What is Red shift: Definition and 121 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. H

    Equation to give the lookback time as a function of redshift

    Hi, I'm currently reading the introduction to cosmology second edition by Barbara Ryden and at the page 105, the author says we get ##t_0 - t_e = H_0^{-1}[z - (1 + \frac{q_0}{2})z^2]## by inverting ##z = H_0(t_0 - t_e) + (1 + \frac{q_0}{2}H_0^2(t_0 - t_e)^2)##. However, I can't figure out how...
  2. J O Linton

    B Cosmological Red Shift in a Perfectly Reflecting Box

    When a photon travels from a distant galaxy to us it undergoes an increase in wavelength due to the expansion of the universe during the time of flight. On the other hand, physical objects such as atoms and galaxies do not undergo a similar expansion because they are bound together by...
  3. C

    B Conflict Between Time Dilation and Red/Blue Shift?

    Experiment 1: Astronaut travels away from Earth at near the speed of light, then travels toward the Earth at near the speed of light. Einstein tells us she barely aged, but red shift/blue shift don't seem to agree with that. While traveling away, both Earth and astronaut observe each other...
  4. D.S.Beyer

    I Red shift / Blue Shift - Gravity Well Roundtrip

    If I shine a white light out into space from Earth, and it bends around a black hole and comes straight back to me so I can see it in my telescope, is the light red or blue shifted, or neither? For sake of the thought experiment, let’s leave out cosmic expansion that would stretch it on it’s...
  5. J

    B Redshift & Energy Conservation: Questions Answered

    Hi, I have read over several threads already on this and have a few questions if someone could please answer that would be great: 1) The threads seem to suggest that energy is not conserved (or at least it isn't a requirement) on the scale of the universe. Why does it not have to be conserved...
  6. jeffinbath

    B Is there more to the Red Shift than we think?

    How can we be certain that it is not the case that a minor proportion of the red shift from distant galaxies is not caused by the doppler effect or by gravitational fields or by dispersion from cosmic dust but is caused by a light aging factor? There is a small general red shift from stars held...
  7. S

    B [Curiosity] Are there any central databases of Red Shift?

    I'm curious to know if there are any central databases of red shift - whether stars, galaxies or other objects.
  8. H

    Is the mass of the electron constant?

    If the mass of the electron has been changing during the evolution of the universe, then the orbits of the electrons would also change, which will shift the light spectrum of each atom. Could this explain red shift of far galaxies, and the shift is not because the universe is expanding? Henry
  9. W

    I How many galaxies do we have red shift data for?

    Im wondering does anyone know roughly how many galaxies we have red shift data for, just a ball park. Is it hundreds? thousands? millions?
  10. AGuglielmone

    B Are all wavelengths of light possible?

    I just had the thought that atoms emit light at quantized levels but that would seem to imply that only certain energy levels could possibly exist instead of a complete spectrum. But, if light is traveling down or away from a gravitational field the frequency gets shifted. Would this make it...
  11. D.S.Beyer

    B Density of the early Universe contributing to the red-shift?

    Does the relative density of the early universe contribute to the red-shift of distant galaxies? If so, by how much? How would this be calculated? Asked another way : Assuming both the early universe and the current universe are flat, could the relative difference of their space time metric...
  12. gmalcolm77

    I Red Shift Energy: Understanding the Light Phenomena

    I find it difficult to conceptualize red shift when thinking of light as a stream of photons. In thinking of it as a wave phenomena, I can see it as a matter of a given energy concentration/area. But why should a photon lose energy as a result of the velocity of the emitter?
  13. DarkMattrHole

    I Does red shift approach infinite near time zero?

    Hi. I'm trying to get an idea how to look at the beginning, before the cosmic background radiation (CBR) and what we can detect with our eyes, and what we can assume about earlier times than light was around. I was looking into the red shift effect effect, as not only a measure of relative...
  14. J

    I Have we measured the change in the red shift over time?

    Have experiments been done to measure the red shifts of distant objects over time? If inflation is correct, the the speed at which an object is receding relative to us is based on how far away that object is located. Therefore, the most distant objects would also have the highest acceleration...
  15. Teslascience

    I Do dark matter and dark energy have an effect on the red shift?

    Do dark matter and energy affect on red shift phenomena? If yes, what, and what is the consequence of it?
  16. M

    I Atomic Oscillations & Redshift in Sun and Earth

    My question is very simple (and I assume it has been discussed before but I cant't find the topic): An atom in the Sun emits a photon detected by an observer on Earth. Disregarding uncertainties and experimental problems relating to the movement of the atom (or assuming we could correct for...
  17. Joe Fatuch

    I How does the speed of light affect expansion acceleration?

    I may have a fundamental misunderstanding of the concept, but I was wondering, how does the accelerating expansion of the universe calculate for the time dilation in light travel? From my understanding, we know that the universe expansion is accelerating because the farthest galaxies that we...
  18. BL4CKB0X97

    B Is blue shift as common as red shift?

    A somewhat trivial question, but I hear of redshift a lot more than blue. And is possible to get a list of galaxies that display these attributes? I would like to use the data for a project. Thanks guys.
  19. T

    I Red shift and the maths for gravity

    I have looked but do not seem to be able to pin down how Hubble's Constant deals with Gravity. Is it considered as Newtonian or Einstein special theory?
  20. just wandering

    I Why don't any stars "look" red? red shift

    Universe is expanding/galaxies moving away from each other and far away galaxies are moving away from us faster. so the light is redshifted. .."at very large redshifts, much of the ultraviolet and visible light from distant sources is shifted into the infrared part of the spectrum. This means...
  21. S

    B Photon red shifting and Doppler effect

    When thinking of sound waves (or more commonly seen waves) the doppler effect is fairly obvious. A car is moving away from you, it's horn's membrane is fluctuating, increasing & reducing air pressure over time. So if it's moving away from you the sound's peak would occur further away...
  22. wolram

    B On the relativity of red shift

    arXiv:1605.08634 (cross-list from physics.pop-ph) [pdf] On The Relativity of Redshifts: Does Space Really "Expand"? Geraint F. Lewis Comments: 6 pages, 5 figures, appeared in Australian Physics Journal-ref: Australian Physics (2016), 53(3), 95-100 Subjects: Popular Physics (physics.pop-ph)...
  23. marcelhattingh

    B How do we know we are not inside a blackhole?

    If you fall into a black hole, that what is closer to the singularity, will fall faster that than what is further. However, space before and after you, is falling in with you. So from each General Relativistic perspective of "you", because there would be no inertial frame to orient yourself (you...
  24. F

    I How can I calculate red shift for intragalactic bodies

    Hello everyone. I am doing an studi on Be stars, all of which are inside the Milky Way, I want to show that the red shift won't be a big deal here, but I am confused about how to prove it; what I have done is combining the equations λr=λe(1+z) and cz=Hd to get λr=λe(1+Hd/c) where z=v/c. I get...
  25. Rob Haskell

    Questions about the balloon analogy

    I've been thinking about the balloon surface analogy and I want to ask some questions. Perhaps I am pushing it too far. But in that case it seems important to find its limits. My understanding is that the this analogy is used to illustrate the expansion of the universe and in particular how it...
  26. wolram

    Can a galaxy at z=8.68 disprove the LCDM theory?

    We see from wikipidea that EGSY8p7 z=8.68 13.2 is only a fraction away from the time of the BIG EVENT how close will a galaxy be to disprove the LCDM.
  27. S

    Light waves with 0 amplitude due to extreme red shift?

    Is it theoretically/mathematically possible for light waves to be red-shifted so much that it's amplitude is equal to zero?
  28. T

    Does Gravitational Lensing Cause Red Shift in Light?

    Does light get red shifted during lensing, when it passes by an object with great mass such as a galaxy or cluster? If so then what are the effects on measurements of the CMBR and most distant red shifted galaxies? If true then presumably some light would be red shifted more than others...
  29. 2

    Deriving Gravitational Red Shift: Questions & Answers

    I am trying to derive the gravitational red shift effect but I think I am going about it all wrong. Specifically, I want to derive the change in frequency/ wavelength when a photon moves away from the surface of a star mass M and radius R. So I tried to use relativistic mass of the photon and I...
  30. Gary Boothe

    Red Shift Q: Where Does Lost Energy Go?

    I understand that because galaxies are receding from us due to the expanding universe, that we see a red shift in the light from these galaxies. If there is a red shift, the light loses energy, but where does this lost energy go? Is it that in the photon's reference frame there is no change in...
  31. T

    Speed of Light & Red Shift: My Understanding

    My basic understanding is that no matter how you observe light, it always a constant, it always travels at the speed of light. So even traveling at near the speed of light you will always measure light traveling at the speed of light. If this is the case? Why do we observe red shift?
  32. jerromyjon

    Exploring the Causes of Red Shift in Cosmology

    Does the expansion of space cause cosmological red-shift or does EM red-shift cause space to expand?
  33. R

    What causes red shift in the universe?

    Hi all, I understand the concept of red shift, and how it helped to explain how the universe was expanding and that most of the observable universe is moving away from us. It happens due to the wavelength of light increasing due to the expansion of space. As I understand it, it's not know how...
  34. I

    How Do Scientists Distinguish Between Red Shift and a Red Dwarf's Natural Color?

    How do scientists know if something is traveling away from us at any given moment, yes, it might be red, but it could be just a red dwarf star? How do they know it is the Doppler Effect in action, not just its natural colour?
  35. afcsimoes

    Exploring the Redshift Phenomenon: Beyond the Expansion of the Universe

    When a photon loses a little bit of energy, its frequency lows a little. That is, it suffers a slight redshift. So, why to interpret the redshift as being only due to the expansion of the Universe?
  36. D

    Red Shift & Blue Shift: Explained

    I apologize if this is a dumb question, or if I am not remembering things correctly, but ... ... wasn't one of the earliert objects (a Cepheid?) Hubble detected as red-shifted located within the Andromeda Galaxy? If so, and since we're on a "collision course" with the Andromeda, why didn't...
  37. Y

    Do contracting objects show red shift?

    If we consider a point on the central part of a contracting object as observed from earth, the point is continuously moving away from us as the object contracts. Moreover, light emerges from a deeper gravitational well as the object contracts. So, shouldn't contracting objects show a red shift...
  38. B

    Red shift of light from galaxies

    Hi. In light of that ridiculous Ham / Nye debate I have someone trying to explain away the red shift of light from galaxies by suggesting it could be due to the photons interacting with electrons or other subatomic particles en-route, thereby loosing energy and increasing their wavelength...
  39. B

    Is there red shift evidence for light?

    I'm aware of the doppler effect for sound waves. But light waves are different, e.g., light travels through a vacuum but sound does not. Has there been any laboratory confirmation for red shift for LIGHT. I could find only ONE paper by Bélopolsky, A. in 1901 using rotating mirrors. But there...
  40. L

    How and how much does is the cosmological red shift affected?

    When we look out into deep space we see a red shift in the light from distant sources because they are more or less all receding from us. How is the red shift distorted by passing through our local galactic gravitational well, before hitting us down here on the ground?(more red/more blue/no...
  41. C

    Dark energy acceleration of space effect on red shift

    I have not seen this discussed so this a question in the form of a statement for clarity. If light left a star 10 billion years ago and space has expanded for 10 billion years before the light is observed the light frequency would be stretched out or red shifted therefore some of the red...
  42. H

    Redshift of Light: Exploring Possibilities of the Big Bang

    I have long been curious as to the Big Bang. Our cosmology is based on Hubble's observation of the red-shift and its interpretation, and our observations are made to fit the interpretation of an expanding universe. What if simply the frequency of electromagnetic radiation decays (lengthens) with...
  43. N

    Mass Increase and Distances: Red Shift or Blue Shift?

    In a hypothetical universe where mass of matter constantly increases, will we observe that distances also constantly increase? This is not a new theory, I'm just asking what classic GM says about that. If we are in a galaxy with constantly increasing mass, measuring distances to other...
  44. T

    Question on Red shift applied to objects within the same galaxy

    Hi everyone, I signed up just to ask you guys a quick question, as most of you will know infinitely more than I do on this subject. I'm not an astrophysicist, I have a strong interest in the universe and cosmology, but it's more of a hobby for me than a central focus, so I apologise in...
  45. G

    Red Shift: Questions About Light & Big Bang

    This might be obvious but i was listening to someone talk about how he believes the big bang is cyclical and the universe is constantly expanding and contracting. now i don't believe this but one of the points he raised was (and i know it doesn't support his argument) we don't know if galaxies...
  46. C

    Is the only evidence for an expanding universe the doppler Red Shift?

    Basically, all more distant galaxies have a Red Shift that increases with the distance to the galaxy, the Universe is expanding. 1) If there are other pieces of evidence that indicate the Universe is expanding, what are they? 2) Any internet links describing exactly how the other...
  47. P

    Red Shift and Receding Galaxies

    Hi, I understand the idea of red shift and how that explains that galaxies are receding, etc. I just don't understand how the measurement is made. I know that, in labs, we can shine white light at hydrogen and analyze the absorption spectrum. Some black lines are going to show up, due...
  48. R

    Calculating the doppler red shift

    Homework Statement I'm working on this problem here: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redshf2.html The answer is already listed, it's the steps I'm worried about. I don't see how they get from step 1 to step 2 here. They don't even tell me what T is. It seems...