What is Red-shift: Definition and 16 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. T

    I How do we know that red-shift is caused by expansion?

    Hi, I am not a cosmologist, but this question has been bugging me for some time. I am an engineer, and I own an aquarium, at least I owned an aquarium with an awesome LED light. Under certain conditions, I found that when the blue night light was on, organics in the water would fluoresce, green...
  2. 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...
  3. binbagsss

    Application of the Fundamental Theorem of Calculus (cosmological red-shift)

    Homework Statement [/B] I am stuck on the section of my lecture notes attached, where it says that equation 4.20 follows from 4.18 via an application of the fundamental theorem of calculus Homework Equations FoC: if ## f## is cts on ##[a,b]## then the function ...
  4. Doctor Strange

    A How is the red-shift of CMB obtained?

    The value for the red-shift of the CMB is 1090 according to the latest Plank study. How do they arrive at this number? You can look at how hydrogen lines of supernovae light are shifted, but how do you tell how far light is shifted when looking at the light from a primordial soup? Is there...
  5. 1

    B Where did the energy of red shifted CMB go?

    i know it sounds stupid, but i can't seem to find an answer to it where did the energy went? you can't just just destroy energy, but when you red shift a gamma CMB into a microwave CMB, the energy has go to go somewhere...? it's not like one gamma photo spitted into many microwave, cause that...
  6. Bruce Wilson

    Do we really need Dark Energy?

    If you measure the red-shift of a star 10 million light years away, you measure the velocity of the star 10 million years ago when the stars were traveling faster (if we are willing to accept that the expansion of the universe is slowing down). This not such an outrageous idea since firstly it...
  7. 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?
  8. Dry

    Expansion is a compounded error based on a wrong assumption?

    I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has. Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...
  9. binbagsss

    Unlocking the Significance of FRW Metric for Cosmological Redshift

    So from a killing tensor the FRW metric is known to possess, for a massless particle we find the well known result that as the universe expands the frequency of the photons decreases . But , what does this do for gr ? Was this known to happen before gr ? Thanks a lot. (I know it is used to...
  10. J

    Red-shift question from another thread.

    Homework Statement This is not a HW questions but from another thread. https://www.physicsforums.com/showthread.php?p=4495692&posted=1#post4495692 The statement I made was that if z increases for a source, then it is accelerating away. Or, it could that if z is constant then the sources...
  11. A

    Seeking clarification on red-shift

    I've recently read that red-shift is caused when a photon escapes a star and loses some of its energy, lowering its frequency and therefore making it appear redder. However, I've become very familiar with the fact that light will red-shift as it travels across the universe; I've been under the...
  12. A

    Define Red-Shift: Explaining BH Event Horizon

    Could some one explain what red-shift really means when we're talking about the EH of a BH? Thank you.
  13. jfy4

    So, what is the function for the redshift z as a function of distance r?

    This may seem elementary, however I have been unable to find a declarative statement that seems reliable. What is the equation for the red-shift z as a function of distance r to the best of our knowledge? That is z(r)=? Thanks in advance.
  14. nomadreid

    What are the Three Sources of Red-Shift in Astronomical Observations?

    There appears to be three sources of red-shift: or are there only two, and two of the following are equivalent? (1) Classical Doppler shift (2) Relativistic Doppler shift (3) Red-shifting from the expansion of space Otherwise put, which ones are calculated for retreating galaxy clusters and...
  15. C

    Origin of the Hubble Red-Shift

    Suppose that some time in the future a clever group of theorists and experimentalists finally devise a way to accurately determine the value of the Hubble Constant. Say by an interferometer connected to a quantum computer. Everything works fine in the lab and is checked and checked...
  16. E

    Is Gravitational Red-Shift a Valid Concept?

    Hey, this is my first post. Just talking about an idea I had. A month ago I was pondering the effects gravity has on light. Specifically if there could be a “gravitational red-shift” of light coming from massive stars. I first started by thinking of gravity as simply causing an object...