What is Redshift: Definition and 379 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. S

    B Redshift of a light pulse between 2 accelerating rockets

    I'm reading book from here. Suppose two rockets are accelerating with the same acceleration ##a## and are separated by some distance ##z##. At time ##t_0## the trailing rocket emits a light pulse. The book tells that pulse reaches leading box after time ##z/c## as seen in background frame. But...
  2. F

    A Convention of units for densities in cosmology

    I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, ##\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{sr}^{-1}\right]## and the corresponding density of galaxies per ##\operatorname{arcmin}^2## for...
  3. Cerenkov

    B What value of z (redshift) equals apparent superluminal recession?

    Hello. I have visited this site recently... https://lco.global/spacebook/light/redshift/ ...and was wondering at what value of z in the table below would galaxies appear to recede at superluminal velocities? z Time the light has been traveling Distance to the object now 0.0000715 1...
  4. BadgerBadger92

    B Difference between special relativity and redshift?

    For quite a while I thought that time dilation was the effect of bodies moving close to the speed of light dilates the passage of time. I also have read about red shift, which seems to be the same thing, though I’m sure they are totally different. I learned in special relativity (the time...
  5. L

    B Which statement is more accurate? (Hubble's redshift vs. Hubble's law)

    A. The two great cosmological discoveries of this century, Hubble's redshift and the cosmic microwave background, have made the Big Bang theory the most credible theory of the origin of the universe so far. B. The two great cosmological discoveries of this century, Hubble's law and the cosmic...
  6. sol47739

    I Is the light from Andromeda truly blue shifted?

    I asked a similar question on Quora not that long...
  7. doudou

    I Gravity does negative or positive work with cosmos redshift?

    Based on the fact of observed cosmological redshift, scientists have proposed different ideas to explain. One interesting question is whether gravity does negative or positive work now: According to universe expanding in Big Bang theory (Lemaitre, 1927), obviously gravity does overall negative...
  8. ergospherical

    I Observed Redshift from Moving Source: Deriving the Result

    If a galaxy is receding from us, then the 1+redshift observed on Earth is the product ##(1+z_{pec})(1+z_{cosm})## of the doppler redshift due to the peculiar motion of the galaxy and the cosmological redshift due to the FRW metric. It makes sense if we think about some intermediate observers...
  9. Tertius

    I Can a scalar field model account for the cosmic redshift?

    A minimally coupled scalar field can model a cosmological fluid model where And where the equation of state can be the standard ## \omega = \frac {p} {\rho}## I can see how this does a fine job modeling matter, because as the scale factor increases, the density will go as ##\frac {1} {a^3}##...
  10. E

    I Calculation of gravitational redshift in an accelerating elevator

    When we derive the formula of the redshift by the equivalence principle we imagine a light ray which goes from the bottom to the top of the elevator and which would take a duration t = h/c to make the journey, with h = height of the elevator. I don't understand why t = h/c, because while the ray...
  11. ergospherical

    I Calculating Particle Redshift at Time t_0 & t_1

    A question about the FLRW solution has confused a few of us. At time ##t_0## a particle has radial speed ##v_0^r## relative to the fundamental observers, and at a later time ##t_1## it has radial speed ##v_1^r##. The task is to show that\begin{align*} \frac{a_0}{a_1} =\frac{v_1^r \gamma_1}{v_0^r...
  12. Oldman too

    I JWST breaks another Redshift record

    A paper just out seems to describe a galaxy formed ~250 myr after the BB. One of many records that JWST will be both setting and breaking as the data pours in. With the papers focus on UV, this is probably going to be a Pop 3 star search. Section 6.2 of the paper describes the studies findings...
  13. 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
  14. M

    Measurement of speed using redshift or blueshift

    The solution on my textbook is 13000m/s toward Earth as the light is blue-shifted I'm able to calculate the magnitude of velocity (13000m/s), but i don't understand why thus is blue-shifted? Since in the lab, the light's wavelength observed is slightly higher than light from Ursa Majoris. So my...
  15. Vick

    I Formula for redshift of photon decoupling

    I have come across an old formula from my notes and I have no reference for it but it is using truncated digits in its formula to calculate the redshift for decoupling. The formula is nearly as accurate as the observed data from Planck 2018. So I would like to figure out the derivation of those...
  16. zrek

    B Experimental evidence on cosmological redshift

    The space expands due the dark energy, so the light wavelength also stretches during its journey between galaxies, causing a measurable redshift. How can I argue that this is the right reasoning for the measured redshift? What if someone states that the photon maybe simple loses its energy over...
  17. Arman777

    A Relativistic Redshift and understanding it's approximation

    I was reading an article, and I saw this expression. $$ 1+z=\frac{(g_{\mu\nu}k^{\mu}u^{\nu})_e}{(g_{\mu\nu}k^{\mu}u^{\nu})_o} $$ Where ##e## represents the emitter frame, ##o## the observer frame, ##g_{\mu\nu}## is the metric, ##k^{\mu}## is the photon four-momentum and ##u^{\nu}## is the...
  18. Kwantumnaut

    Accurate depiction of cosmological redshift?

    Summary:: Accurate depiction of cosmological redshift? Hello. I'm thinking about getting a tattoo representing cosmological redshift, and just wanted to make sure my custom image is correct and accurate, and if you would recognize and be able to tell what it represents. If not then any...
  19. M

    I Collision between the Milky Way and Andromeda and Redshift

    Hi ! My question is the following: If there is a redshift in astronomical observations with other galaxies, as our Milky Way galaxy is approaching with the Andromeda galaxy, they must surely be in an accelerated approach due to the force of gravity between the two galaxies, and as our galaxy...
  20. 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...
  21. F

    A Angular power spectrum dependence in redshift z and k

    Hi, I wanted to have a precision about a question that has been post on this relation between P(k) and C_l The author writes the ##C_\ell## like this : $$C_\ell(z,z') = \int_0^\infty dkk^2 j_\ell(kz)j_\ell(kz')P(k)$$ I don't undertstand the meaning of ##z## and ##z'## : these are not...
  22. S

    Astrophysics Redshift Question

    I have attempted to link the equations, but I don't really understand how the data given fits. Does the angular radius get plugged in as the deflection angle?
  23. 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...

    Cosmology: Hubble's law and Redshift

    I tried to attempt this by taking z=dv/c but I saw that relation Liddle's book on Cosmology and I really don't know where this came from. Secondly, I will need some series ( maybe Bionomial) in terms of z from which I can show small z. I think it's simple but here it's not clicking to me.
  25. C

    B Redshift speed calculation

    If many quasars are at redshift 10, and another group is at redshift 6, what is the speed of the second group relative to the first? The redshift 10 group can be considered "at rest", so the redshift 6 group is heading in our direction at what rate? For this scenario, let us assume all the...
  26. C

    I Material source of the quasar redshift

    When I see discussions about quasar redshift, exactly what are they talking about? I assume a quasar is similar to a black hole, so emits little radiation from the main mass. The light from a quasar comes principally from its jets of accelerated material, no? Which means the jet pointed...
  27. E

    B The accelerated expansion of the universe; redshift and time/distance

    I am certain that my confusion here rests in a misunderstanding on my part and not in a mistake having been made by countless physical theorists. Nevertheless, I have had a hard time wrapping my head around it. Here is the crux: We observe that light from distant objects is more redshifted...
  28. Z

    I Gravitational Redshift: Units, Velocity & Ratio Explained

    I noticed in physics papers that gravitational redshift is expressed in m/s or km/s. I assume that this must be the equivalent velocity to produce that same redshift. So for example, if the gravitational redshift was measured as 3x10 ↑ -4 then; z= v/c 3x10^-4= v/c v = 9x10^4 m/s v=90km/s...
  29. G

    B Eddington's 1919 Eclipse: Photon Deviation & Redshift

    Hi all, I've been wondering: Thinking of Arthur Eddington's relativistic oriented 1919 eclipse observation, would the photon deviation due to the Sun's gravitational imposition have caused the photons to exhibit a qualitative redshift due to the time photons had spent within the Sun's...
  30. 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...
  31. 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? Is...
  32. 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...
  33. J

    B Red-Shifted Light: 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...
  34. 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...
  35. X

    I Challenges to the Redshift Method and Hubble's Law

    Since Hubble's Law has been around a long time, so, after almost 100 years, to challenge its validity looks like dumb and stupid. So let me be that dumb guy, or maybe the bad boy that spoils the whole thing. Here let me start with the Redshift in general. There are three Redshifts proposed till...
  36. D

    I Some redshift from prism effect of a gravitational lens?

    A quick search turned up Simaciu, Ion. (1997). Chromatic aberration of gravitational lens. 10.13140/2.1.1133.6003. The math is beyond me so I first made a basic assumption that chromatic aberration of gravitational lenses worked somewhat analogous to a prism in that red is bent less than other...
  37. 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...
  38. 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}##...
  39. Arman777

    B Gravitational Redshift and its effects on Photon

    Let us assume that we have a large gravitational field, then the gravitational redshift can be expressed as, $$\frac {v_{\infty}} {v_e} = (1-r_s/R_e)^{1/2}$$ In this equation ##v_{\infty}## represents the frequency of the light measured by an observer at infinity, ##v_e## is the frequency of...
  40. H

    I Question about the redshift

    As light interacts as a particle and when being absorbed or emitted by an electron that is done instantaneously, that is, no time between the photons energy not being contained (I’m unsure as to whether or not this is the correct word) within the electron to being contained within it, how does...
  41. Ennio

    I Redshift Along Galactic Plane: Study & Research

    How do the redshift change along the galactic plane or ortoganal to the galactic plane? If we measure a redshift of a star on the backgorund of other side of our galactic plane (where most stars are concentrated) are the redshifts avaraged higher? Is there a study or research discussing this...
  42. 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...
  43. M

    I Redshift of Star Light: Proportionality to Distance & Other Theories

    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...
  44. itoero

    B What is the cause of Cosmological redshift?

    What causes Cosmological redshift? Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona
  45. F

    I BAO : Relation between redshift, Hubble constant and radial

    From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : ##c\Delta z = H(z)\Delta \chi\quad\quad(1)## with ##z## redshift, ##H(z)## Hubble constant at redshift = ##z## and ##\chi## radial coordinates. One...
  46. 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?
  47. 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: https://www1.maths.leeds.ac.uk/~serguei/teaching/cosmology.pdf I have a couple of simple questions on the derivation. The are placed at points during the derivation.I note the...
  48. 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...
  49. T

    B Gravitational Redshift: Will Two Lasers Operate Differently?

    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?