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Homework Help: Silly question about redshift, relativity

  1. Mar 8, 2004 #1
    when we use redshift to determine direction, velocity of distant radiation sources do we not take into account the redshift due to gravitational forces?

    that is, if gravitational fields can lengthen wavelengths of light (as well as bend them) wouldn't more distant objects like far off galaxies naturally be more inclined towards the red end of the spectrum because of accumulated gravitational interference?

    am i missing something?
  2. jcsd
  3. Mar 8, 2004 #2
    Why should they be shifted specifically toward the red end? Using the same argument you implied, you could say that they should be blue shifted, since they're passing stars on the way here!

  4. Mar 8, 2004 #3


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    You can turn the question on its head - if the redshift were due to gravity, how much mass would there be? The answer is, far, far more mass than other observations imply.
  5. Mar 8, 2004 #4
    right, right, i've made a rather large assumption there...

    relativity teaches us that light leaving a large gravitational force will be red shifted and similarly, light entering a large gravitational force will be blue shifted thus we can think of gravity as pulling at light just as it pulls at matter, not effecting it's speed but it's wavelength.

    the large assumption i have made is that much older galaxies will be that much more dense - in fact generally proportional to their distance/age.

    as for gravitational red shift implying much more mass than we observe - isn't that the most fundamental mystery of cosmology at the moment? observing gravational effects that imply much much more mass than we observe as normal matter?
  6. Mar 8, 2004 #5


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    In the Astronomy & Cosmology sub-forum, there's a sticky thread (called A&C reference library?) which includes an extensive discussion of 'dark matter', which is what I think you're referring to. Please take a look at that, it may answer many of your questions.
  7. Mar 8, 2004 #6
    thanks nereid, i will definitely have a look at that
  8. Mar 8, 2004 #7


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    "the large assumption i have made is that much older galaxies will be that much more dense - in fact generally proportional to their distance/age."-- says Billy

    Are you thinking that as the universe ages and expands, a typical galaxy also expands in proportion, and thus becomes less dense? And therefore that as you look at a galaxy farther away, and are thus seeing it as it was in an earlier era, its stars will tend to be closer together?

    In some of his popular astrophysics books, John Wheeler has stressed that atoms, solar systems, and individual galaxies do not partake in the expansion. But distance between typical galaxies, he maintains, does increase over time.
    Last edited: Mar 15, 2004
  9. Mar 8, 2004 #8
    if you think of a galaxy as a discrete gravitational system it is logical to assume that universal expansion has about as much effect on galaxies as atomic bonds, yes, this makes sense.

    i was simply thinking that more older galaxies would have formed in closer proximity to the big bang and so formed in a much denser universe and had much more material to work with.
  10. Mar 9, 2004 #9
    If I'm not mistaken, all galaxies were formed out of very very big gas clouds (or even a big big gas cloud formed as soon as the universe started to cool down), so you could also argue that early galaxies should be less dense.
  11. Mar 9, 2004 #10


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    While a lot of details about how galaxies formed are unclear, and different researchers have different emphases and perspectives (sometimes very different), the general idea is as follows (much detail omitted; caveats apply!):
    - at the time of matter-radiation decoupling (when electrons combined with protons to form hydrogen atoms, ~the surface of last scattering, which we now see as the CMB), some regions of the universe were slightly denser than others, and some were slightly less dense (how this came about is another story)
    - ('density' here means density of mass, which is predominantly 'dark matter'; ordinary matter - 'baryonic matter' - is a minor constituent)
    - the 'overdense' parts would attract more matter, and shrink
    - the baryonic matter part of these blobs would shrink to gas clouds; the dark matter part wouldn't shrink
    - parts of the gas clouds embedded in dark matter blobs would contract and collapse to form massive stars
    - these collections of massive stars in gas clouds in dark matter blobs are 'protogalaxies', and several examples may have recently been observed
    - the massive stars would evolve very quickly, become supernovae, and collapse to black holes
    - some black holes would merge, and form the seed for galaxy nuclei
    - protogalaxies would collide and merge, to form galaxies
    - the expansion of the universe was not great enough to pull the gas clouds and protogalaxies apart (or, gravity was stronger than the universal expansion)
    - as the overdensity regions were of many different sizes, so some galaxies were able to form clusters, and some clusters to form super-clusters (being larger, these took longer to form than the proto-galaxies!)

    [Edit: fixed typo :frown: ]
    Last edited: Mar 11, 2004
  12. Mar 9, 2004 #11
    thanks for all the info nereid, i'm still very interested in dark matter (as a lot of people) and dark energy and really anything that might be affecting radiation. just to clear up - we do NOT take shifting of radiation due to gravitational interference into account because there is nowhere near enough mass in the cosmos to make it a significant variable? even allowing for a liberal estimate on distribution of dark matter?

    and thanks for all the replies guys - i'm just starting to get into this site, there's so much excellent and exciting stuff to read, so many knowledgable people here!
  13. Mar 15, 2004 #12


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    I missed this the first time.
    You appear to be referring to the Sachs-Wolfe effect, "When [photons] pass through a region which contains a relatively greater concentration of matter, they undergo a redshift. Conversely, they undergo a blueshift relative to the average background as a consequence of passing through a region of low density." This link has a more detailed account.
    That's true for galaxies, but not for the CMB, where the Integrated Sachs-Wolfe effect must be considered. Remember that (crudely) a) it's really dense concentrations of matter which will give rise to noticable redshifts (galaxies, on the whole, are largely empty space), b) gravitational redshifts have been detected in astronomical observations, just where they're expected (near black holes).
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