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Could anything else cause redshift?

  1. Mar 21, 2010 #1
    I know about cosmic expansion, but I was just curious, could the observations of redshift be caused because space is cooling, not expanding.

    From what I understand, as things cool they redshift.

    If the universe started cooling after the big bang, wouldn't everything we saw from the early universe be redshifting?
    Last edited: Mar 21, 2010
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  3. Mar 21, 2010 #2

    Vanadium 50

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    That's not the case. Spectral lines don't move with temperature.
  4. Mar 21, 2010 #3
    Since it is possible to redshift light with gravity, why do we think the universe is expanding just because we see redshift in distant galaxies. Couldn't the redshift be caused by all the gravity between us and the distant galaxy? Is there other evidence that the universe is expanding?
  5. Mar 21, 2010 #4
    I think it's a good question, as there's usually more explanation on this topic than there is understanding.

    The Big Bang model predict cosmic microwave background radiation (CMBR). The predicted CMBR has been found.
  6. Mar 21, 2010 #5
    See the thread on Curvature Cosmology for a possible alternative.
  7. Mar 21, 2010 #6


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    The light isn't going away from a large source of gravity, it is just going through space, passing many sources of gravity. It'll be blueshifted and redshifted and overall end up about what it was when emitted (if the universe is static and the mass roughly uniform).
  8. Mar 23, 2010 #7
    Cooler objects apear red, hot objects appear blue, wouldn't a cooling object be turning redder? Isn't that the definition of redshift? Shifting towards red light? Isn't that how we can tell that certain stars are hotter then others?

    I could be wrong, but I can't find anything that says otherwise. If you can cite something that specifically states that temp doesn't effect light that would be very helpful.

    Thanks :)
  9. Mar 23, 2010 #8


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    In particular, what we actually see shifting is spectral absorption or emission lines. These lines happen at a very well defined wavelength, which is INDEPENDENT of temperature! So when we see them systematically redshifted, it's a good bet that they're receeding.

    There is some truth in the statement that cooler objects appear redder. This is because of the black body spectrum curve, which, for lower temperatures, peaks at lower wavelengths. But either in a 40000K star or a 4000K, the h-alpha line is still going to be at 656nm, regardless of the surrounding black body curve.
  10. Mar 23, 2010 #9


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    To illustrate what Nabeshin is talking about, check out the attached image. It shows an un shifted and shifted spectrum. The bright lines are the emission lines for an element. Note how they move towards the red end in the bottom, shifted spectrum.

    Reddening of the light due to cooling would result in the spectrum being dimmer at the blue end, and thus looking redder overall, but the emission lines would stay where they were.

    Attached Files:

  11. Mar 25, 2010 #10
    Thank you, I think I understand the difference now. Glad I posted ;)

    Although all the answers here were very good, I think this one from Yahoo was also good and thought I would add it for completion.

    I think you are confusing things like the red giant portion of a star's life cycle with the Doppler effect, which is a different issue. When a star grows cooler, it is true that its color fades into the red end of the visible spectrum. But the individual discrete spectral lines that represent the energy changes within each element remain fixed. The red ones become more prominent (more low energy red photons, fewer high energy blue photons) as the temperature changes, but the signature frequency of the individual lines for each element do not change.

    When a Doppler redshift occurs due to recession, the entire spectrum changes - so a specific element that was characterized by a discrete and identifiable pair of lines in the green portion of the spectrum can now be found - looking identical, but shifted into the red portion of the spectrum. The whole spectrum then looks like a similar star that shows no relative motion, but the entire set of spectral lines is shifted downward in frequency. That's different than the simple change in the nature (amplitude) of individual photon wavelengths characterized by cooling.
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