Redshift Definition and 385 Threads

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

    Misconceptions re: Cosmological Redshift

    Apparently my recent thread on the cosmological redshift assumed more general insight into this subject than is the case. So the purpose of this thread is to help dispel some popular misconceptions about the cosmological redshift and hopefully spur a vigorous discussion. This thread is NOT...
  2. 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...
  3. 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...
  4. 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.
  5. 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...
  6. 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...
  7. D

    Does time dilation cause redshift?

    This may be an easy answer for AtsroPhys vetrans. But I'm assuming since the doppler effect of an accelerating galaxy causes reshift, then the time dilation from a gravitational field would do the same. This is referring to very powerful fields, and also to redshift in all waves (not just...
  8. B

    Accounting for Cosmological Expansion in Distance Measurements

    Some objects, Quasars for example have extremely high redshifts, and come from a time long past, and place far away. When determining distance on such large scales, how do you also factor in the cosmological expansion of the universe, meaning 10 billion years ago the universe was much smaller...
  9. N

    Gravitational redshift and black holes

    Questions: Gravitational redshift and black holes I have some questions: 1. What does gravitational redshift do to light trying to escape a black hole? Is the light destroyed? 2. And what is the physical cause of this redshift? (I’m not interested in equations and math, only the physical...
  10. codelieb

    There is no Transverse Gravitatonal Redshift

    There is no "Transverse Gravitatonal Redshift" There has been a lot of noise on this forum lately about A. Mayer's so-called "Transverse Gravitational Redshift." It is not difficult to prove that no such effect exists, yet I have not seen anyone post a calculation to prove it. I have therefore...
  11. C

    CMB Redshift Question (Visible Wavelengths)

    I've looked quite a bit on this forum and also elsewhere online and I cannot really find any information on my inquiry. If this is a double post please delete and send me in the right direction. Now I may be totally off on my thought process, but as I understand it the CMB first started off...
  12. J

    Redshift at the epoch of matter-radiation equality

    I have a homework question I am not sure how to approach: We are told that 1) CMB provides about 10^9 photons/baryons. 2) For matter dominated era, assume flat university with H_o = 73km/s/Mpc 3) Stefan-Boltzman law for energy density: ρ_rad = αT^4 where α = 7.56x10^-15 ergs / cm^3 K^4...
  13. U

    What Redshift Occurs When Photon and Baryon Densities Equalize at Recombination?

    Homework Statement We are to assume the recombination happens at redshift z_{rec} when the number density of photons n_{\gamma}(z_{rec}) capable of ionizing hydrogen is exactly equal to the number density of baryons n_{b}(z_{rec}) . Use the measured number density of baryons, the...
  14. V

    Sunyaev Zel'dovich effect - redshift independence

    So, I have read that the SZ effect is virtually independent of redshift. I follow the argument that the factor of (1+z)^-4 in the surface brightness cancels the (1+z)^4 factor in the photon energy density at the cluster (three factors from space being smaller in each dimension, one from the...
  15. Y

    Is Light in a Perfectly Reflecting Box Redshifted Over Billions of Years?

    If light from billions of years ago were trapped within a perfectly reflecting mirrored box, will it be redshifted in a way comparable to light from galaxies billions of light-years away? I had made up this http://blog.chungyc.org/2009/03/redshift-puzzle/" as a supposed illustration of how the...
  16. O

    How much does dust affect light from high redshift quasars?

    As I understand it, the conclusion that our spatially flat universe is expanding at an accelerating rate is supported by observations of 'standard candles' (type S1a supernovae) which appear less bright than expected when their z exceeds about 0.3 --- . Robert Kirschner has discussed these...
  17. B

    Where Does the Lost Energy Go in Cosmological Redshift?

    This is my first time on physics forums, and I am glad to have the chance to participate. My questions have been asked many times by others, but after lots of research, including among these threads on physics forums, especially “hurk4” on Feb 14-07 and Will Kastens, I can find no definitive...
  18. wolram

    How Do Large Galaxy Fluctuations Challenge Standard Cosmological Models?

    arXiv:0902.0229 [ps, pdf, other] Title: Large-scale fluctuations in the distribution of galaxies from the Two Degree Field Galaxy Redshift Survey Authors: Francesco Sylos Labini, Nikolay L. Vasilyev, Yurij V. Baryshev Comments: 20 pages, 31 figures, to be published on Astronomy and...
  19. P

    Redshift for Proton-Antiproton Pair Creation in Early Universe

    Homework Statement The question: Explain why pair creation is expected to occur in the early universe, and estimate the redshift at which proton-antiproton pairs are created.Homework Equations The Attempt at a Solution My logic for the reason is that the process requires high energy gamma...
  20. S

    Are Quasars and Galaxies Redshifts Truly Quantized?

    http://arxiv.org/PS_cache/arxiv/pdf/0712/0712.3833v2.pdf Fourier spectral analysis has been carried out on the quasar number count as a function of redshift calculated from the quasar data of the Sloan Digital Sky Survey DR6 data release. The results indicate that quasars have preferred...
  21. N

    How Does Radial Redshift Affect the Energy of Light?

    I find it helpful to conceptualize radial redshift as nothing more than a longitudinal stretching out or elongation of an incoming light beam segment, as measured in the observer's rest frame. For example, a light beam segment which has been redshifted by a factor of 2 has been stretched to...
  22. N

    SR Doppler Redshift & SR Time Dilation

    Light sources A & B begin at the same location. Then, source A begins moving directly (and soon inertially) away from Observer at 0.8c, while source B begins moving directly (and soon inertially) toward Observer at 0.8c. SR Doppler redshift causes the light received by Observer from A to be...
  23. P

    Calculating Wavelengths of Redshift in Cosmology

    [i asked this question in my blog..but i know it would be better answered here] what are the calculations of the wave lengths of a cosmological redshift? (meaning them all) i just wanted to know what precises wavelength determines which red shift.. thanks guys
  24. C

    Hubble Redshift: New Paper on Its Implications

    A new paper on the Hubble redshift: [PLAIN]www.ptep-online.com/index_files/2009/PP-16-L1.PDF[/URL]
  25. wolram

    Is GRB 080913 the Highest Redshift GRB to Date?

    arXiv:0810.2314 Title: GRB 080913 at redshift 6.7 Comments: 14 pages, 7 figures, ApJ (subm) Subjects: Astrophysics (astro-ph) We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND which led to the discovery of its...
  26. I

    Alternative hypoteses for cosmological redshift

    I heard there are many alternative hypoteses for cosmological redshift, e g Fred Hoyle' s variation of mass and tired light. There are other ones? Thanks.
  27. wolram

    Kinematic Origin of Cosmological Redshift: Bunn & Hogg

    arXiv:0808.1081 (cross-list from physics.pop-ph) [ps, pdf, other] Title: The kinematic origin of the cosmological redshift Authors: Emory F. Bunn, David W. Hogg Comments: 14 pages. Submitted to Am. J. Phys Subjects: Popular Physics (physics.pop-ph); Astrophysics (astro-ph); General...
  28. J

    Superluminal Recession & Cosmological Redshift

    I'm taking the liberty of revising and restating this topic which started in a separate thread. Comments are welcome. A lively debate is underway today by mainstream cosmologists as to whether the expansion of the universe implies that empty space between galaxies is also expanding. When...
  29. I

    How Relativity say about redshift in terms of curvature?

    How Relativity say about redshift in terms of curvature?
  30. J

    Redshift Ques: Applying Doppler Formula Can Lead to Error

    The formula for redshift according to the special relativity is f = \sqrt{\frac{c-v}{c+v}} f_0. Here f\to 0 when v\to c. So for arbitrarily strong redshift effect we can always solve the corresponding velocity v<c of the emitter moving away. Why then does Carrol explain this: "As an example...
  31. M

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

    Hey! :smile: I have a few questions regarding redshift. Firstly are there example of a shift in frequency of light other than the one seen in redshift or blueshift. I mean are there experiments that have been conducted that prove that this noted shift to the red end of the spectrum mean that...
  32. M

    Gravitational Redshift: Exploring Effects of Time & Space

    Based on the quote below and the link to Wikipedia, I believe the implication is that gravitational redshift will occur when a photon climbs out of the gravity well, while blueshift will occur when falling into the gravity well. However, I am trying to clarify whether both time dilation and...
  33. C

    Deriving gravitational redshift using the energy change approach

    An extremely elementary question but... Lets say we had a spaceship sitting on Earth under the influence of gravity. Light travels from the ship's floor to the ceiling. An observer 'A' stands near the ceiling. Hence the light loses gravitational potential energy, and therefore decreases...
  34. 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?
  35. 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...
  36. 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...
  37. 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...
  38. Y

    Calculating the redshift at which radiation energy density equaled mass density

    how do you calculate the redshift at which the universe shifted from radiation dominated to mass dominated i was told to assume a flat universe with H0 = 70 also the temperature, to which i was given the formula density rad = aT^4 for the first part, i tried using density is propotional...
  39. 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...
  40. 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...
  41. 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...
  42. 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...
  43. 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?
  44. 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...
  45. 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...
  46. 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...
  47. C

    Redshift - does energy fade or get absorbed?

    Is it possible that redshift, though correct in it's theory, is not what we are viewing in terms of expansion of the universe? Is it possible that what we think is redshift is actually the strength of the light/energy fading, being absorbed or blocked over hundreds of millions of light years...
  48. 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...
  49. B

    No Redshift from Distant Galaxies: Implications for Universe

    So what if no distant galaxy showed any redshift of blueshift? what would that mean? What would this imply about the universe? What-if any –effects would this have on the Earth? I figure this would support the steady state theory and that it was static and infinite. anything else?? thanks
  50. F

    How Does Redshift Change Over Observational Time?

    Greetings--this is from Ryden's Introduction to Cosmology, question # 5.2. Let me restate the question: Here w is the equation of state parameter for whatever the universe is made of (e.g. w = 0 for matter, 1/3 for radiation). Here is my best attempt so far: Simplify notation by...
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