What is Redshift: Definition and 382 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. A

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  2. T

    I Gravitational red blue shift

    Why is gravitational red blue shift ruled out as a cause of what we see in stars and galaxy's and such?
  3. S

    B Redshift of a light pulse between 2 accelerating rockets

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  4. F

    A Convention of units for densities in cosmology

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  5. Cerenkov

    B What value of z (redshift) equals apparent superluminal recession?

    Hello. I have visited this site recently... https://lco.global/spacebook/light/redshift/ ...and was wondering at what value of z in the table below would galaxies appear to recede at superluminal velocities? z Time the light has been traveling Distance to the object now 0.0000715 1...
  6. BadgerBadger92

    B Difference between special relativity and redshift?

    For quite a while I thought that time dilation was the effect of bodies moving close to the speed of light dilates the passage of time. I also have read about red shift, which seems to be the same thing, though I’m sure they are totally different. I learned in special relativity (the time...
  7. L

    B Which statement is more accurate? (Hubble's redshift vs. Hubble's law)

    A. The two great cosmological discoveries of this century, Hubble's redshift and the cosmic microwave background, have made the Big Bang theory the most credible theory of the origin of the universe so far. B. The two great cosmological discoveries of this century, Hubble's law and the cosmic...
  8. sol47739

    I Is the light from Andromeda truly blue shifted?

    I asked a similar question on Quora not that long...
  9. doudou

    I Gravity does negative or positive work with cosmos redshift?

    Based on the fact of observed cosmological redshift, scientists have proposed different ideas to explain. One interesting question is whether gravity does negative or positive work now: According to universe expanding in Big Bang theory (Lemaitre, 1927), obviously gravity does overall negative...
  10. ergospherical

    I Observed Redshift from Moving Source: Deriving the Result

    If a galaxy is receding from us, then the 1+redshift observed on Earth is the product ##(1+z_{pec})(1+z_{cosm})## of the doppler redshift due to the peculiar motion of the galaxy and the cosmological redshift due to the FRW metric. It makes sense if we think about some intermediate observers...
  11. Tertius

    I Can a scalar field model account for the cosmic redshift?

    A minimally coupled scalar field can model a cosmological fluid model where And where the equation of state can be the standard ## \omega = \frac {p} {\rho}## I can see how this does a fine job modeling matter, because as the scale factor increases, the density will go as ##\frac {1} {a^3}##...
  12. E

    I Calculation of gravitational redshift in an accelerating elevator

    When we derive the formula of the redshift by the equivalence principle we imagine a light ray which goes from the bottom to the top of the elevator and which would take a duration t = h/c to make the journey, with h = height of the elevator. I don't understand why t = h/c, because while the ray...
  13. cosmologyscience

    Exploring the Universe: Galaxy Formation, Redshift, and Cosmology

    Hello, I'm an independent researcher in cosmology and working on topics like cosmological redshift, galaxy formation and galaxy cluster dynamics. Came across the forum searching for details about the solar system orientation in the galaxy rotation disk. Very inspired and enthusiastic about the...
  14. ergospherical

    I Calculating Particle Redshift at Time t_0 & t_1

    A question about the FLRW solution has confused a few of us. At time ##t_0## a particle has radial speed ##v_0^r## relative to the fundamental observers, and at a later time ##t_1## it has radial speed ##v_1^r##. The task is to show that\begin{align*} \frac{a_0}{a_1} =\frac{v_1^r \gamma_1}{v_0^r...
  15. Oldman too

    I JWST breaks another Redshift record

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  16. SH2372 General Relativity - Lecture 5

    SH2372 General Relativity - Lecture 5

    0:00 What is spacetime? 20:20 Kinematics, basic definitions 35:18 Motion of test particles 41:18 Frequency shift of light 1:10:19 Simultaneity
  17. M

    Measurement of speed using redshift or blueshift

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  18. Vick

    I Formula for redshift of photon decoupling

    I have come across an old formula from my notes and I have no reference for it but it is using truncated digits in its formula to calculate the redshift for decoupling. The formula is nearly as accurate as the observed data from Planck 2018. So I would like to figure out the derivation of those...
  19. zrek

    B Experimental evidence on cosmological redshift

    The space expands due the dark energy, so the light wavelength also stretches during its journey between galaxies, causing a measurable redshift. How can I argue that this is the right reasoning for the measured redshift? What if someone states that the photon maybe simple loses its energy over...
  20. Arman777

    A Relativistic Redshift and understanding it's approximation

    I was reading an article, and I saw this expression. $$ 1+z=\frac{(g_{\mu\nu}k^{\mu}u^{\nu})_e}{(g_{\mu\nu}k^{\mu}u^{\nu})_o} $$ Where ##e## represents the emitter frame, ##o## the observer frame, ##g_{\mu\nu}## is the metric, ##k^{\mu}## is the photon four-momentum and ##u^{\nu}## is the...
  21. Kwantumnaut

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  22. M

    I Collision between the Milky Way and Andromeda and Redshift

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  23. Bolhuso

    I Galaxy redshift and thermodynamics

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  24. F

    A Angular power spectrum dependence in redshift z and k

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  25. S

    Astrophysics Redshift Question

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  26. E

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  27. AHSAN MUJTABA

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  28. C

    B What is the relative speed of two groups of quasars with different redshifts?

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  29. C

    I Material source of the quasar redshift

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  30. E

    B The accelerated expansion of the universe; redshift and time/distance

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  31. Z

    I Gravitational Redshift: Units, Velocity & Ratio Explained

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  32. G

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  33. Arman777

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  34. Arman777

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  35. JTorn

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  36. J

    B Red-Shifted Light: Speed Less Than C?

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  37. George Keeling

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  38. X

    I Challenges to the Redshift Method and Hubble's Law

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  39. D

    I Some redshift from prism effect of a gravitational lens?

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  40. Y

    I What will eventually happen to all EM waves / photons?

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  41. J

    Cosmological Redshift

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  42. Arman777

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  43. H

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  44. Ennio

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  45. Ken Gallock

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  46. M

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  47. itoero

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  48. F

    I BAO : Relation between redshift, Hubble constant and radial

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  49. Deepblu

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