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Doppler effect and expanding universe

  1. Apr 4, 2009 #1
    Astronomers believe that the universe is expanding because of the measured doppler effect redshift of light emitted from distant stars in other galaxies, correct? Do they have other reasons for believing that the universe is expanding? The stars in our own galaxy are known to not be traveling away from us, but do they also red shift due to expanding space as well?
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  3. Apr 5, 2009 #2


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    Not correct. The cosmological redshift is not in any simple sense a doppler effect. It does not correspond to the rate the distance to the thing was increasing on the day it emitted the light. Nor does it correspond to the rate that the distance to the thing is increasing today.
    The cosmological redshift is the cumulative effect of the whole expansion history of the universe during the time the light was in transit.
    It is equal to the ratio by which largescale distances have expanded during the light travel time----minus one.

    redshift = z = expansion ratio - 1.

    So if distances have expanded 7-fold while the light was traveling, the redshift number z = 6.

    Yes! Expanding universe was conjectured, just from Einstein general relativity, in around 1923, long before the cosmo redshift pattern was discovered by Hubble.
  4. Apr 5, 2009 #3
    And this is because the cosmo. redshift is dynamic and changing? Is the current spacial expansion rate measurable in a lab? Would it be theoretically possible that the reason that light is redshifting be due to something else such as some unknown frictional force, rather than expanding space?

    But I thought that Einstein originally thought that the universe was static and only later changed his mind after Hubble's observations were published?
    After Hubble's discovery was published, Albert Einstein abandoned his work on the cosmological constant(...)prevented him from predicting the expanding universe.

  5. Apr 5, 2009 #4


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    Have another look at my post. I didn't say that Einstein figured out the expanding solution in 1923.

    It was a younger guy named Alex Friedman who worked out the expanding universe model and published it in 1923. He took Einstein's 1915 Gen Rel theory and simplified the main equation and solved it and essentially got the big bang.

    Later a second person, George LeMaître got the expanding distances picture.
    Einstein clung to the static, non-expanding picture and so missed the boat.

    Friedman is sometimes spelled with a double N, as Friedmann. Cosmology's main equation is the Friedman equation. Look it up on wiki. There are actually two Friedman equations.
    Brilliant guy. At one time held the world record for high altitude ascent.

    Einstein didn't like Friedman's 1923 paper, said it couldn't possibly be right. Stuck in his own view. Came around later of course---with Hubble results and all.
  6. Apr 5, 2009 #5


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    It is possible in principle, but only if theory is wrong. It is not possible theoretically, if by theory you mean consistent with present knowledge of physics.

    You can postulate "unknown forces" to do anything you like. There's no evidence for them, however. There are known "forces" that will redshift light, but none of them fit the specific observations we have in this universe for the cosmological redshift.

    Yes, that is true. Einstein had to introduce a special cosmological constant term to avoid the expansion that was predicted from his theory, and he did this well before Hubble's observations, precisely because expanding space IS a prediction of general relativity. He just assumed that the rate of expansion was zero and derived as a consequence his "cosmological constant term".

    (Addendum. And as Marcus points out: other physicists worked out the implications of GR in the absence of a presumed stability; all still well before Hubble.)

    The text omitted from your wikipedia quote makes it a bit misleading. He was NOT prevented from predicting expanding universe by Hubble. He was prevented by his own choice to assume that there was no expansion, and introduce the cosmological constant term, all well before Hubble came along. What wikipedia says is as follows:
    After Hubble's discovery was published, Albert Einstein abandoned his work on the cosmological constant (which he had designed to allow for a static solution to his equations). He later termed this work his "greatest blunder" since the assumption of a static universe had prevented him from predicting the expanding universe.

    The full text is clear that it was Einstein's own assumptions that held him back from predicting what Hubble had later observed.

    The full story is a bit complex, because in fact Hubble's original observations got the Hubble constant wrong by a factor of nearly 10. Hubble's initial estimates of the Hubble relation suggested that the universe was much too young. Hubble himself preferred a model that did not involve actual velocities -- either ballistic or an expansion of space term.

    Cheers -- Sylas
  7. Apr 17, 2009 #6
    I think
    Einstein couldn't believe on his general theory of relativity. but friedman believed his theory more than him and general theory of relativity explained that universe can't be static thats why friedman made two postulates ,that explains that universe is expanding and it happened before 1929 whan hubble practically proved that universe is expanding.
    the universe looks identical in whichever direction we look, and
    that this would also be true if we were observing the universe from anywhere
    Even Einstein,
    when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static that he modified his theory to make this possible, introducing a so-called cosmological constant into his equations.
    Only one man, it seems, was willing to take general relativity at face value, and
    while Einstein and other physicists were looking for ways of avoiding general relativity’s prediction of a nonstatic universe, Alexander Friedmann instead set about explaining it
    so he believes more in theory of relativity than Einstein himself.
  8. Apr 17, 2009 #7
    Unless something like "light friction" proved to be true, which was what my original question was about. Since then, I came across http://en.wikipedia.org/wiki/Tired_light" [Broken], which explains some of my questions and happens to explain that it probably isn't true. Plus, even though I don't really understand the math behind it (but still know that its predictions have been proven true in experiments), relativity predicts an expanding universe.
    Last edited by a moderator: May 4, 2017
  9. Apr 20, 2009 #8
    There were several thought provoking articles that hinted at velocity-distance expansion before it became a Hubble credit - The velcoity distance law was implicit it Weyl's 1922 publication. De Sitter exploited the cosmological constant in 1917 to conjure an exponentially expanding empty universe - which was later shown by Howard Robertson to behave somewhat along the lines we now observed if mass were sprinkled throughout the volume. I read somewhere - can't recall the reference, that Einstein himself actually interrogated a few astronomers as to whether there was any data that would indicate the nebula appeared to moving away by virture of some universal motion - - unfortunately, the data that was beginning to be accumulated by Slipher and others had not then been properly interpreted. So he introduced the cosmological constant - but on the other hand - Hubble was not too sure the red shift data really represented velcocity - Perhaps the Law should be renamed - Hubble seems to have received more than his share of the credits
  10. Apr 20, 2009 #9


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    What 1922 publication of Hermann Weyl, pray tell? :smile:
  11. Aug 30, 2009 #10
    You know, technically, because the theory of relativity predicts the expanding universe, and Einstein created the theory, he did predict the expanding universe. But then yes I know he changed it so he could feel better about what it predicted. Just wanted to be a bit anal/podantic.
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