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Photon coming from a far away star is redshifted?

  1. Jun 22, 2006 #1
    How do we know that a photon coming from a far away star is redshifted?
    Since we only can measure wavelength here on earth how do we know that its wavelength was shorter when emited?

    thanks
     
  2. jcsd
  3. Jun 22, 2006 #2
    Exicted atoms emit specific, unique spectra. It is these fingerprint-like patterns which are shifted. Since these patterns are conserved, we can determine the redshift.
     
  4. Jun 22, 2006 #3

    LURCH

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    There's a picture at wikipedia;

    http://en.wikipedia.org/wiki/Redshift

    That might help. You can see the pattern of lines in a nearby object (the sun) and compar them to the lines in a distant star. The pattern of lines is nearly identical, and if you take the near object and move all its spectral lines just a little bit toward the red, they match up perfectly with the spectral lines of the more distant light source. The very likely conclusion; the two are emitting the same spectrum of light, and light from the more distant objected is shifting towards the red.
     
  5. Jun 23, 2006 #4
    Basically, we don't.
    Trouble is that these photons set off a very long time ago. Arp says that maybe some of these constants like the Plank constant are time dependent and had a different value a few million years ago.
    After all, we have only been measuring them for 100 years so how can we extrapolate back and say that since they have been constant for the last 100 year or so (- within expermental error) they have been constant for all time?
    E = hf,
    if h changes the frequency changes and so wavelength was different when they set off. hence redshfted
     
  6. Jun 23, 2006 #5

    SpaceTiger

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  7. Jun 24, 2006 #6

    Chronos

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    Most scientists assume the laws of physics are the same at all times and places in this universe. Of course, this assumption is unprovable. But, when you apply that assumption to observational evidence, you get answers that make sense. On the other hand, if you attempt to get cute and 'tweak' things, like h, you get bizarre results that throw everything out of synch - sort of like the string landscape [sorry, I coudn't resist].
     
    Last edited: Jun 24, 2006
  8. Jun 24, 2006 #7

    russ_watters

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    That is not true. Since a look into space is a look back in time and the universe looks the same everywhere, we can say with a high degree of certainty that the laws of physics are the same everywhere and have been for at least most of the way back to the Big Bang. If things like the Planck constant were even a little different a billion years ago, the unverse of a billion years ago would look vastly different from what we see.
     
  9. Jun 24, 2006 #8
    Space Tiger will be proud of me here.
    In the BB, as one looks further and further out one looks further and further back in time. Heavier elements are formed by supernovae explosions and so one would not expect to find iron in distant early, unformed galaxies. And we, followers of the BB, say that that is what you find. That is, you do see aging (or should that be un ageing?) as you look at galaxies further and further away.
    Without it the BB would be incorrect.
    Cheers,
    A truly converted Ratfink.
     
  10. Jun 25, 2006 #9

    Chronos

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    Iron enrichment is not really an issue in the early universe. Population III stars can easily explain that observation.
     
  11. Jun 25, 2006 #10

    Garth

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    Actually it is an issue - the accretion disks of some high-z quasars show an iron abundance 3 x solar, e.g. APM 08279+5255at z = 3.91 whose estimated age is 2.1 Gyr when the universe was only 1.6 Gyrs old (according to LCDM model expansion).
    There are several more high-Fe high-z quasars known.

    Although PopIII stars can explain other high-z metallicity, it is difficult to explain how so much iron formed so early as iron is the last element to be created in the nuclear fusion process.

    This point is emphasised by Drs. Norbert Schartel, Fred Jansen and Prof. Guenther Hasinger in their ESA web-page article Is the universe older than expected?
    What might this be telling us?
    You do not have to throw out the 'z' - 'observed age' relationship, that depends on the cosmological scale factor R(t), which occurs in the Robertson-Walker metric.

    Perhaps R(t) simply needs adjusting? (again!)

    Garth
     
    Last edited: Jun 25, 2006
  12. Jun 25, 2006 #11

    russ_watters

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    I recognize that we see differences in age as we look further away. I wasn't saying we don't.
     
  13. Jun 25, 2006 #12
    So are you taking this back?
    Cheers,
    Ratfink
     
  14. Jun 26, 2006 #13
    Hubbleshift & Dopplershift

    Talking about redshifted radiation in our universe it seems good to me to take into consideration the 2 sorts of shift mechanisms.
    The first one is the Doppler-effect (speed difference dependent) en the second one is the Hubble-shift (distance difference dependent). Whenever the expansion stops, its related frequency shift (Hubble-shift) of radiation during its trip through the universe will be frozen while its related Doppler-effect disappears. Other Dopper-effects of relatively moving radiation-sources will continue.
    I suppose I am right.
     
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