Redshift Definition and 45 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).

View More On
  1. 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
  2. Bolhuso

    I Galaxy redshift and thermodynamics

    I just finished rereading the great "A Brief History of Time". To me, what stands out the most in this book, is its ability to keep raising questions while you read it. This thought came up. It's been stuck in my mind for days, so I will humbly post it here to get some feedback. Please forgive...
  3. E

    Redshift & Total Energy Density

    1. If today vacuum and matter contribute 71 % and 29 % to the total energy density of the universe, at what redshift z were they contributing equally? 2. If today vacuum, matter, and radiation contribute 71 %, 29 %, and 0.01% to the total energy density of the universe, at what redshift z were...
  4. Arman777

    I Voids and the Hubble Constant

    It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have measured the Hubble constant lower. Since when the light passes an underdense region it gets...
  5. Arman777

    I Which redshift value is used in the velocity measurement of distance

    Let us say that we have a stellar object so its total velocity is defined as $$ v_{tot} = v_{pec} + V_{rec}$$ Where $$V_{rec} = H_0r$$ and $$V(z) = \frac{cz}{1+z}[1+\frac{1}{2}(1-q_0)z - \frac{1}{6}(1-q_0-3q_0^2+j_0)z^2]$$ for small z. So my first question is what is the $z$ value here...
  6. JTorn

    Why doesn't a real Infinite Redshift Limit occur at R+ for Kerr BHs?

    As I have studied before, I found that Infinite Red Shift occurs where gtt = 0 but this exercise says that on Kerr's Black Hole it doesn't really work like that. Right now I'm blocked because I didn't find anything on the internet about it so I don't know how to show this phenomenon. Any help...
  7. J

    B Is red-shifted light traveling at a speed less than c?

    The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source. If so then the original quantity of...
  8. George Keeling

    I Have I got the right picture for cosmological redshift?

    Summary: I have a question on cosmological redshift which I have just learned about from Sean Carroll. After calculating it for an expanding universe he does a thought experiment to show that it is different to Doppler redshift which would be detected if two galaxies were flying away from each...
  9. Y

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

    I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...
  10. J

    Cosmological Redshift

    Homework Statement Consider a point in the intergalactic medium at some cosmic time ## t_{obs}##, the time of arrival of a photon of wavelength ##λ_{obs}## as seen by a hydrogen atom at that location. The source of this photon a comoving distance ##r## away emitted it at wavelength ##λ_{em}##...
  11. Ken Gallock

    A QED: redshifting light and infrared divergence

    I am looking for some resources describing the following content: A light with wavelength ##\lambda## is propagating in flat spacetime. The light redshifts as its wavelength gets larger and larger. In quantum field theory, this causes an infrared divergence of the field. What I want to know...
  12. M

    I Redshift of star light

    A question regarding the redshift of star light being proportional to the star's distance from us. I suppose there were other, competing explanations for this when it was first observed (e.g. that light somehow loses energy/frequency extremely slowly over large distances), in addition to the...
  13. Deepblu

    B How do we know that the universe is still expanding

    This idea has always bugged me: If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?
  14. redtree

    I Redshift and the Friedmann metric

    My discussion of the Friedmann metric comes from the derivation presented in section 4.2.1 of the reference: I have a couple of simple questions on the derivation. The are placed at points during the derivation. I note the...
  15. redtree

    I Curve fitting the luminosity distance and redshift data

    Can anyone recommend papers that directly curve-fit redshift as a function of luminosity distance for type Ia supernova and gamma ray bursts? I am looking for papers that do not curve-fit the data via an assumed model, even one as simple as Friedmann–Lemaître–Robertson–Walker (FLRW) metric. I...
  16. T

    B Gravitational Redshift of a laser

    If we run two identical lasers and put one at sea level and one on top of a high mountain, will they operate at different frequencies?
  17. ASmartDude

    B Is there a specific unit used to measure redshift?

    i have been learning about using redshift to measure the distance of far-flung objects, but in all my research i never saw a specific unit used to quantify how redshifted an object is. to me it seems like an oversight, because such a unit would be very useful for the study of distant galaxies...
  18. R

    I Why are non-relativistic particles not redshifted?

    Hey! I was reading some script and when it comes to the cosmological redshift, it says, that only relativistic particles are affected by cosmological redshift. This does feel quite natural, however, I haven't been able to come up with an explanation that shows it with proper physics and...
  19. Orodruin

    Insights Coordinate Dependent Statements in an Expanding Universe - Comments

    Greg Bernhardt submitted a new PF Insights post Coordinate Dependent Statements in an Expanding Universe Continue reading the Original PF Insights Post.
  20. nearc

    I Best way to filter NED redshift data

    i'm using to extract redshift data. while I'm very happy there is now lots of data available, data requests need to be limited in size or you get an error. i would like to examine the patterns of redshift vs longitude, but that is too much data, i...
  21. hilbert2

    I New possible explanation for Hubble redshift

    I just saw this news article on the home page of the university I'm working in: It seems to be about a study that can potentially explain the redshift of distant stars with a mechanism that's related to the interaction of photons with the...
  22. S

    I Systematic redshift?

    Hi everyone, I am new to observations and observational terms! I am reading the paper "constraining the time variation of the fine-structure constant" by Srianand et. al in the section "constraining alpha with QSO absorption lines" there is a sentence saying "... rest wavelengths of MG II ...
  23. redtree

    I Gravitational redshift

    I am trying to find a derivation of gravitational redshift from a static metric that does not depend on the equivalence principle and is not a heuristic Newtonian derivation. Any suggestions?
  24. S

    Plotting Density Parameters as a Function of Redshift

    I'm trying to plot the density parameters against redshift in Python, so I suppose this is kind of a cross over of programming and physics. I've been given the following two equations in order to do so $$r(z) = \lambda_H \int_{0}^{z} \frac{dz'}{E(z')}$$ $$E(z) = \frac{H(z)}{H_0} = \sqrt...
  25. Jim Hasty

    A Redshift data on stars the same distance from earth

    Can anyone in the cosmology community direct me to hard research data that specifically demonstrates: there is no detectable redshift difference in stars that are the same distance from the Earth but in all different directions. This is of course related to 'does the universe have a preferred...
  26. P

    I Scale factor/redshift formula wrong at the end?

    In this video: The professor at the end (at about 7:28), used the formula for scale factor and redshift as a(t) = 1/z, instead of the actual one a(t) = 1/1+z. And when we apply both of them, they give very different results. So, how could the professor use the first formula, which we were...
  27. P

    I How can we measure a galaxy's velocity via a star?

    In this video (), we first took the spectrum of a star and then of a galaxy which look like the following: However, what I don't get is: if we take the spectrum of a star and then of the galaxy the star is in, shouldn't it give me the same spectrum as they're moving in the same speed? And if...
  28. P

    I How can we measure redshift?

    As far as I know, when we use the spectrum of a star, we see where the absorption lines are and using this, we can detect the elements that are present in the star. We also measure whether those absorption lines are supposed to be for a particular element. But why is it not possible that the...
  29. BiGyElLoWhAt

    How to calculate redshift from the schwartzchild metric

    Homework Statement I'm doing a project on the redshift from a star system (I chose a binomial system because why not). I might be going a little overboard using topology to calculate redshift, but whatever. First off, can I just treat a binomial system as the superposition of 2 sources which...
  30. C

    D(z)/d(t) of high z galaxies

    I am interested in locating information of the evidence of z evolution that has been collected of the spectra of relatively high z sources over the previous 75(35) years. Before I get too far into the question, I thought I would try to find what research might have been done on the subject...
  31. G

    Solar Sail/ Mirror Redshift & Blueshift

    Hi all, I was hoping you could help me out with this one. Im trying to calculate the redshift or blueshift of a photon that strikes a mirror and reflects. I know E=pc, and that 2p is the momentum transfer. I also know that kinetic energy is 2p/M (of the mirror). However, this says that...
  32. T

    Spectroscopy and the doppler effect

    So the prof shows us how to tell if an object is red shifted, by comparing two spectra (i assume of the same object). The emission lines had similar gaps but were shifted right, therefore moving away from us. How can you obtain two different spectra of the same object if it's constantly moving...
  33. E

    Signal time in Twin Paradox: Fast clocks? Redshift/blueshift

    NOTE: This is NOT a homework problem. I created this one myself based on some problems I have seen, with specific numbers used to make calculation clean and easy. Tl/dr version: in a round trip to a star, will the "moving clock" run slower on the way there but faster on the way back due to the...
  34. J

    Onset of Universe acceleration

    Homework Statement Assume the cosmological model with H0=72,Omega_M=1-Omega_lamda=0.3,(so dark energy with w = − 1) and a flat universe.) a) Find the redshift z at which the universe starts accelerating (that is, when it transitions from decelerating to accelerating). b) How long ago did this...
  35. J

    Link redshift with luminosity distance?

    Homework Statement Plot luminosity distance and redshift z Homework Equations $$d_L(z)=(1+z)r(z)$$ where d_L(z) is luminosity distance and r(z) is the comoving distance. and we have $$r(z)= \frac{H_0^{-1}}{\sqrt\Omega_K}*sinn[\sqrt{\Omega_K}\int_0^z\frac{dz'}{\sqrt{\Omega_M(1+z')^3}}]$$ where...
  36. P

    At what redshift does the density of M > density of DM

    Here is the problem link picture I know that omega=density/critical density so I know I can find the value of all of those. DE is dark energy and M is matter (I'm pretty sure). I assume I have to sub that formula into equation 1, and then rearrange it to create a...
  37. Pejeu

    Spaghettification and gravity in theory of relativity?

    Per my layman's understanding, the apparent force of gravity, as explained by the general theory of relativity, is actually the result of rectilinear motion at uniform speed through warped spacetime. 1. If that's the case, what actually causes gravity for objects at rest with respect to the...
  38. P

    Paul Schuurmans

    I am looking forward to joining discussions here. My Backgroud is 2 years "applied physics", 5 years education for teaching in visual communication (art), 15 years experience as a database software developer. Now I am looking for a job as a software tester. Software Testing is similar to being...
  39. C

    Hubble redshift and calculation of galactic rotation curves

    I am interested in whether it is necessary to account for the effects of the Hubble Redshift in determining the rotation velocities of galaxies exhibiting keplerian motion and, in particular, whether the associated spatial expansion of the Universe is known to result in spectral shifts that...
  40. D

    Transverse Doppler Effect

    Homework Statement A star travels in a direction transverse to the line of observation from Earth, with a speed 0.5c. It also emits light with wavelength ##\lambda_0## in the rest frame of the star. Calculate the wavelength of the light as observed on Earth, and also the angle at which the...
  41. R

    Measuring redshift from type 1a supernova

    I know that you can measure redshift from stars by looking at the shift in there balmer lines or more generally there line absorption spectra and seeing how far these lines have shifted from actual balmer lines or line absorption spectra of the same elements on earth. But how would you...
  42. 2

    Dark energy and type 1a supernovae?

    I am trying to wrap my brain around the evidence for accelerating expansion of the universe from type 1a supernovae. From what I understand, it was first realized that the universe was expanding at an increasing rate from discrepancies between the calculated distances to type 1a supernovae using...
  43. T

    Would gravity waves redshift?

    would gravitational waves redshift with the expansion of the universe ?
  44. LachyP

    How to calculate the mass of a star using redshift?

    Hi everyone, Is there a simple formula/equation for calculating the mass of a star simply by measuring it's redshift. I know there is a way to do it, but have been unable to find any clues on the web.. Thanks for your help! :)
  45. M

    Gravitational Lensing

    Homework Statement A quasar at redshift 0.3 is gravitationally lensed into two images by an elliptical galaxy at redshift 0.18. The two images of the quasar are separated from the center of the galaxy by 1.1 and 1.6 arcseconds, on opposite sides. One of the quasar images flares up in intensity...