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Is the universe still expanding?

  1. Sep 11, 2005 #1
    When the HST looks out into the far off regions of space, the data it recieves (the light from distant galaxies etc.), has been travelling for millions of light years. When this data shows red shift, and therefore that the source of the light is moving away from us, does this mean that at the time that light was emitted (millions of light years ago), its source was travelling at the velocity indicated by the red shift, or does the red shift indicate the velocity of the source of the light at the present time? If we can only measure the velocity from the time the light was emitted, then how do we know whether the source is still moving away from us or not?
  2. jcsd
  3. Sep 12, 2005 #2
  4. Sep 12, 2005 #3
    I get the basic gist of the doppler effect, but what i want to know is, does the motion of the light source affect the wavelength of the beam of light along it's whole length, considering that we are talking about a beam of light that extends through millions of light years of space. As an example, if the source of light suddenly disappeared, it would be millions of years before we would know that it had disappeared. Would we still see the red shift and assume that the source of light was speeding away from us?
  5. Sep 12, 2005 #4
  6. Sep 12, 2005 #5
    Ahh!! Now it makes more sense, Thanks. When you understand that it's the space between us and those distant objects that is expanding rather than just the objects themselves moving away from us it's easier to see why the whole length of the light beam is stretched. I think i will learn a lot from these forums.
  7. Sep 13, 2005 #6
    I have been thinking about this subject some more, and referring back to my second post, if the source of light did disappear, (explode or collide with another star etc.) what would happen to the light that we observe from that source?
  8. Sep 14, 2005 #7


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    what was already on its way would keep coming as if nothing had happened.
    we would only learn of the disappearance, or explosion, or collision, later when the light from THAT event had a chance to reach us
  9. Sep 14, 2005 #8

    So we would still assume that that object was there, and that it was speeding away from us at the velocity indicated by the redshift?
    Is most of what is known about the universe based on similar assumptions.
    What i mean is could much of what we have observed out there be long gone, or are there other ways of telling whether these objects are still there?
  10. Sep 14, 2005 #9


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    who is WE?

    astronomer/cosmologists would not need to assume that
    their models only require you to assume what is consistent with the laws of physics

    (what is required by physical law is already a bit fuzzy, there is broad agreement on a lot but some disagreement and some gradual change in the consensus view)

    If a star explodes, and a certain amount of energy escapes as light, neutrinos, etc. then (conservation of mass-energy law) whatever doesnt escape is going to still be there as material remnants, a cloud of crud, a neutron-star cinder etc.

    the models cosmologists use DONT DEPEND ON THE MATTER BEING IN PARTICULAR FORMS , they are mathematical models that dont need to assume that a particular object remains that object----the atoms and particles are still there regardless of what arrangement they happen to be in.


    so I guess the answer to your question is NO we dont need to assume details that we dont actually know. The models are mathematical and not verbal. You just plug in what you have observed---from the light that already reached us----and you assume some math equations called physical laws----and you draw conclusions from that.

    the conclusions are of a limited qualified nature. you cant deduce with certainty things you havent seen yet, only make predictions about what will be observed over there in the future "we saw it explode so that glowing expanding cloud of crud that we see now, if nothing has interfered with it, is probably still out there glowing and expanding according to the laws, and we can predict that we will still see it tomorrow"

    then if ever anything happens that goes against prediction, everybody gets very excited because it is a chance to revise the laws!
    Physical laws are used to make predictions until they are proven wrong (or of limited applicability) by making a wrong prediction and can be replaced or corrected by improved laws.

    Fortunately or unfortunately, the laws only change very slowly. newton's laws are still more or less true (only slightly modified by einstein) and keep on being used, and keep on working. one just has to use a little common sense about not applying them to some extreme situations where they need relativistic correction.

    that is good in the sense that a great deal of stuff is routinely predictable

    but it is less fun for the theorists because they get their kicks when experiment and observation proves that some law is wrong and then the theorists get to look for how to tweak it so it will fit the data better, or replace it by some more clever law.

    I am not all that knowledgeable about this -----basically your question goes into Philosophy of Science, or Foundations of Science, issues. So maybe somebody else here will answer in some more satisfactory way. this is the best I can do at the moment anyway.

    ========SUMMARY OF RESPONSE=====

    NO, if "we" means scientists, then we do not claim to know about future observations, we know only what we have observed.

    we make a lawful model that fits the past observations

    and then we use that model to PREDICT about future observations

    and that then serves to TEST the model and the laws that went into it

    and if future observation goes against the predictions then everybody has fun because they get to refine the model and maybe even modify the laws.
    but that doesnt happen very much because the laws have gotten pretty good and keep predicting right.
  11. Sep 14, 2005 #10


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    that "speeding away" is not such a good verbal image
    did you read the Sci Am article by Chuck Lineweaver and Tamara Davis about expanding universe? it clears up a lot of common confusion
    we have some links to that around here somewhere.

    how do you imagine the recession velocity is "indicated" by the redshift?

    if you think you can calculate it as a doppler effect, with the doppler shift formula, then you have misunderstood expansion

    if you want to read that recent Sci Am article, ask and someone will get links----or you can probably google with Lineweaver Davis and get it
  12. Sep 14, 2005 #11
  13. Sep 14, 2005 #12


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  14. Sep 14, 2005 #13


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    Simetra, see if these work!
    MAIN Lineweaver and Davis article in March 2005 SciAm

    SIDEBARS with pictorial diagrams and a question together with right and wrong answers explained.

    What kind of explosion was the big bang?

    Can galaxies recede faster than light?

    Can we see galaxies receding faster than light?

    Why is there a cosmic redshift?

    How large is the observable universe?

    Do objects inside the universe expand, too?

    If these links dont work because they have gotten old, please let me know.
  15. Sep 15, 2005 #14
    Life becomes much simpler if you assume Hubble was wrong.
    That the universe is not expanding!
    That the red shift we see is partly due to the earths rotation on its axis,
    round the sun, orbits the centre of the milky way.
    We are always moving away from a lot of the sky.
    But REd shift is also caused by light passing through gravity fields, light slows down.
    Shapiro effect . Compton effect , Einstiens weak light.
    The universe is infinite and not expanding!
    Last edited: Sep 16, 2005
  16. Sep 15, 2005 #15


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    Indeed, ignorance is bliss.
  17. Sep 15, 2005 #16
    The majority of people in the world now believe the big bang is a fact,
    they are ignorant.and do not look at evidence to the contrary.
    They fell safer in a finite universe with a beginning .
    Infinity frightens them.
    Nevertheless the universe is infinite.
    nasa is coming to the same conclusion by lokking at the geometry of the observable Universe.
  18. Sep 15, 2005 #17


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    Please, share with us this evidence you're so knowledgable of.

    NASA has not found anything inconsistent with the Big Bang. The observable universe is flat, exactly as we've expected for over 20 years.
  19. Sep 15, 2005 #18

    Yes, please do....I'm all ears (eyes).
  20. Sep 16, 2005 #19


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    Evidence favoring Big Bang theory: The cosmological microwave background radiation, primordial elemental abundance, WMAP power spectrum. large scale structure of the universe... Big Bang theory does not require a finite, or infinite, universe.
  21. Sep 16, 2005 #20
    If scientists are so blissfully happy, why are they so paranoid
    when the big bang THEORY is critisized?
    In reality they are afraid of infinity because it cannot be described mathematically.
    The present math is inaccurate and innappropriate to infinity.
    They are also afraid of the idea of no beggining!
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