Redshift Definition and 385 Threads

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. Dr. Strange

    Question about Recombination Redshift values

    Wikipedia gives the redshift of recombination at z = 1500 (roughly 4,000 K). The Plank report list a 'redshift for which the optical length equals unity' and gives a value of 1090. What is the difference between the two?
  2. G

    I Diffraction Redshift and Emission Theory

    In the emission theory of light, light waves can move at any speed. We can still apply the Doppler effect, but to the best of my knowledge, only the frequency changes, not the wavelength. The pattern for a diffraction grating only depends on the wavelength right? And we have observed...
  3. 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...
  4. J

    An argument that massive particles don't redshift?

    I start with the spatially flat FRW metric in conformal co-ordinates: $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)$$ This metric has the following non-zero Christoffel symbols: \begin{eqnarray*} \Gamma^0_{\alpha \beta} &=& \frac{\dot{a}}{a} \delta_{\alpha \beta} \\ \Gamma^i_{0j} &=& \Gamma^i_{j0} =...
  5. J

    When Did the Universe Start Accelerating?

    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...
  6. J

    Has neutrino redshift been observed?

    I understand that neutrinos are emitted when supernova explode. I presume that there are quite accurate models that predict the energies of those neutrinos at the time of emission. Has the phenomenon of redshift been observed when those neutrinos are subsequently detected?
  7. 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...
  8. P

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

    Here is the problem link picture http://i.imgur.com/0BtcXJk.png 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...
  9. U

    Redshift effect And photon energy loss

    I've been searching around the web to figure out why photons shift towards the longer wavelengths as they travel from stars and other light sources but I haven't figured out why they loose energy as they travel ( and after reading some web pages I was told that they don't even loose the energy...
  10. 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...
  11. stevebd1

    Insights Time Dilation and Redshift for a Static Black Hole - Comments

    stevebd1 submitted a new PF Insights post Time Dilation and Redshift for a Static Black Hole Continue reading the Original PF Insights Post.
  12. J

    Does Redshift Change Over Time for a Constant Recession Speed?

    [Moderator's note: post spun off from previous thread.] I'd really like some verification on the following. After thinking about light redshift, I came to the following conclusion: Is it true that λObs / λEmit for 1 particular star doesn't stay constant over time even if its recession speed...
  13. P

    Exploring Nature: Joining Discussions on Software Testing and Astronomy

    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...
  14. 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...
  15. U

    How is Frequency Redshift Related to Sphere's Proper Area and Flux Ratio?

    Homework Statement (a) Show the relation between frequency received and emitted (b) Find the proper area of sphere (c) Find ratio of fluxes Homework EquationsThe Attempt at a Solution Part (a) Metric is ##ds^2 = -c^2dt^2 + a(t)^2 \left( \frac{dr^2}{1-kr^2}+ r^2(d\theta^2 + \sin^2\theta)...
  16. wabbit

    Redshift of non-comoving galaxy

    Homework Statement In a flat FRW universe, for a source moving radially at velocity v (at emission time) relative to the local comoving frame, what is the redshift observed by a comoving observer? Homework Equations ##c=1## Proper time to cosmological time ratio...
  17. Q

    Measuring Gravitational Redshift due to Galaxies without GR

    Hi guys. How do astrophysicists measure the redshift of electromagnetic waves from galaxies due to gravity without the use of General Relativity? If I can be more specific, how do astrophysicists know that the gravitational redshift of light emitted from some part of a galaxy or galaxy cluster...
  18. F

    Cosmological Redshift: Calculating Wavelengths

    Hi all! I've got a question about the cosmological redshift. We're given the metric ds^2 = c^2\,dt^2 - a(t)^2 \left[ dr^2 + r^2\,d\theta^2 + r^2\sin^2 \theta\,d\varphi^2 \right] Now light moves on null geodesics, so c^2\,dt^2 - a(t)^2\,dr^2 for radially moving light. For a GR exercise, we are...
  19. D

    Transverse Doppler Effect for a Moving Star

    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...
  20. I

    Redshift, Earth's orbital speed, and the speed of light

    The Earth is moving around the Sun at around 30km/s, a change of 60km/s over the course of the year. Presumably, measurements of the speed of light would show a redshift up to ±30km/s? Are redshift measurements this accurate? Likewise, the speed of the Solar System around the Milky Way, is...
  21. throneoo

    Cosmological and doppler redshift

    cosmological redshifts are caused by the metric expansion of space, whereas doppler shift is a result of relative velocities through space. So before the development of the modern cosmological models, how did astronomers originally distinguish these two causes of redshifts (e.g. when they tried...
  22. U

    Redshift in Terms of k | General Relativity

    Hi, I have been studying general relativity using Hobson's lately, particularly about the FRW universe. I know that for a matter universe with curvature, H^2 = \left( \frac{\dot a}{a} \right)^2 = \frac{8\pi G}{3} \rho_m -\frac{kc^2}{a^2} Another expression I came across is also 1 = \Omega_m +...
  23. 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...
  24. bitznbitez

    Observed Redshift Variation Over TIme

    Recently we got to watch G2 as its orbit took it around the black hole at the center of the galaxy. This showed a visual change in direction along with a corresponding redshift to blue shift change on a relatively short timeline. It was really a great event. Binary stars exhibit the same...
  25. 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...
  26. S

    High redshift galaxies in the HUDF

    In the Hubble Ultra Deep Field ( http://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field ) there are almost 10.000 galaxies and some of them with redshift Z>6 . Making some approximations i need to compute the number of galaxies with redshift z = 7. If the source density ( number / arcmin^2 ) at...
  27. J

    Redshift of supernova light curve

    I am trying to understand how the width of a supernova light curve depends on the redshift of its component frequencies. Let us make the simple assumption that the light curve is Gaussian. The inverse Fourier transform of a Gaussian is given by: $$\large e^{-\alpha...
  28. T

    Gravitational Waves and the Expansion of the Universe: Redshift or Not?

    would gravitational waves redshift with the expansion of the universe ?
  29. 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! :)
  30. avito009

    How to determine the mass of a planet using redshift?

    The size of the shift (Redshift or Blue shift) combined with measurement of the period of orbit tells us how massive an object is. How? How can we know the mass of a planet using redshift or blue shift?
  31. Wes Tausend

    Theoretical Extent of Redshift - How Far Can It Go?

    How far can the Redshift phenomenon theoretically shift frequency? I believe I understand the doppler-like effect, but have never seen this question regarding extreme RF shift asked before. One might think that if a relative object moved away very near the speed of light that the radio...
  32. M

    Solving Gravitational Lensing Homework for Hubble Constant

    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...
  33. A

    Why is the Cosmological Redshift z Confusing?

    I am confused about the physical meaning of the redshift. Let say the non-relativistic one z=v/c. When I read Barbara Ryden, Intro to cosmology, she demonstrate the z is not related to the space expansion between the source and the receiver, but 'it does tell us what the scale factor was at the...
  34. ellipsis

    Cosmological redshift: Where does the energy go?

    A few small questions: Observationally, why do we conclude that the metric of space is increasing, rather than that light increases in wavelength as it travels cosmological distances? Or are these two conclusions isomorphic? Since wavelength is negatively correlated with energy, where does...
  35. aditya ver.2.0

    Gravitational Redshift: Exploring Frequency and Light Changes in G-Fields

    Is the change in frequency an actual change or is it like the Doppler effect? Does the speed of light also change in a g-field?
  36. R

    Gravitation Redshift for very dense stars

    Homework Statement In deriving the expression ##\frac{f'-f}{f}=\frac{gH}{c^2}=\frac{GM_s}{R_sc^2}## , it was assumed that ##\triangle f=f'-f## was small, and that the photon had a constant mass of ##\frac{hf}{c^2}##. Suppose that a star is so dense that ##\triangle f## is not small. (a) Show...
  37. E

    Gravitational redshift with time dilation?

    Hello, thanks for reading this. I need help trying to find a way to calculate the time dilation due to gravity, from a satellite 50,000km above the surface of the Earth, traveling 10,000km/h relative to a stationary observer on Earth. A signal is being sent from the satellite to the observer...
  38. M

    Understanding Redshift: Mathias' 15-Year-Old Query

    Hello guys! I know what redshift is and all concerning it, so i have a good understanding of it. Though there is something that my 15 year old little head can't really seem to understand... When a star emits its photon, it travels to us and due to the accelerating distance between it and us...
  39. C

    Are Redshifts Used to Specify Locations in Cosmology?

    I have heard cosmologists use the phrase "at redshift", presumably indicating the location of something. Are redsifts used to specify locations in cosmology, and if so, how is that done?
  40. TumblingDice

    Expansion, redshift, and conservation of energy

    I've often read posts where the conservation of energy is questioned regarding redshift due to expansion. The question arises because the energy level of photons is directly related to the light's frequency, and this alludes to photons "losing" energy as the frequency lowers. The reasoning I've...
  41. B

    Calculating Redshift for Objects in the Universe

    We know that z redshift (+1) ir the ratio between the observed and emitted wavelength. Could you tell me, in practice, how you can you find z for a star? or can you give me a link where a particular case is explained in detail? Any object (star, (proto-)galaxy,quasar) emits EMR in a wide...
  42. K

    Gamma Ray Burst datasets that consist of distance modulus & redshift

    Hi guys I am looking for a dataset to use in my thesis. I am working on UNION 2.1 sample of supernovea Ia data. That dataset consist of redshift and distance modulus with uncertainty (sigma) for 590 supernovea. Do you guys know where can i find similar dataset for GRBs?
  43. Q

    Redshift and blueshift in relativistic doppler effect

    I'm looking for an intuitive explanation for the redshift and blueshift phenonema that occurs when a light ray is emitted transversely. According to wikipedia: Assuming the objects are not accelerated, light emitted when the objects are closest together will be received some time later...
  44. Matterwave

    Cosmological Redshift: Electron Energy Loss in Expanding Universe?

    So I have been thinking. Light gets redshifted because of the cosmological expansion of the Universe. This would mean that other, material particles, should get "cosmologically redshifted" as well right? So, for example, if an electron were flying towards us from some distant galaxy (and we...
  45. P

    Gravitational Redshift and Hubble.

    Hey guys, I feel like an idiot for asking this. However, I wanted to make sure that my head was screwed on straight before I asked my old astronomy professor permission to use some of his old lecture notes. Is gravitational redshift one of the causes of Hubble's Law? Is the Redshift of the...
  46. S

    Derivation of gravitational redshift: Mass of a photon?

    This is not exactly a homework question. In a physics textbook, they derive an expression for gravitational redshift of a photon emitted by a star at a large distance from the source by taking photon as a mass traveling up, against a gravitational potential and hence expending its...
  47. tom.stoer

    Cosmological redshift as kinematical Doppler effect

    In http://arxiv.org/abs/0808.1081 / Am.J.Phys.77:688-694,2009 Bunn and Hogg explain how cosmological redshift can be interpreted as accumulation of infinitesimal Doppler shifts. This suggested to invert z = z(v) and interpret v(z) as relative velocity of two objects with redshift z. It seems...
  48. H

    Evaluate the change in redshift over 10 years

    Homework Statement I am given a model where for an accelerating universe the redshift changes according to the following equations (given in part b). For this model and assuming that H0=70km/s/Mpc, evaluate the change in redshift over 10 years, for a source at z=1 and the change in recession...
  49. B

    Comoving redshift and area of sphere

    Homework Statement Standard candles may be used to measure the "luminosity distance", using DL = (L/4F)1/2, where L is the source's intrinsic luminosity, and F is the observed flux. Inthis problem you will relate the luminosity distance to the previously discussed angular diameter...
  50. WannabeNewton

    Solving Redshift of Two Kinds in Schwarzschild Metric

    Redshift of two "kinds" Hello! Consider the following problem: A spaceship on a circular orbit at radius ##r## in a Schwarzschild metric emits a photon with the rest frame frequency ##\omega## at an angle ##\alpha## outward from the tangential direction of the motion, in the plane of the orbit...
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