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    What is the difference between cosmological redshift and just "redshift"?
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    There isn't. Both are just shifts in wavelength emission towards the red end of the spectrum (longer wavelength) due to the object moving away therefore 'stretching' the waveas it is emitted.
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    but isnt cosmological redshift due to space, itself, stretching?
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    no space isnt stretching as such. things are moving "away from a single initial" point. red shift comes in a box with the Hubble constant which each then support the big bang theory. based on this, everything is moving outwards. i would't say space itself is being stretched. this is more philosophical than scientific - the "where is the edge of the universe and is it growing - if so where is it growing?" question.

    if you imagine a long strip of paper being moved past you to your left at 3x10^8m/s and then you standing there moving a red pen up and down on the paper surface, as the paper moves along you are creating a wave shape on the paper. this looks like standard wave formation. then, if you move along the paper (to the right) still making the up and down movement at the same speed, every second that passes you cover more paper so your wave becomes stretched - a greater wavelength. using this analogy if the paper is space and you are the wave source, space isnt actually being stretched.
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    Sorry corkscrew, but mathemagician is right, ordinary redshift is put down the the doppler effect you described, however cosmological redshift is as a result of a combination of spacetime stretching AND the fact that the photons lose energy and thus decrease their frequency so increase their wavelength
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    That's perfectly fine. I've done my astrophysics paper and that's the last time I have to do it now. He hopes.
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    (Original post by rpotter)
    Sorry corkscrew, but mathemagician is right, ordinary redshift is put down the the doppler effect you described, however cosmological redshift is as a result of a combination of spacetime stretching AND the fact that the photons lose energy and thus decrease their frequency so increase their wavelength
    Photons losing energy? How? Photons don't lose energy unless they interact with matter, for example Reverse Compton Scattering.
    Space itself is expanding, in a process imaginitively known as, expansion. This should mean that the Big Crunch won't happen right? Well no, we don't know how long space will continue expanding for, that is if there is a finite bound to that question.
    The best example of the difference between redshift and c.redshift, is in the cosmic background radiation. These are photons from (I don't know how many seconds after the Big Bang), when the Universe became transparent to light. The thing is, the universe was quite hot at the time. In theory (without expansion, since it was everywhere to begin with, so it can't have redshifted) this should result in a 3000K backbody radiation. Which it is not. It is about 2.7K today. This is purely due to expansion.
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    I dont think they meant the actual indevidual photons lose energy when they said that, but that when a wave is redshifted the wavelength is increased and because
    E = hc / wavelength ... the photos have less energy.

    But yeah about the two types of red-shift im not sure..... in a past paper there was a question which said

    'what is the difference between terrestrial doppler effects and cosmological redshift?'
    and the answer was
    'Terrestrial is change of wavelength due to relative motion blue shift for approaching sources'
    'Cosmological red shift is change of wavelength due to expansion of spcae'

    hmm... kinda makes sense i guess??
    Hope that helps

    Soph
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    They also tal of 'gravitational redshift' in past exam papers too.... guess this must just be gravity stretching a wave or something??

    Soph
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    (Original post by kingmonkeywoman)
    I dont think they meant the actual indevidual photons lose energy when they said that, but that when a wave is redshifted the wavelength is increased and because
    E = hc / wavelength ... the photos have less energy.

    But yeah about the two types of red-shift im not sure..... in a past paper there was a question which said

    'what is the difference between terrestrial doppler effects and cosmological redshift?'
    and the answer was
    'Terrestrial is change of wavelength due to relative motion blue shift for approaching sources'
    'Cosmological red shift is change of wavelength due to expansion of spcae'

    hmm... kinda makes sense i guess??
    Hope that helps

    Soph
    merci
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    yea I guess that as space expands the wavelength of the light in it expands.. weird though. I guess it's similar to when you have a sound wave travelling through a room, and you suddenly make the room twice as large, the distance between particles - the medium - is increased and so the wavelength will be increased as well. btw I just made that analogy up on the spot so it's probably not too accurate.

    but when the wavelength of light falls, then it has lost energy.. where this energy goes I don't know,
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    (Original post by mik1w)
    yea I guess that as space expands the wavelength of the light in it expands.. weird though. I guess it's similar to when you have a sound wave travelling through a room, and you suddenly make the room twice as large, the distance between particles - the medium - is increased and so the wavelength will be increased as well. btw I just made that analogy up on the spot so it's probably not too accurate.

    but when the wavelength of light falls, then it has lost energy.. where this energy goes I don't know,


    the energy isnt 'lost' is it?
    just the photons dont get given as much energy in the first place.... if the wave has been spread out then so has the energy?
    maybe? i could be wrong.
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    but if you think of the photons, there are the same number of photons as before, none are created or lost. but as the wavelength of each increases, its frequency and therefore energy falls.

    due to convervation the energy must go somewhere. maybe it's lost to the electromagnetic field as the photon escapes gravitational fields?

    maybe I'm confusing this with gravitational redshift (or maybe they are the same thing)
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    You need to consider quantum theory, light is neither a particle nor a wave, so although in a way there are the same number of photons, as the light wave spreads out each point on the light wave has less energy, so the photons don't physically lose energy, but the probability of a photon having a high energy is decreased. Sorry probably not the best explanation but I hope people understand what I'm saying
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    Yeah that makes sense, kinda what i was trying to say, although the way you said about probability is the correct way to talk about it.

    Whats gravitational redshift?

    is it as simple as it sounds?... gravity causing redshift due to force on waves???
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    Apparently a star rotating causes broadening of spectral lines because the left hand side gets blue-shifted and the right hand side gets red shift, with the centre being broadend.... What has roation got to do with the doppler effect???

    Just thought id ask seeing as we're talking about the same kind of thing...

    Soph
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    Since one side of the star is effectively moving towards us it will be blue-shifted and vice versa for the other side
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    Ah yeah, makes sense, cheers
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    medical physics sounds better
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    Nah medical physics is boring
 
 
 
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