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. P

    Redshift is due to the expansion of the universe

    Okay, from what I understand, the cosmological redshift is due to the expansion of the universe. I'm confused as to how this works. I don't understand why, if the universe is expanding, that the waves don't just have farther to travel. I also don't understand what is waving or why other...
  2. D

    Redshift, Distance & Expansion rate question

    I'm a bit confused about redshift and the apparent increase in acceleration. If I understood it correctly redshift is caused by the expansion of the universe (space itself). What I don't understand though, if galaxies further away have a higher increase in redshift vs distance than galaxies...
  3. A

    GR. Mach and cosmic redshift

    ....This is just a concept I have been playing with for awhile... As my knowledge of GR is limited and my knowledge of Mach's principle is limited to its application to inertia I am going to use my own generalizations from them both. If my understanding or interpretation of either is wrong...
  4. N

    Cosmological Redshift in FRW Coordinates – Energy Conservation & Time Dilation

    I want to discuss energy conservation and time dilation relative to the cosmological redshift. Because the elements of the redshift are coordinate specific, I’ll focus just on the FRW metric using proper distance coordinates (not comoving coordinates). As explained in Tamara Davis’s cover...
  5. C

    Does the Zeeman Effect Alter Clock Rates in Different Magnetic Fields?

    Lets say we emit a photon from the ground out towards the sky , and as the photon travels away from Earth it gets red-shifted . In stead of canceling the shift by using the Doppler effect like they did in the pound-rebka experiment , we use the zeeman effect to alter the discrete energy...
  6. K

    What is the relationship between temperature and redshift?

    I was just curious as to if there are any graphs or data available to see a more detailed outline of the temperature vs. redshift or temperature vs. time available in any published papers, books, or even online.
  7. stevenb

    Exploring Historical Context of GR Predicted Gravitational Redshift

    I was just reading the old document by H.A. Lorentz " The Einstein Theory of Relativity" which is freely available in ebook and text format. I find it interesting to read some of the comments from those times, and Lorentz provides a nice summary of the thoughts of the days soon after a second...
  8. L

    Understanding the accelerating expansion of the universe based on redshift

    Understanding the accelerating/decelerating expansion of universe based on redshift.. I am trying to understand how the expansion of the universe is accelerating, if indeed it is. It's confusing, as some people say that the expansion is accelerating, whereas I have a video lecture where the...
  9. W

    Magnitude vs Redshift Supernova 1a

    The following two graphics are supposed to be the same, but they are not (there is something I am not getting right). After the calculation of m ( z ) (magnitude as a function of redshift) I get the plot of the first link: -Plot at the bottom (NOT the last one) of...
  10. G

    Black holes and redshift (2 questions)

    I'm new to the Physics forum and thought I'd start with a couple of stupid questions. 1) How do we know that black holes are not stellar nuclei that have just collapsed to a volume less than their mass dependent Schwarzschild radius (the radius at which the speed of light is the escape...
  11. Z

    Gravitational time dilation, redshift and doppler

    If as a result of the equivalence principle we can derive the gravitational red shift entirely from the perspective of an accelerating body with no reference to gravity and no reference to gravitational time dilation then one is essentially just working out the Doppler shift of a signal...
  12. TrickyDicky

    Cosmological redshift and doppler redshift

    I am a bit confused here. Is cosmological redshift the same as doppler redshift? This is from wikipedia: "The redshift z often is described as a redshift velocity, which is the recessional velocity that would produce the same redshift if it were caused by a linear Doppler effect (which...
  13. MikeGomez

    Does the redshift effect contribute to gravitational attraction?

    We can imagine a sphere floating freely in space. Perhaps it is something like a ping pong ball. We attach a piece of string to a random position on the surface of the sphere. If we vibrate the string, the sphere will be pulled in the direction of the string. If we repeatedly move the...
  14. A

    Cosmological redshift as a Doppler effect or metric evolution?

    In several threads where I've seen the redshift issue discussed there's been some confusion about this point, Must we treat cosmological redshift as a purely kinematic (relativistic)doppler effect or as the time dymamics of the metric space? Or both views can be made to converge?
  15. Z

    Gravitational redshift equation

    Gravitational redshift is given by the following approximate equation; \frac{\lambda}{\lambda_o} = 1 - \frac{GM}{r c^2} From http://scienceworld.wolfram.com/physics/GravitationalRedshift.html Where \lambda is the shifted wavelength and \lambda_o is the rest wavelength. r is...
  16. K

    Cosmological Redshift: Why Does k Remain the Same?

    condidering a photon in expanding cosmos, it 's said that the wavenumber k remains unchanged, the wavelength \lambda increases, proportional to the scale factor a(t) of the universe, and the frequency w decreases in the opposite way, that is the cosmic redshift. so, why does k...
  17. H

    CMB Redshift: 13.7B Yrs Ago, 10.4x Speed of Light Today

    The CMB is believed to have started [or ended] 13.7 billion years ago. Recent estimates of Hubble constant are around 70 km/s/Mpc, or 228 km/s/Mlyr. When we look at CMB we are looking back 13700 million years into the past. Multiplying 228 by 13700 gives us 1.04 times the speed of light as...
  18. S

    Redshift first year astronomy

    redshift ""first year astronomy"" how do i find distance using redshift? please keep in mind that this is first year astronomy course i am currently taking having some trouble doing it. don't know where to start
  19. M

    Is gravitational redshift taken into account in Hubble's Law?

    From what I understand: 1. The more massive the object, the stronger the gravitational field. This leads to the light being emitted from the surface to shift down in frequency. 2. The rate of expansion of the universe causes a redshift proportional its distance away. I'm new to this, but...
  20. I

    What is the Speed of Galaxies by Redshift?

    Homework Statement I am trying to calculate, from their redshifts, the speed at which each of the following galaxies is moving away from us. Give your answers in both km/s and as a fraction of the speed of light. The galaxies are: Galaxy 1: Observed wavelength of hydrogen line is 659.2...
  21. R

    Calculate the redshift of a galaxy that is 650 Mpc away

    How do I calculate the redshift of a galaxy that is 650 Mpc away if I am only told its spectral lines should show a redshift z=.15?
  22. S

    Quasars: Are We Measuring Them Wrong?

    Why is it that Quasars don't red-shift? Any ideas or speculations? http://www.physorg.com/news190027752.html Surely all the measurements across all these quasars can't all be wrong. Is it possible that the Big Bang theory is wrong? Is it possible that the universe is not expanding? If...
  23. R

    Calculating Redshift of Universe Domination

    Homework Statement Hey :-) I want to calculate the redshift at which the Universe became vacuum dominated, with given values for the energy densities of Omega_{m} and Omega_{Lambda}. Homework Equations I know that the scale factors rho_{m} is proportional to a^{-3} rho_{r} is...
  24. V

    Cosmological redshift and photon energy loss in SR

    This might be a tired topic but please help me to understand. Assume a photon moving in vacuum, ignore potential interstellar medium absorption and re-emission since it is not relevant to discussion of space expansion in this context. Redshifted photons will undergo energy loss between *right...
  25. K

    Blue Shift & Red Shift: Explaining the Universe

    I have a question.. So far, from what I've browsed on the forums, I haven't found an answer to it, and I figured this would be the best place for it. I'm aware of the balloon analogy, but I have a question regarding blueshifts and redshifts. If you take the balloon analogy, everything in the...
  26. E

    Calculating Redshift: A Guide to Using Hubble Constant and Distance

    A redshift survey has identified a galaxy cluster for which it infers a di stance of 520 Mpc. At what redshift is this cluster observed to be? You may use H0 = 72 km s−1 Mpc−1 Now call me old fashioned but don't i need some kind of relativistic speed to calculate the redshift, i mean i have a...
  27. A

    Could anything else cause redshift?

    I know about cosmic expansion, but I was just curious, could the observations of redshift be caused because space is cooling, not expanding. From what I understand, as things cool they redshift. If the universe started cooling after the big bang, wouldn't everything we saw from the early...
  28. Z

    Calculating age of the universe using redshift?

    Hello. This is one of my coursework questions I was wondering if I could get some insight here.. here is the question: The size of the Universe if conveniently parameterized by a scale factor, a(t), which simply describes how big the Universe is at other times relative to its present size...
  29. Z

    Calculate Redshift Where Radiation and Matter Energy Densities Equal

    Hello. This is question for my course work, I was wondering if I could get some insight, here is the question: Assume that the vast majority of the photons in the present Universe are cosmic microwave radiation photons that are a relic of the big bang. For simplicity, also assume that all the...
  30. L

    Cosmic background radiation - redshift, blueshift, anisotropies

    I've been trying to find out more about cosmic background radiation, but I am unable to find any information on the variations in the measured cosmic background radiation. I'm talking about these WMAP images. Most of the images you'll find on Internet are the versions after being corrected...
  31. marcus

    Current eff. universe limited to redshift 1.73

    the figure 1.73 is approximate...given uncertainties, somewhere around 1.7. We see galaxies out beyond z = 7, of course, and the ancient background light has redshift 1090, so we see ancient matter emitting light, before it even formed stars and galaxies. But as of now, the standard model...
  32. M

    Big Bang Big Schmang, astronomical redshift is an artifact of distance

    http://img76.imageshack.us/img76/2711/virgocluster.jpg Look at this image of the Virgo cluster of galaxies, for images of galaxies extending as far as the eye can see, some appear to exist in a swarm around the sub cluster at the core, others are in streamers and wisps of galaxies extending...
  33. J

    Redshift Factor in a Symptotic Stationary Flat Space

    I am reading Wald's book. There is a problem confused me: page 158, problem 4,(b). An symptotic statoinary flat space, two stationary observer connected with a rope. One observer A is at finite r and the other B is at infinity. Observer B is really stationary by other forces and holding the...
  34. N

    What is the role of gravity in the cosmological redshift?

    I’ve made several efforts to provide a simple description of how the cosmological redshift works. Each has been closer to the mark than its predecessor, and now I think I’ve got it nailed. The premise underlying the redshift is simply that a photon must travel at a local velocity of c through...
  35. D

    Where does the missing energy go in redshift?

    If an atom in a distant galaxy emits a uv photon, the photon would have had enough energy to cause photoelectric emission from a negatively charged zinc plate. By the time the photon reaches me on Earth it has been sufficiently 'redshifted' so that it could not cause photoelectric emission if...
  36. A

    Expansion redshift VS gravitational redshift?

    While objects closer to us tend to shift both in direction red or blue, depending on their movement in relation to us, distant objects such as galaxies tend to only shift to the red. As I understand this is the base of the idea that the universe is expanding. But how are we sure that is the...
  37. cepheid

    Variation of Density Parameters with Redshift

    I was looking at section 7.6 of Longair's Galaxy Formation, and in it, he is talking about the flatness problem, or fine-tuning problem. In it, he shows that if you define a general (i.e. varies with cosmic time and hence with redshift) density parameter \Omega_m for matter by analogy with...
  38. K

    Seyfert Galaxies & Quasars: Redshift Differences Explained

    I have read on a NASA website amongst other places that "Many astronomers believe that Seyfert galaxies and high-energy quasars are basically the same type of objects, but we are simply viewing them differently". But on the same website it also says that "by measuring their redshifts, we find...
  39. Jorrie

    Redshift and Particle Momentum Decay

    Here is an interesting observation, which I would like to know the validity of. The momentum of a relativistic 'cosmological' particle in a homogeneous universe can be written as ^{[1]} L = \gamma m a v_{pec} = K where \gamma=(1-v_{pec}^2/c^2)^{-0.5}, m the rest mass, v_{pec} the peculiar...
  40. C

    Curving light undergoing redshift

    If light is moving on a curve, for instance, if light were bent around a massive body thus curving due to gravity, does it undergo any redshift of blueshift? I ask this because the universe may not be expanding (what we see as redshift), but may just be affected by gravity, and curving, thus...
  41. T

    Redshift as a distance indicator

    Surely it is possible to take a spectrum, deduce the redshift, then use this as a definitive distance indicator for all distances. Why is it we use the distance ladder?
  42. P

    Redshift, Quantum Theory, and the Standard Model

    I am stuck on a few cosmological points I could use some help on. The first being "redshift" as an indication of inflation. To me red shift just seems like it's a result of the degradation of light quanta over long distances. Since light slows down in environments close to absolute zero...
  43. N

    Gravitational redshift and time dilation

    It seems quite common to more or less equate gravitational redshift to gravitational time dilation. For example, the two might be explained as follows. We have two observers. A is standing on the surface of the earth, B on the top of a tower. A sends light up to B with some frequency, B...
  44. Z

    Gravitational redshift and time dilation

    Gravitational redshift seems to be measured as if gravitational time dilation has no effect upon it. What effect if any does gravitational time dilation have on the measurement of gravitational redshift?
  45. M

    Are There Alternative Theories to Explain Redshift and Universal Expansion?

    Are there any alternative theories/explanations of why light from distant galaxies is redshifted by an amount proportional to the distance away from us, other than the popular universal expansion explanation? Maybe some theory about light losing energy when traveling over such large distances...
  46. E

    How does the expansion of the universe affect redshift?

    does the expansion itself redshifts photons (if so, does the expansion affect the photon in his frame of referance?) or is it because the distant object that emits them is seemingly moving away from us (in cases where the expansion is much more dominant of course)? The latter doesn't makes much...
  47. N

    What is the relation between redshift and absorption lines in galaxies?

    I understand the concept, but not the math. For example, let's say a galaxy is moving away from us, and we observe one absorption line at 400 nm, and another absorption line at 500 nm. Let's also say that we determine that the radiation was emitted originally at 200 nm and 300 nm...
  48. T

    Which statement is true about Redshift and the Hubble constant?

    Which statement is true? A The apparent speed of recession of a galaxy is given by the product of the Hubble constant and the distance to the galaxy. B The further away a galaxy is, the lower its apparent speed of motion away from us. C A galaxy with a redshift of 5.37 is situated at a...
  49. R

    Gravitational Redshift for Photon around Kerr Black Hole

    I have a hopefully straightforward question. It is well known that in the Schwarzschild metric the gravitational redshift is given by 1+z=(1-r_{s}/r)^{-1/2}. Clearly this is just the ratio of observed to emitted frequencies (or energies). I understand this so far. However, for the case of the...
  50. 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...
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