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. Drakkith

    Cosmological Redshift Question

    Let's say that an event releases a 1 second burst of EM radiation that propagates outward from the source in a spherical wavefront. If the EM wave travels long enough so that expansion doubles its wavelength, is that 1 second burst received as a 2 second burst? Has the total energy of the EM...
  2. 7

    Gravitational redshift inside a crystal - Pound-Rebeka experiment

    Homework Statement Some atoms emit X-rays at transition from excited to ground state. Energy of an excited state of the atom ##\scriptsize^{57}_{26}\textrm{Fe}## is ##\scriptsize14.4keV## larger than than the energy of it's ground state. The mass of the atom is...
  3. G

    The vicious circle of standard redshift explanations

    We should all know that photons only exist in quantum mechanics. In fact, the idea that energy comes bundled in discrete units is what actually caused QM in the first place. For some reason, however, I've spent my entire life under the assumption that cosmologists make true statements about...
  4. N

    How many parsecs is it when the redshift z=1

    How to convert red shift to parsecs? Is there a simple converting equation?
  5. Mordred

    Lightcone 1.0 basic redshift article development

    Developing a basic explanatory manual for the Light cone 1.0 calculator. This is as a supplement to give a basic understanding on what the terms used in the calculator mean. The user manual is separate as is the advanced manual which shows the math forms used in the calculator. The CMB...
  6. bitznbitez

    Differentiating Redshift Type by Spectral Shift Signatures

    Spectral redshift is currently understood to be of a variety of sources, velocity / doppler redshift, gravitational redshift, cosmological redshift, etc. Is there any ability to tell,purely from the spectral analysis of the light itself, without knowing what the source of the light is, what...
  7. K

    Distance modulus and redshift Ia supernovae Catalog

    Hello! I am looking for a catalog that contains redshift, modulus distance and position (if it is possible) of many type Ia supernovae as possible. I have found this: http://www.cbat.eps.harvard.edu/lists/Supernovae.html, but this link only has the position data. Does anyone know wher I...
  8. J

    Cosmological redshift interpretation

    Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a photon's wavelength somehow expands while it travels to us from a distant galaxy. Metric expansion...
  9. tom.stoer

    Is There a Universal Redshift Formula for Arbitrary Spacetime Metrics?

    Quick question: is there a general formula for the redshift zC[g] for a photon traveled in a spacetime with arbitrary metric g along a light-like geodesic C? I do neither want to use a special symmetry for g (spherical, homogeneous and isotropy) nor do I want to use a specific expression for...
  10. P

    Redshift at 11.9 and the Hubble Constant

    Recently, astronomers from Caltech and Edinburgh University discovered galaxies with a redshift of 11.9. With the Hubble constant at 67.8 km/s/Mpc ( according to the most recent survey with the Planck Satellite ) this means that the galaxies are at a distance of 14.24 billion light years ! With...
  11. P

    Cosmological redshift, how fast can the universe expand?

    Ok, I'm writing up something and I do something I always hate doing... I start arguing myself into a corner. The argument of the night is the expansion of the universe and how it applies to redshifts. Which has brought a series of questions which I can't seem to grasp right now. I) Is...
  12. Mordred

    Redshift FAQ article development

    I have been working on an article covering Redshift. The article is in this thread https://www.physicsforums.com/showthread.php?p=4307715#post4307715 I would like opinions, recommendations and any other contributions that forum members feel would be good to add.
  13. D

    Galaxies: Redshift, velocity and distance

    Ok, so I need to calculate the age of the universe using hubbles law. Problem is that I cannot find enough data: Information I'm looking for: 40 galaxies including all of this three: Redshift(z) velocity(v) and distance(d) Can anyone could direct me to any professional/reliable...
  14. marcus

    Normal&good question about redshift

    I got a PM with the sort of reasonable question about REDSHIFT that naturally occurs to people, and variants of which come up regularly. So rather than answer by PM, I take the liberty of replying in open forum. Other people may want to contribute, or may be interested in the different...
  15. C

    Exploring Redshift Anomalies: Shape Distortion and Non-Visible Light Experiments

    Anyone interested in going over some crude experiments and having a look to see if anything stands out as incorrect or impracticle? I'm not interested in any discussion as to the validity of doing the experiments, kindly keep that nonsense to yourself, I'm only interested in any input to...
  16. samsara15

    Redshift & Far Distant Stars: Effects on Temperature

    Wouldn't the Redshift result in decreased heat from far distant stars, resulting in an equilibirum temperature, much lower than the average star?
  17. Egregious

    Help: Explaining Expanding/Accelerating Universe Without Redshift Fallacy

    Preface: I am not a physicist, but I do tend to accept the theories of an expanding and accelerating universe. I am not inquiring to dispute these ideas, but it recently occurred to me that there is an obvious fallacy in the arguments for these ideas as presented in the popular media. I wish to...
  18. P

    Distribution of stars by Metallicity and Redshift

    Is there a large database that contains, for many stars, their estimated metallicities as well as their redshifts (or, equivalently, distances from us) and masses? What I'm trying to do is estimate the chemical abundances at different ages of the universe. If I have the metallicity of a star...
  19. L

    Redshift / blueshift of light in a gravity well

    I appreciate that as light approaches (say) a star, the light is blueshifted by gravity, and that as it leaves the area of the star, the light is redshifted as it climbs out of the gravity well. However, given that spacecraft execute gravity assist manoeuvres to increase / reduce speed, does...
  20. B

    Simple Redshift Question/Orbital Velocity

    Homework Statement When studying the optical spectrum of a very distant quasar (quasi stellar object), they found that a certain spectral line appears at a wavelength of 559 nm instead of the regular 446 nm. In terms of the speed of the light, what is the radial speed of the quasar with...
  21. Jonathan Scott

    Redshift of spinning star surface as seen on axis

    I think I heard long ago that the redshift of all parts of the surface of a spinning star as seen from a distant point on the axis is expected to be the same, at least in theory, because of the following argument. Please can anyone confirm or refute this? A body which is capable of being...
  22. X

    Confusion about redshift in a universe with accelerating expansion

    Hello, I've been trying to internally conceptualize as much of physics as I could, and doing so I realized that something I thought I understood at first does not actually make any sense in my head anymore. And since it doesn't, I'm afraid that I might have misunderstood a lot of other concepts...
  23. Chronos

    Another nail in the coffin of alternative redshift theories

    Cosmological redshift is often targeted by crackpot physics zealots. Here is a sober discussion worthy of review: Direct Determination of Expansion History Using Redshift Distortions, http://arxiv.org/abs/1210.6596
  24. C

    General Relativity gravitational redshift

    Homework Statement The gravitational redshift tends to decrease the frequency of light as it travels upwards a distance h,\frac{\Delta{f}}{f_{0}}=\frac{-gh}{c^2} integrate both sides of this equation (from the surface of the gravitation body out to infinity) to derive the expression for the...
  25. K

    Exploring Redshift of Distant Galaxies - Experimental Data

    Why it's not explained just by Doppler redshift caused by faster movement of those galaxies billions of years ago when that light was emitted? Would the speeds of the galaxies necessary for Doppler redshift to explain all of the observed galaxies redshift be unreasonable or is there something...
  26. S

    Gravitational Lensing and Redshift

    hi there, I was just at a seminar in which the speaker was talking about using gravitational lensing to investigate galaxy clusters my question is pretty simple: is there any effect on the redshift of the image of an object which is "lensed" by the gravitational lensing phenomenon? I would...
  27. S

    Mechanism of the cosmological redshift

    Hello, I have read that `cosmological redshift` means a photon is being `stretched`by space dilation and thus gets to have `longer wavelength` I am unsure if this is true and if yes, why is it true Many thanks
  28. C

    Question about gravitational redshift

    Here's my question first: If radio waves are transmitted from earth, are they actually redshifted, or is it that they appear redshifted when measured by an identical clock in space? Here's where I'm coming from. I'm trying to reconcile things I know. I think I know that (1) clocks further...
  29. L

    Could Redshift not result from the Doppler effect?

    Could "Redshift" not result from the Doppler effect? Redshift is the astronomical phenomenon where galaxies and other far-away objects appear shifted to the "red" side of the visible light spectrum. The farther an object is from us, the more pronounced this shift is. This phenomenon is most...
  30. D

    Non-expanding solution to redshift observations

    I do not have the solution referenced in the title, but I seriously ask if one could possibly exist. Given the observed redshift data, can there be a solution to any mathematical model which DOES NOT result in an expanding universe? Credit to user Chronos sig line: If a theory appears to...
  31. S

    Comprehensive Supernova Catalog: Redshift, DM, DM Err & Position Data

    I need a list of type Ia supernova with redshifts, Distance Modulus, Distance modulus error, and position in galactic coordinates, but I havn't been able to find one that has all of that information (I have found several with some of the information, but not all of it). Does anyone know where I...
  32. MikeGomez

    Gravitational redshift question

    A photon emitted from a star with a large mass (strong gravitational field) will be redshifted more than a comparable photon emitted from a star with a weaker gravitational field (comparable meaning the two photons would otherwise be expected to have exactly the same values in both cases, i.e...
  33. C

    Calculating distance from redshift

    Hello everyone I've starting to learn about Hubble's law and I have a very simple question. How are the velocities to distant objects calculated from a redshift? I understand the basic principle, that faster objects have longer wavelengths, but I'm not sure about the formula which links the...
  34. andrewkirk

    Derivation of the formula for cosmological redshift

    I was hoping somebody could point me towards a derivation of the following formula for cosmological redshift: z = R(t0)/R(te)-1. Wikipedia just presents the formula as a fait accompli and the only explanation is a vague reference to "stretched photons", which is not helpful. I was...
  35. J

    Redshift Time and the Start of the Solar System

    Einstein tells us time and space are linked, could redshift be caused by a change in time rather than space? Including uncertainties how close is the start of this period of universal expansion to the start of the solar system? Could the 2 coincide?
  36. V

    No redshift in a freely falling frame

    Why is there no redshift in a freely falling frame? The photon in a freely falling frame also rises in the gravitational field, so isn't it supposed to be redshifted?
  37. J

    What is the equation for high (cosmological) redshift?

    The equation for cosmological redshift where z << 1 is is commonly given as z = λobs / λemit -1 What is the equation for high-z, accounting for how light surpasses the spatial expansion it leaves behind, but abstracting from gravitational influences? I'm particularly interested in how CMB can...
  38. S

    Can alternate radiation model explain redshift?

    What radiation model is used for understanding the cosmological redgarbage? It seems that neither a planar wave nor a point source would represent what we think we are really observing when looking at distant galaxies or exploding stars. Could the observed redshift be a property of the radiation...
  39. E

    What Does a Redshift Calculation of 2.5 Mean?

    I have my answer of the doppler shifting as 2.5 meaning redshift, but what does this mean, 2.5 what? if its 2.5 'units' then how can I work out the speed of the object going from my Stationary position? it's all nice knowing for example my object could be going at 0.1m under the speed of...
  40. MattRob

    Redshift and Expansion of the Universe

    So, IIRC, the big deal about dark energy is that the universe appears to be expanding. How do we know the universe is expanding? As far as I know (and that's not much), we know because of redshifting of light on the spectrum as it moves away, as per the Doppler effect, which has been observed...
  41. D

    Supernovae Type Ia: Redshift, Luminosity & Speed

    Homework Statement Supernovae type Ia has a red shift z=1.1. Observed luminosity of the star has changed in the interval of 60 days. What time interval would an observer measure in the near vicinity of the star? What is the speed that the star is moving away from us? What is the distance of the...
  42. J

    Re: CMB Redshift Question (Visible Wavelengths)

    The earlier thread was closed for some reason. It prompted this question: Given the formula for CMB temperature Tobs = Tem/(1+z) (analogous to the formula for z) it seems the age of the universe at the emission of the CMB would have been about 12.5 Myr (if 13.75 Gyr presently), not 380,000...
  43. A

    Cosmological redshift alternative explanation?

    Could the cosmological redshift be explained also as a change of the fundamental physical constants with distance? Say Planck's constant changing over billions of light years, instead of interpreting it as a Doppler effect due to a recession speed? Yes, I know, Occam's razor would favor the...
  44. B

    Gravitionel redshift – How is it working?

    Experiential data shows that light on its way out of a gravitionel field is losing energy and become redshifted. Q1. But what about light that moves into a gravitionel field, - Is that gaining energy (getting more blue shifted) ? Has we ever measured what happens with light that for example...
  45. Drakkith

    Redshift Effect on Absorption/Emission Lines

    Is the width of spectral emission/absorption lines stretched in either cosmological or doppler redshift?
  46. ShayanJ

    Velocity determination through redshift

    As you know,astronomers calculate the velocity of distant stars by measuring their light's redshift.This suggests that they have a way to know each star's light's wavelength in a different way from observing its light.What is that way? thanks
  47. M

    Redshift and blueshift in expanding universe

    At first, people thought the universe was stationary. This seemed logical, why would any object tend to move in one direction or the other if there was the same amount of mass in all directions? Then they noticed the redshift of distant galaxies and concluded that the universe must be expanding...
  48. C

    Redshift (Ryden 5.2), confusing step.

    Homework Statement A light source in a flat, single-component universe has a redshift z when observed at a time t_{0} . Show that the observed redshift changes at a rate \frac{dz}{dt_{0}} = H_{0}(1+z) - H_{0}(1+z)^{3(1+w)/2} Homework Equations H_{0} = (\frac{\dot{a}}{a})|_{t = t_{0}} =...
  49. J

    Cosmological Redshift: Equation & Universe History

    What is the equation for the cosmological redshift as a function of the history of the universe? Basically as a function throughout all of time.
  50. G

    Calculating distance using redshift

    Hi. I have pretty rudimentary math skills, and am hoping someone can explain this formula for me, found online a number of places including: http://www.astrophysicsspectator.com/topics/overview/DistanceExtragalactic.html for calculating distance (d) in megaparsecs using redshift (z), the...
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