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Red Shift - From Dark Material Or Recession?

  1. Mar 22, 2006 #1
    Hi! I'm new here and I have a question I hope you can help me with.

    I'm curious about the conclusion that our Universe is expanding and ask if the evidence of Red Shift is reliable enough - or, could the apparent Red Shift be caused by interference from Dark Energy/Matter distorting our optics or slowing down light waves?

    For example, look at a distant ship through a pane of glass and you will see it clearly enough - but should the ship be further away and there be panes of glass every few metres to look through, how accurate will the image of the ship be then. Project that the ship is a galaxy on the edge of the observable Universe and an infinite number of panes of glass in the way - but the panes are now particles of Dark Energy or Dark Matter that make up to 96% of the Universe - how clear and accurate will the image be then? May the increasing build up of dark stuff (more of it in the way the further out you look) possibly slow down the light waves, so creating a Red Shift caused by interference which we may think is caused by a speed up of universal expansion? If so, then the greater the distance to the object being observed the correspondingly greater would be its Red Shift caused by the extra dark stuff in the way, but there may be no acceleration or expansion really!

    There are probably all sorts of reasons why this scenario doesn't work but I'm curious as to what they are. Therefore, please can someone explain them to me in layman's terms so I can understand. Thanks.
  2. jcsd
  3. Mar 22, 2006 #2


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    Well, for starters, light doesn't ever slow down. It always travels at c in a vacuum. (Since you're essentially proposing an ether, light would always travel the same speed in that ether.)

    Second, redshift isn't caused by any variation in the speed of light. Redshifting, as the name implies, refers to a change in the frequency of light towards the red end of the spectrum.

    Third, intervening material will certainly change the spectrum of starlight shining through it. For example, interstellar dust clouds attenuate certain parts of the spectrum, and make some stars appear redder than they otherwise would.

    However, this form of absorption reddening is a very different mechanism that that responsible for what is called 'redshift.'

    You are hopefully familiar with atomic spectra; when you excite atoms of hydrogen, for example, they only produce photons with a specific set of wavelengths. These specific wavelengths form a "signature" for the element. Even from billions of light-years away, the relative spacing of these wavelengths stays the same, so we can identify this "signature" from very distant objects.

    What we find is that all of the spectral lines are still there (implying that there is very little absorption happening), but they're all shifted downwards in frequency. There's no way an absorption process can cause that, but it's easily explained by the expansion of the universe.

    - Warren
  4. Mar 23, 2006 #3


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    Hi Seraph316, welcome to PF!

    Chroot gave a nice explanation to your well reasoned question. I merely wish to expand upon it. Dark matter and dark energy do warp the curvature of space, thus conspiring to create redshift. And this is exactly what we observe. Light blithely travels along null geodesic paths throughout the universe at the same old boring speed of light, but, the curvature [mainly due to the stretching of space] of the universe causes it to behave badly compared to local clocks. That is called redshift.
    Last edited: Mar 23, 2006
  5. Aug 12, 2009 #4
    So if space does indeed act like a prisim or lens for electromagnetic radiation, bending it this way and that, what absolute proof do we have that the redshift we see is NOT caused by that process?

    Do we actually have concrete proof that the Universe is expanding?

    Or is it based on the fact that dopler shift is the easiest/most convenient explanation?

    Is it possible that the CMBR is simply radiation from outside our observable universe that has been redshifted to almost infinity? If the furthest galaxies we see are so heavily redshifted, what would happen to radiation that comes from far beyond them? If one assumes that space does act like a prisim or lens, would it not make sense that radiation from beyond them would be even further redshifted?

    I hope these are logical questions?
  6. Aug 13, 2009 #5


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    It is illogical to perceive the CMB as photons reaching us from outside our observable universe.
  7. Aug 14, 2009 #6
    Hmm I'm not so sure, the first indications from the MAGIC telescope are that 'the fabric of space-time is not silky smooth as Einstein and many others have presumed, but rough, turbulent and fundamentally grainy stuff'.

    If this turns out to be true then it brings us back to the OP's question; could redshift be caused as a function of space itself, not recession? In other words, as we look backwards we can only see a given volume of space with increasing redshift towards the edges caused by space itself, not recession. But that does not preclude the fact that there might be stuff outside our observable bubble, merely that we are unable to detect it yet.

    I think the variation we see in the CBR could be a function of differing degrees of redshift distortion depending upon in which direction we decide to look. As another guess I think the data from MAGIC and the CBR map might support this, ie- vector with more turbulence = area of lower CBR temperature, and vica versa.

    Meh, be interesting to find out.
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