >1 for redshifts, <1 for blueshifts).

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  1. N

    The Mechanism of the Cosmological Redshift

    It is well established that the cosmological redshift causes both the wavelength and the separation of photons to increase in proportion with the increase in the cosmic scale factor a. The traditional explanation for the mechanism of the redshift is that ‘expanding space’ progressively forces...
  2. P

    Redshift: Is the Universe Slowing or Accelerating?

    Just a physics buff so bear with me, this has been bothering but I am sure there is a simple answer. I watch a lot of physics and science programs on TV. When discussing red shift the explanation is that the light from objects that a further away is more red-shifted than light from closer...
  3. A

    Redshift of Sound in a Gravitational Field

    Hi, Do sound waves experience a redshift similar to that of light in a gravitational field? Does anyone know the equation governing the change in frequency of the sound? Is this the same as the Doppler shift for an accelerating source of sound? Thanks.
  4. N

    Superluminal velocities & redshift

    We all know that nothing can travel faster than the speed of light in a local frame. We also know that distant galaxies in our observable universe have recession 'velocities' relative to us which are far in excess of the speed of light, c, despite the fact that the velocity in their local...
  5. E

    What causes the gravitational redshift phenomenon?

    The item of gravitational redshift in the WIKIpedia says that this redshift is referred to the light of others forms of electromagnetic waves. This puzzles me. Other waves except electromagnetic waves have not this phenomenon, like sonic wave ? Is the gravitational redshift a result of...
  6. D

    Does time dilation cause redshift?

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  7. B

    Accounting for Cosmological Expansion in Distance Measurements

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  8. N

    Gravitational redshift and black holes

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  9. codelieb

    There is no Transverse Gravitatonal Redshift

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  10. C

    CMB Redshift Question (Visible Wavelengths)

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  11. J

    Redshift at the epoch of matter-radiation equality

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  12. U

    Redshift at recombination

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  13. V

    Sunyaev Zel'dovich effect - redshift independence

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  14. Y

    Trapped light: redshift or not?

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  15. O

    How much does dust affect light from high redshift quasars?

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  16. B

    Where Does the Lost Energy Go in Cosmological Redshift?

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  17. wolram

    How Do Large Galaxy Fluctuations Challenge Standard Cosmological Models?

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  18. P

    Redshift for Proton-Antiproton Pair Creation in Early Universe

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  19. S

    Are Quasars and Galaxies Redshifts Truly Quantized?

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  20. N

    Conceptualize radial redshift

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  21. N

    SR Doppler Redshift & SR Time Dilation

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  22. P

    Calculating Wavelengths of Redshift in Cosmology

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  23. C

    Hubble Redshift: New Paper on Its Implications

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  24. wolram

    Is GRB 080913 the Highest Redshift GRB to Date?

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  25. I

    Alternative hypoteses for cosmological redshift

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  26. wolram

    Kinematic Origin of Cosmological Redshift: Bunn & Hogg

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  27. J

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  28. I

    How Relativity say about redshift in terms of curvature?

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  29. J

    Redshift Ques: Applying Doppler Formula Can Lead to Error

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  30. M

    Is Redshift an Accurate Indicator of the Universe's Expansion Rate?

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  31. M

    Gravitational Redshift: Exploring Effects of Time & Space

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  32. C

    Deriving gravitational redshift using the energy change approach

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  33. S

    The redshift is how far/long ago?

    How far into space do we need to look before we can see the first signs of redshift?
  34. O

    Hubble's Redshift: Conclusions and Vacuum Solutions

    The far away galaxies are moving faster which Hubble concluded from their redshift. Could he also have come to the conclusion that the observed redshift is of gravitational (or more generally: geometrical, as resulting from a "weird" metric) origin ? Does anybody know what a vacuum solution...
  35. marcus

    Redshift 9, oldest galaxies seen yet

    http://space.newscientist.com/article/dn12233-baby-galaxies-sighted-at-dawn-of-universe.html great. (of course must be cautious with first report and wait to see if confirmed...but looking good) galaxies from when the universe was only half a billion years old SciAm has this...
  36. T

    Gravitational Redshift Contribution to Quasar Redshift: Explaining Hubbles Law

    Does anyone know how much percentage of the total redshift of a quasar is contributed by Gravitational redshift considering light from the Quasar is emitted by heavy active galactic nuclei ? If there is a significant contribution from gravitational redshift then how will the Hubbles law hold...
  37. Y

    Calculating the redshift at which radiation energy density equaled mass density

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  38. B

    How to calculate the age of the universe with redshift

    Hi i am confused as to how to calculate the age of the universe with redshift say for example The age of the universe now is 13.4 billion years old (and a critical universe). How do i find the age of the universe if it was a redshift at say 0.6?? Do i have to find the scale factor first...
  39. B

    Calculating the Transition from Radiation to Matter Dominated Universe

    Hi, I really need help with this question I've tried everything! let's assume a critical/flat universe. Ho=70 km/s/Mpc. I computed Boltzman law for energy (volume) density by integrating Planck's law p rad= alpha*T^4. alpha= 7.56*10^-15 ergs/cm^3/K^4 (Which I computed to be 7.56*10^-16...
  40. B

    Calculating Age of the Universe with Redshift - Help Needed!

    Hi i am confused as to how to calculate the age of the universe with redshift say for example The age of the universe now is 13.4 billion years old (and a critical universe). How do i find the age of the universe if it was a redshift at say 0.6?? Do i have to find the scale factor first...
  41. K

    Age of Universe, Redshift & Scale Factor

    Homework Statement Assume that the current age of the universe is 13.4 billion years old, and that we live in a matter-dominated, omega_m = 1, critical universe, what is the age of the universe at redshift 0.6? HINT: use the current age of the universe to pin down the proportionality...
  42. P

    Redshift Anomaly: Progress & Confusion

    Has there been any progress in clarifying this anomaly? I am a bit confused as to how people can ignore this problem?
  43. B

    What is the redshift and velocity of the galaxy relative to us

    hey everyone... so I am tryna solve this problem : a galaxy shows an emission line at a wavelenght of 6610.2 A, which we identify as the H-alpha line in the Balmer series of Hydrogen. what is the redshift and velocity of the galaxy relative to us. period= 4.3 days magnitude= 26.25...
  44. A

    Does Redshift Indicate Mutual Movement Between Distant Galaxies?

    I don't understand whether this is true or false. It goes like this: if a distant galaxy has a substantial redshift ( as viewed from our galaxy) then anyone living in that galaxy would see a substantial redshift in a spectrum of the Milky Way galaxy. I know redshift is when the object is...
  45. N

    Correct formula for gravitational redshift

    Based on general relativity (see book Gravity; an introduction to Einstein's General relativity by James B. Hartle), the frequency of light (emitted at R) is modified by the factor SQRT (1 - 2 G M / c**2 R) (see page 191). On the other hand, based on the equivalence principle, one arrives at...
  46. C

    Redshift - does energy fade or get absorbed?

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  47. marcus

    Got a link for the angular-size redshift relation?

    http://en.wikipedia.org/wiki/Angular_size_redshift_relation the Wiki entry for this declares itself a stub What they have is just a few sentences serving as a place-holder until somebody contributes a real article. the thing about angular-size redshift relation is that out beyond a certain...
  48. B

    No Redshift from Distant Galaxies: Implications for Universe

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  49. F

    Change in Redshift vs. Obs. Time

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  50. B

    Understanding redshift surveys

    ...With Galaxy Explorer 2.0 Hi! I'm doing some personal reaserch on Sloan Digital Sky Survey, and I was wondering if anyone here uses this software? If so, could you tell me what the center is? For details of the SDSS project check the websites: GalaxyExplorer: a 3D...
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