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. V

    Derive Redshift Parameter Expression using Cosmological Principle

    Homework Statement The cosmological principle can also be used to derive an expression for the redshift of light (from far away galaxies) in terms of the scale factors at emission and observation. Use the fact that the wavelength in terms of the co-moving coordinates Δx is independent of...
  2. D

    Redshift as evidence of expansion

    why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
  3. T

    Redshift and time dilatation of Hawking radiation

    Micro black holes should in principle be observable by emitting Hawking radiation. However, as this takes place extremely close to the event horizon, shouldn't one expect then extreme redshifting (z = 1/(1-Rs/R)^-1/2 -1, Rs = Schwarzschildradius, R = Radius of Emission) and time dilatation? In...
  4. S

    Redshift to Megaparsec Formula

    z x c = H x d is the formula I am using to try to convert redshift to megaparsecs, where H I'm taking to be 71km/s/Mpc and c as 299,792.46 km/s. When I plug in .1 for z I get 1.377 Billion Light years (422.2429 Mpc), which sounds right. But when I plug in 10 for z I get the huge distance of...
  5. T

    Gravitational Redshift really verification of GR?

    If we assume: E = mc^{2} and for photons: E = hv Then we can derive an effective mass: m = \frac{hv}{c^{2}} And using simple classical gravity obtain: hv - \frac{GMm}{r} = hv - \frac{GMhv}{c^{2}r} = Constant You can derive the constant by evaluating the equation above at...
  6. Z

    Redshift and Weak equivalence principle

    Firstly: Does anyone know at our current level of technology what "small enough not to detect tidal forces" equates to? Near the surface of the earth(as it probably depends on the amount of curvature of space). Secondly: Does anyone know at out current level of technology what the minimum...
  7. T

    Astrophysics/Cosmology question - redshift

    Homework Statement Our universe is observed to be flat, with density parameters Ωm,0 = 0.3 in non-relativistic matter and Ω\Lambda,0 = 0.7 in dark energy at the present time. Neglect the contribution from relativistic matter. At what redshift did the expansion of the universe start to...
  8. I

    Deriving Hubble redshift in closed Universe from Maxwell equations

    Homework Statement I should derive the Hubble law redshift from Maxwell equations in closed Universe. Homework Equations The metric of closed Universe is ds^2 = dt^2 - a^2(t)\left(d\chi^2 + \sin^2 \chi d\theta^2 + \sin^2 \chi \sin^2 \theta d\phi^2\right). The Hubble law redshift: \frac...
  9. L

    What are the limits of redshift?

    Does anyone know what the limit is for light from distant objects to be so redshifted that they become undetectable? Could there be a way in detecting objects beyond this barrier? Would any wavelength at all be detectable?
  10. TrickyDicky

    Gravitational redshift, energy and the EP

    It is common knowledge that of the classical tests of GR, gravitational redshift is normally considered to be testing the Equivalence principle and conservation of energy rather than the full GR theory. So it is considered a more fundamental test of our basic principles of GR and physics in...
  11. M

    The redshift from recession or expansion

    Almost all textbooks tell us that the cosmic redshift is arised from the cosmic expansion rather than the recession of the celestial objects. I think the conclusion is basically based on the assumption of Copernican Principle, i.e. there is no favored position in the whole universe. But what if...
  12. T

    Proof of the expansion of the universe-cosmological redshift?

    Hi everybody! I'm going to make this quick, so i won't waste your time! SO: What are the proofs of the expansion of the universe?? And what were the firsts? Is the cosmological redshift a proof of that? A little more complicated one: From where, in the einstein equations, should i see...
  13. bcrowell

    Detecting a Dipole in the Handedness of Spiral Galaxies at Redshift z ~ 0.04

    Longo, "Detection of a Dipole in the Handedness of Spiral Galaxies with Redshifts z ~ 0.04," 2011, http://arxiv.org/abs/1104.2815 If this is right, then I guess it would be the first possible hint of a nonzero rate of rotation for the universe. He doesn't attempt to state his results in terms...
  14. J

    [cosmology] Find the comobile distance of a galaxy given redshift and H0.

    Homework Statement Calculate the comobile distance of a galaxy with z=7.3, H_{0}=72 km/s/Mpc, universe with \Omega_{0}=\Omega_{0,m}=1 Calculate the scale factor when the galaxy emitted the light we receive today. Homework Equations Friedmann equation (\frac{\dot{a}}{a})^{2}=(H_{0})^{2}[...
  15. Vorde

    3D Model of the 2MASS Redshift Survey

    Does anyone know of a 3D model of the data from the 2MASS Survey? I have to imagine it would be incredibly easy to code, but I haven't found one. Thank you.
  16. E

    Why Does Redshift Occur?; Other Factors That Cause Elongation Of Wavelength?

    Me and my Dad were having a conversation about the elongation of EMR wavelengths over an extended period of time. He began to explain to me what Red Shift was but I couldn't understand what he was trying to get at (he's a poor explainer, or I'm bad listener). But his explanation did spark an...
  17. P

    Rearranging redshift formula for v

    Hey there I would just like some help in rearranging the formula: [tex] z=\frac{1+\frac{v\cos \theta}{c}}{\sqrt{1-\frac{v^2}{c^2}}}-1 [\tex] for v. Thanks, PP
  18. S

    Quasars with high redshift in nearby galaxies

    http://www.sciencedaily.com/releases/2005/01/050111115201.htm Please see this article. I've been trying to find the thread with 41 questions posted aces days ago. One of the questions was relating to high redshift objects like quasars appearing in near by galaxies. Someonevwanted a reference...
  19. T

    Uniform redshift in inflationary model?

    This question might be a result of perceptual ignorance but still: In explaining an inflationary cosmological model that postulates a prime singularity (i.e. starting with a Big Bang) some tend to associate it with "dots on an inflating balloon". Doesn't this imply that the "dots" that are...
  20. J

    Questions on Redshift, Hubble’s Law, spacetime expansion and the standard model

    The current state of NASA’s version of cosmology is amazing to watch – According to modern cosmology, which NASA endorses, the Milky Way is moving through space at 600km/s and the Earth is moving through space at 30km/s and the sun is moving through space at 250km/s. So if we total up these...
  21. E

    Redshift: Speed of Light & Beyond?

    Just a little confused about amounts of redshift, I read somewhere about redshift and how some galaxies are moving at redshift 3 (I think), which was said to be at or near the speed of light, then later in the article is said a galaxy was found to be moving at redshift 6. I understand how the...
  22. H

    The mesure of distances using the redshift

    Hi all, in order to determine the recessional velocity of same galaxy we use the redshift law: [(observed wavelength) -(source wavelength) ]/source wavelength=v/c The problem I have found here:confused: is how we can know the source wavelength. thinks.
  23. S

    H-alpha line in galaxies/ hubble redshift

    As an assignment I am trying to calculate the Hubble redshift by looking at spectra of galaxies... Obviously the very first thing I want to do is find a suitable emission line...i am trying to find the halpha line in all the spectra I am analysing. First off, there is a very large spike in the...
  24. soothsayer

    At what redshift did the Universe beging accelerating?

    Homework Statement Starting with the equation for a'', derive an expression for the redshift at which the deceleration of the Universe turned into acceleration. Your expression should contain only fundamental cosmological parameters at the present day such as \OmegaM0 and \Omega\Lambda0...
  25. C

    Effect of ratio of energy density of matter and radiation on redshift

    Homework Statement If the ratio of the energy densities of matter and radiation was much higher than it is today (by several orders of magnitude), in a few sentences describe what effect this would have on the redshift (or scale factor) when the universe switched from being...
  26. V

    How much of galaxy redshift is due to gravity?

    Hi, I'm trying to get my head around theories of the universe, and am wondering if Hubbles Law has been mis-understood. I know that light from far away galaxies has been redshifted i.e. has lost energy. But I'm not convinced that this means those galaxies are moving away from us...
  27. C

    Redshift from space expansion, & conservation of energy?

    Greetings, When a photon travels through expanding space, it's frequency decreases and it has less energy. The energy of photons of the cosmic background radiation have decreased dramatically. Where does the energy go? Is it lost? Thanks
  28. M

    How is the observed redshift caused by a recessional velocity?

    I know that the issue of cosmological redshift has been discussed in this forum before, e.g. https://www.physicsforums.com/showthread.php?t=368958", but I would appreciate any knowledgeable insights regarding the model outlined below. This model is only considering light in terms of a stream of...
  29. C

    WMAP Data Redshift and Background Radiation

    Let's assume that the universe has been expanding & accelerating since the big bang, we all know this redshifts any radiation directed toward our planet. We also know it's safe to say that the universe is ~13.7 billion years old given the fact that that's as far back as we can see in the...
  30. M

    Understanding the Causes of Photon Redshift

    Hi, Basically, I am trying to understand whether there is an accepted and consistent description of the cause of http://en.wikipedia.org/wiki/Redshift" when discussed in terms of photons propagating through the vacuum of expanding space. An initial search on the subject of redshift seems to...
  31. L

    Why does redshift indicate that space-time is expanding

    Since nobody answered my last question I''l try it this way: Why does redshift indicate that space-time is expanding and not that galaxy's are just flying away from each other?
  32. C

    Question about the GR gravitational redshift experiment

    GR predict that a photon climbing in the Earth's gravitational field will lose energy and will consequently be redshifted. A test was done by Pound and Snider in 1965 using the Mossbauer effect They measured the redshift experienced by a 14.4 Kev rays from the decay of Fe in climbing up...
  33. L

    Redshift: expanding space-time or galaxy moving away form each other?

    I know that Hubble observed redshift which led to the conclusion that space was expanding. But why did redshift lead to such conclusion? Why didn't they just think that galaxy were "flying" away from each other instead of thinking that it was the space that was actually expanding? What made...
  34. J

    Value for redshift change in acceleration

    If the universe was decelarating and then started to accelerate can we calculate when this happened, and if so how can we do this?
  35. Z

    Expansion, redshift, energy loss

    Maybe this question has already been asked, anyway, here it goes. The universe is expanding and according to relativity, the radiation is redshifted, meaning there is a loss of energy. If energy is conserved, where does that loss of energy go?
  36. B

    Redshift and Universal Expansion

    Correct me if I'm wrong, but the only real evidence showing that the universe is expanding (and accelerating it's expansion) is by observing the so called "standard candle" supernovae which always have the same luminosity and observing that the farther they are, the more red shifted their light...
  37. 1

    Realspace and redshift space correlation function questions

    why is the redshift space correlation function smaller than the realspace correlation function at small scales and the opposite on large scales?
  38. B

    What is the formula for gravitational redshift and its time derivative?

    Gravitational redshift formula Hi, I am looking for the formula that describes the gravitational redshift as frequency2 = function(frequency1, radius1, radius2, gravitating_mass) What I find on the net is http://www.wolframalpha.com/input/?i=gravitational+redshift+from+r1+to+r2" the...
  39. A

    Is redshift unreliable as a measuring tool?

    If there are 2 objects emitting light to each other and the light fills the spacetime between them and then that spacetime expands the waveform becomes stretched. However, if the middle of the spacetime between the 2 objects has yet to have light enter it from either object and that spacetime...
  40. M

    Maximum redshift - can it move light off the scale?

    Light is redshifted and the greater the z, the further the spectral shift is to the left. Is there a point at which low-frequency EM (radio/micro waves) shifts below the spectra and disappear? The real question - Is there a z, and thus an implied distance, at which the highest energy...
  41. redtree

    Published data sets for distance modulus versus redshift

    Does anyone have a recommendation for a paper/source for data relating distance modulus and redshift data over a large range of redshift, such as 0 < z <= 5? I have found that combining data from different publications can be difficult given the variety of ways that distance modulus can be...
  42. B

    Optimizing Gravitational Redshift Measurements: Techniques and Targets

    what is the best method to measure gravitational redshift using a small telescope ? and in which stars the effect is the strongest ?
  43. marcus

    Galaxy at redshift z=8.6 (the actual paper)

    http://arxiv.org/abs/1010.4312 This is the actual paper that was published yesterday in Nature magazine. Good show ESO! Galaxy is now 3.1 billion years old. The baby picture that European Southern took of it is when the universe had been expanding for only 600 million years.
  44. T

    How is measuring Redshift possible?

    I am very interested in physics but have no background education on it so forgive me if this question is amateur. I am trying to grasp this redshift thing. It's the measurement of a shift in the wavelength of light. The only variable I can think of is the wavelength of the light received on...
  45. TrickyDicky

    Gravitational redshift from doppler shift

    Hi, I'm interested in the derivation of the gravitational redshift formula from the Doppler shift in an accelerated frame formula (or viceversa), that are linked by the Equivalence principle, it should be pretty straightforward but I get stuck. Anyone can show me this or point me to a link with...
  46. E

    Hawking radiation redshift (?)

    I think it was proven that firing a photon (laser) straight up from the surface of Earth did in fact redshift the photon and decrease its frequency. And now, when we consider a black hole and Hawking radiation... I mean - the gravity inside the event horizon is so strong that EM cannot escape...
  47. TrickyDicky

    Converting Redshift to Velocity: The Accurate Formula Explained

    What is the formula used to convert the measured redshift into a velocity?, not the approximated formula for low speeds v=cz , but the more general and accurate one. Thanks.
  48. L

    BMR decay question and redshift

    One of the proofs that there was a Big Bang is the measurement of microwave backround radiation. It has been measured to be around 2.7 degrees kelvin and the explanation given why it's that the BB created a spectrum of frequencies and they have been stretched out by space expansion over time to...
  49. R

    Estimating Redshift of Photon Gas in Universe Transition

    Homework Statement The problem is translated from a different language, so I hope I am not missing anything. I need to estimate the redshift of the photon gas in the universe, at the time of transition from radiation to matter domination. Cosmological parameters: k=0 (meaning a flat universe)...
  50. W

    What causes variations in redshift measurements?

    Hello! I have a small question, in my textbook and everywhere I look online it states that to find the redshift of a receding galaxy from the spectrograph you divide the change in wavelength by the original point you are measuring from. The problem I have is, if there is a 20nm shift for...
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