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. V

    Red shift and blue shift as relating to time dilation

    I was listening to a lecture about the expansion of the universe and found myself confused about the light we see from very distant galaxies. These galaxies are red shifted because they are moving away from us and the photons are being stretched. Does this mean that the galaxy will appear to...
  2. N

    Why do people think red shift is caused by big bang?

    Question: as title says. I'm not a physicist at all and I do not know much about how people obtained the big bang theory from red shift. So here is what I thought from red shift: In a time period t1, a distant body emits a light wave with a certain amount of energy with n oscilations(I don't...
  3. D

    Energy of photons and the red shift

    Was just learning red shift for my A level final exam and thought about this: Okay so I have a galaxy with a fixed amount of stars (for the duration of this thought experiment) producing a fixed power of light, which can be assumed to be from its centre. The galaxy is receding from the Earth at...
  4. K

    Expansion of space - Red shift

    I have always been puzzled by how we see an object from the past and can tell what it is doing now if it is billions of light years away. Given that; 1. the shift in the measurement of the light spectrum indicates the speed at which an object is retreating or getting closer. 2. that...
  5. K

    The meaning of z number in red shift

    Hello I understand that red shift is a measurement of relative speed. What I am looking for is what the number after the z means. Is it a percent of c or, Or is it it just a scale that physics uses to note the difference between different objects. Thanks for your time. Kevin
  6. B

    Proof of Red Shift that indicates Expansion

    What experimental evidence or optical measurements are used to prove that red shift is due to gravitational fields of suns, and is not simply an artefact of light passing through and being diffracted by the matter in the heliosphere of many suns before reaching earth?
  7. F

    Understanding Red Shift: A Derivation from Classical and Relativistic Physics

    This is another one of those concepts where I understand what happens at the macroscopic level, but don't understand the why, or rather what is happening at the fundamental level. Now my understanding of QM has improved a fair amount in the last week thanks to some help from ZapperZ, but I'm...
  8. S

    Red shift and observable light

    Since light emitted farther away from our point in the universe is more and more red shifted, would this mean that at a certain time we wouldn't be able to observe light further than a fixed distance since it's been red shifted beyond the electromagnetic spectrum?
  9. D

    Conservation of energy in gravitational red shift

    When an em-wave red shifts due to leaving a gravitational field it loses energy, right? Where does that energy go, into gravitational potential energy of the photon? If so does this mean that the photon can "fall" back on the gravitating body to reclaim this energy? What happens to the energy if...
  10. J

    A Problem with (Almost) Universal Red Shift

    It is my understanding that, except for a couple of local galaxies, everything in the universe is moving away from us as well as from each other (relationships between inhabitants of local galactic groups excepted). If my information is acurate, how is it that a large collection of randomly...
  11. T

    Can measurements of red shift be affected by the shape of spacetime?

    I have read on some threads that there are other possible shapes for spacetime and I wondered could any of these spacetime shapes, or their change over time, affect our measurement of red shift? Also does the matter density at the location where a photon is emitted relative to our own...
  12. J

    Gravitational Red Shift at the center of a mass

    I understand we can calculate that gravitation is neutralized at the center of a mass - say, a planet or star (or anywhere within a spherical shell); I wondered if the GRS is also neutralized, or if being pervaded by gravity - however neutralized - the dilation of space-time persists therein.
  13. L

    Intrinsic Redshift: Current Status and Research

    I came across this Wikipedia article ,http://en.wikipedia.org/wiki/Intrinsic_redshift, that describes the idea of intrinsic red shift but the references seem old. What is the status of this idea today?
  14. J

    Conservation of energy and red shift

    If due to redshift, light shifts towards the red end of the spectrum and the energy of photons = hc/\lambda, then how does this reduction in energy of the photons not violate the conservation of energy law?
  15. S

    Exploring the Differences Between High and Low Red Shift Galaxies

    How do high red shift galaxies differ from low red shift galaxies? Dow e see significant differences? if so what ?
  16. S

    Amplitude of wave stretched as well as red shift?

    The expansion of spacetime stretches and red shifts the wavelength of light. Is the amplitude of the wave stretched as well? So that very distant stars appear brighter, and therefor nearer?
  17. Kawakaze

    Red shift and narrowband filters

    Im planning on trying out some narrowband filters to hopefully dodge the light pollution here and get some nice photos, I was just wondering if red shift ever comes into play, could a distant object be shifted enough to be actually blocked by the filter? I know it depends on the filter and so...
  18. G

    Opinions on Red Shift? Discuss Here

    in my simple opinion the red shift is caused by the increasing distance of the light source and the light destination. its accepted by everyone that light has a constant speed in space, but i think that the constant speed is relative to the source object (kinda like sound). so to keep it simple...
  19. C

    How Does Redshift and Lambda-CDM Model Explain the Universe's Expansion?

    I would first like to thank everyone on this website. So much knowledge i haven't been able to leave. I am researching the time line of the universe and have read a lot of great articles about red shift and the Lambda-CDM. I found a Cosmology Calculator where you can find out the distance in...
  20. A

    How Is Redshift Calculated in Changing Epochs?

    Homework Statement The redshift of a galaxy is measured at the present epoch t0 and again at the infnitesimal future epoch t0 + δt0. Show that the rate of change of z with t0 is given by \frac{dz}{dt_{o}}=(1+z)H_{o}-H(z) Homework Equations Hint - Start From: \frac{\delta z}{\delta...
  21. R

    Question regarding red shift and tired light hypothesis

    The idea that distant objects are receeding from us and thereby producing a proportional redshift --seems to be a fundamental tenant in terms of current cosmological thinking and theory. There have been alternate ideas proposed in the past including the tired light hypothesis-(whereby light...
  22. E

    Exploring the Red Shift in Stars: Uncovering the Mysteries of Our Galaxy

    When I look at the stars in the sky with my undressed eye, they are blue. But pictures of stars taken by scientist always show them as red. Does this mean that the stars in our galaxy are fairly stationary and/or moving closer to earth? Will I find that most red shifts will be seen in stars...
  23. A

    Photon Red Shift Energy losses in expanding universe theories

    The evidence points to an expanding universe, we tell this by looking at the redshift/distance relationship, objects further away are receding faster, with their redshift and distance at an almost linear relation. My question is, the photons emitted by those objects that we are just now...
  24. mrspeedybob

    Light, gravity, and red shift.

    Light responds to gravitation. Light has momentum. Given these two postulates it follows that if a photon passes a massive body both the photon and the body will be effected, though admittedly the effect on the massive body will be extremely slight. Put another way, a photon has its own...
  25. R

    Red shift of light and expanding universe - Your help needed.

    This relates to Doppler's effect. The phenomenon: It is quite simple and says that light emitted from a moving object will be redshifted when the object is moving away from us (earth) and blueshifted when the object is moving towards us (earth). I was thinking of this phenomenon and arrived...
  26. S

    Red shift, gravity, and dark energy.

    When matter is transformed into light energy within a star, the gravity associated with the matter is now associated with the light's energy. As space expands, the redshift is dilluting the light's energy and reducing the gravity associated with that energy. Would the energy dillution further...
  27. R

    How do I calculate the gravitational red shift for a white dwarf star?

    Homework Statement After a star like the Sun has exhausted most of the hydrogen in its core it expands and cools to form a red giant. Eventually, when it has exhausted all its nuclear fuel, it sheds its outer layers and contracts and becomes a white dwarf of similar size to the Earth as...
  28. E

    Red Shift & Time: Can Time Be Expanding Too?

    Hello! I'm brand new to the forum and have a question that has been nagging at me for some time. I've read a great number of articles regarding the red shift, as well as time being a dimension. I was wondering if anyone has addressed the possibility that time might also be subject to the red...
  29. L

    Red or Blue Shift: Moving Light Sources Forward/Backward

    I want to know if the red or blue shift apply if I am moving away or toward a light beam and if it still apply if I am moving the light beam source with me forward or backward.
  30. J

    Gravitational lensing and red shift

    Hi I'm new to the forums. I have a question I hope you guys can help me understand. When gravitational lensing produces multiple images of the same object, do they undergo any red/blue shift? And if so, is the frequency shift the same for each image?
  31. L

    Red shift photons: where does the energy come from?

    The photon energy is given by E = h*f. Now, i am aware of the dual wave/particle character of photons. But let's say i emit ONE SINGLE photon, when moving at, say, 0.99c towards the observer. The observer will observe a huge blue shift in this photon. In fact, with respect to the source, it...
  32. L

    Calculating Cosmic Redshift & Age of Universe

    The cosmic microwave background (CMB) radiation detected by Penzias and Wilson at Bell Labs in 1965 has the spectrum of a blackbody, with an observed peak at λ = 1 mm. It is believed that when the light was emitted the spectrum peaked at a wavelength of λ = 1000 nm (close to the peak of the...
  33. B

    Red Shift and Blue Shift: Is It Possible?

    I was thinking today and had an idea but didn't know if it was possible. When an object moves toward or away from an observer, the light (such as visible light) "shifts", the wavelengths become closer together (moving toward you) or further apart (moving away from you). Say for example we...
  34. P

    Red shift and the train thought experiment

    It seems there is usually no mention of "redshift" when train thought experiment is used to teach relativity. Is there a reason for not bringing redshift (blueshift) into the experiment?
  35. H

    Red shift distance relationship and dark energy

    It is my understanding that there is evidence from the red shift/distance relationship that while the rate of universal expansion was once decreasing (from the Big Bang until about 6-7 billion years ago -BYA for short), at about 6-7 BYA it began increasing. This, as I understand it, is the main...
  36. ChrisXenon

    Expanding Universe & Red Shift

    We hear that the universe is expanding, and this accounts for red shift as all objecsts are accellerating away from each other. But I have also heard that it is the space itself, of which the universe made, that is expanding - not that things are moving in that space. If so, then the...
  37. Z

    The source of gravitational red shift

    On the following website; http://www.cfa.harvard.edu/Walsworth/Activities/Atomic%20clock/old_atomicclock.html" I read that; The gravitational red-shift is a consequence of the local position invariance of the Einstein Equivalence Principle. I know what gravitational red shift is and...
  38. J

    What is the approximate gravitational red shift for light emitted by the sun?

    The book gives the equation: \frac{\Delta\textit{v}}{\textit{v}} = \frac{GM}{c^{2}R} the problem gives me the mass of the sun, it's radius, and the wavelength of light being emitted. So basically I can solve the right side of the equation, but I'm just not sure about how to express the amount...
  39. A

    Why do we have red shift and blue shift, and not only one?

    I hardly understand anything about relativity (as would be evident from my threads and posts), and thus, this dumb question. If there is only length contraction, and not length expansion, why do we have two shifts, namely redshift and blue shift? should not only there be only blue shift due to...
  40. K

    The red shift - what does it indicate?

    First of all - please don't give me warnings if I write something that is not correct here. I'm just trying to understand the Big Bang theory, and I have some problems with it. OK, the red shift. What does it indicate? I don't believe it indicates velocity and distance, because there are...
  41. E

    Redshift Issues: Understanding the Light-Metric Coupling in Cosmology

    I have some issues with the current understanding of the cosmological red shifts and their interpretation using the spatial variation in the metric with time.To make the argument clear, I'm also posting pictures. The universe is depicted as the surface of a sphere but that doesn't affect the...
  42. W

    Red Shift = Expanding Universe?

    Has anyone ever considered the possibility that the perceived Doppler effect of light from distant stars/galaxies is not caused by movement of the emitting bodies, but rather a side effect of light traveling extraordinarily long distances under the week gravitational force of the entire...
  43. wolram

    Measuring Red Shift to Confirm Hubble Flow

    Over a period of several years could the red shift of some ideal body be used to confirm the Hubble flow?
  44. G

    Photon Energy vs. Red Shift, Inflation Theory, Acceleration

    Energy of Photon/Red-shift, inflation, acceleration I have a few questions (following a short premise) below. I have been reading medieval philospohy recently and also some about the "Big Bang." What I am struck by is number of assumptions made in both fields. There is a large amount of...
  45. J

    Red shift and zero point energy

    Is there a relationship between red shift of galxies and zero point energy? Has the zero point energy changed over time?
  46. B

    UV Star Redshift 8: Doppler Effect Impact on IR Spectrum?

    Hello friends! Would a star (O-Type) with max output peaking in the UV range, which was subject to (at distance) redshift 8 end up in the IR spectrum when its light approached earth? Would this be taking into consideration the doppler effect? BTW, yes this is somewhat an extension of my...
  47. wolram

    Measuring Red Shift with Accuracy

    We have measures that define the volt, amp, kilo, pressure, etc, etc, compared to these measures how accurate are red shift figures? i know that tolerances are only guaranteed in a machine shop if the temperature is within limits, and all the above can be tested time and time again, but how...
  48. Bob3141592

    Gravity Red Shift: Can it Happen?

    Can gravity be red-shifted? I've never seen this addressed directly, but I don't see how it couldn't be. If gravity isn't affected by the expansion of the universe, why isn't it? If it is red shifted, would this dimunition of gravity be like a repulsive force?
  49. M

    Cosmological Red Shift: How Does z Change With Time?

    I know that the variation of cosmological red shift with the distance is given by z= H_0 l (1+\frac{1}{2} (1+q_0) H_0 l) Where l is the luminosity distance, H_0 is the Hubble parameter at the corrent epoch and q_0 is the deceleration parameter. I would like to know how does z...
  50. S

    New form for GR's gravitational red shift & a novel interpretationof the underlying physics

    [Moderator's note: the message is posted because it had to be an enormous work to write it. LM] I am posting an unusual version of the standard relation for the gravitational red shift embodied in general relativity. This new version uses the Klein-Gordon equation, sans psi notation, and gives...
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