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How do we really know the universe is expanding?

  1. Jan 23, 2007 #1
    I know about the blue shift of all the galaxies that we see, but does that really prove that galaxies are expanding, or just that something in the intergalactic space is causing a blue shift in the light from distant galaxies to ours?

    Thanks, and I hope this is the correct forum!
  2. jcsd
  3. Jan 23, 2007 #2
    You mean red shift :P
  4. Jan 23, 2007 #3

    Gib Z

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    Something in space must be blue shifting light thats coming in from ALL directions huh, and it must be doing it more the further away the lights coming from huh?

    Even better proof, General Relativity predicts it
  5. Jan 24, 2007 #4
    Hi guys, it is red-shift!
    As the universe expands, the light waves got stretched to a longer wavelength and turns red.
    That is the mainstream relativistic view.
    There is an alternative view called the tired light theory: the universe does not expand, the light photons collide with interstellar material and lost energy - got tired - by quantum theory the frequency dropped to red color.
  6. Jan 24, 2007 #5


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  7. Jan 24, 2007 #6


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    Additionally to redshift, there are other proofs for the expansion of space, as for example the cosmological time dilation of the supernova luminosity curves, the Tolman surface brightness test, the number density of galaxies and their evolution with distance, the variation of the temperature of the cosmic microwave background, the cosmic microwave background itself, the light elements abundances, etc. All of these are based on the validity of general relativity. However, there exists modifications to general relativity that are beyond the accuracy of the current standard tests, in which the expansion of space can be interpreted as a phenomenon dependent on the standards of measurement (alternatively one can interpret a contraction of masses instead of expansion of space). See for example this.
    Last edited: Jan 24, 2007
  8. Jan 24, 2007 #7
    Thanks gurus!
    I have done some checking and I can now understand :-
    a) cosmological time dilation of the supernova luminosity curves . (This one is best for demonstrating relativity!)
    b) the Tolman surface brightness test

    Can you explain how the number density of galaxies demonstrate expansion of the universe?
    Thank you.
  9. Jan 24, 2007 #8


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    The number density of objects for a homogeneous and isotropic distribution of light sources under a limiting observed flux is different in an euclidean, static and flat, space than in an expanding flat space. This is sometimes called log N - log S test and at least in principle it can be performed to tests the geometry and cosmological parameters. I can provide some mathematical details if you are interested. However, honestly, I am not sure whether it can be performed successfully, because it assumes completness of the population taken as a sample and also non-evolution of the population and its luminosity. I am sure that all other tests I have mentioned have been actually successfully performed, but not about this one.
  10. Jan 24, 2007 #9
    Hi Everyone,

    Sorry for putting blue shift... i was talking to my friend about it as i wrote, and we were talking about the Andromeda galaxy as I was typing, so it messed me up, but anyway... These answers are way too filled with big words and vague terms. Can someone explain some or all of the main other pieces of evidence for universe expansion, but in a simple way?

    For example if I were going to explain the theory I mentioned I would say something like the following.
    Many astronomers think that our universe is expanding because nearly all of the light that is coming from observable galaxies is red-shifted. When light is red-shifted, it means that the source of the light is rapidly moving away from the observer as it emits the light which stretches out the wavelength and causes the light to move toward the red end of the spectrum. Because almost all of the galaxies we see have this effect, it seems that they are all moving away from us, which only makes sense if the entire universe is expanding.

    There are way more complicated ways of saying the above explanation, but this lets anyone who is reading it understand the basic idea of what is being said. Are there any other pieces of evidence that support the expansion of the universe theory that can be explained like this?

    Thanks Very Much.
  11. Jan 24, 2007 #10


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    Hi cdm1a23,

    You may be interested in reading more about the cosmic microwave background radiation and density variations in the early universe. More over, you may also want to read about the Theory of Inflation (cosmic inflation).
  12. Jan 25, 2007 #11
    It might be important to understand that the 'vague' words that you are alluding to, are actually fairly precise, operational definitions. It is also important to understand that much of what we know is derived both mathematically and empirically through observation and that most of these concepts are quite complex and intricate. It is often misunderstood it seems, by many people, that they can learn a few very simple concepts and then try to use that as a basis for proof. However, the nature of proof is extremely rigorous, formal, intricate and elegant.

    At a certain point, you will have to learn what these words and concepts mean, how they relate and what their impact is (and perhaps some mathematics wouldn't hurt). This is probably best achieved through reading and thinking or through a formal course. It is also important to understand that at a certain point, without having an understanding of some mathematics and how they model physical systems or phenomena, understanding physics can prove to be difficult.

    I am not certain how deep you would like to extend your physics endeavour, however, if you are interested, I am sure some of the members here can offer to you some introductory reading which might help construct a more accurate cognitive model for physical intution.
    Last edited: Jan 25, 2007
  13. Jan 25, 2007 #12
    I do not claim it is wrong but I am skeptical that the expansion of the universe is a done deal.

    Why would space-time expand in only the spatial directions and keep the temporal direction intact? It implies some preferred fame.

    And furthermore spatial expansion must be equivalent with temporal contraction if one wants to take GR seriously. So where is the "obviousness" that it must be the spatial dimension that is expanding and not the temporal dimension that is contracting or any mixture in-between?

    It is my impression that there is an enormous lack of skepticism about expansion of the spatial dimensions.
    While only skepticism doesn't get you anywhere, a total lack of it can make a community blind.

    Where am I wrong?
  14. Jan 25, 2007 #13


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    The cosmological principle for the matter distribution in space but not in time imposed as a condition in general relativity leads to the cosmological solution of an expanding or contracting space. There is a preferred frame in the sense that it observes the universe to be homogeneous and isotropic but its valid as any other. The cosmological principle has a lot of observational support (microwave background, x-ray background, quasar distribution, galaxy surveys, peculiar velocities, etc.), and of course there you have all other proofs of expansion that I have mentioned. I am not aware of any reasonable cosmological model today that considers static space.

    Why should it be equivalent to temporal contraction?
    Last edited: Jan 25, 2007
  15. Jan 25, 2007 #14


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    To the above explanation you might change "many astronomers" to "most astronomers" or perhaps even something stronger. In addition, you might add the following if you want to get into more detail.

    Some people (including some early astronomers such as Zwicky) have suggested that the red-shift we observe is not due to the galaxies moving away, but due to the light losing energy on its way from these distant galaxies to the Earth. This class of theory (that light loses energy on its journey) are known by the name "tired light theories".

    While "tired light" can explains the observed redshift, it does not explain other observations that have been made since the days of Zwicky.

    For instance, distant supernovae appear to happen in "slow motion" compared to nearby supernovae. This is an expected result if the red-shift is due to relativistic effects, but cannot be explained by tired light theories. There are other problems with tired light theories, for instance, most such theories predict a bluring of distant objects which is not observed, they also have problems correctly predicting the brightness of distant galaxies.

    If you want to add some sources for the above I would suggest

    http://www.astro.ucla.edu/~wright/tiredlit.htm for casual readers

    and the following arxiv papers for less casual readers
  16. Jan 27, 2007 #15
    Thank you to everyone for your help and information. I appreciate those who tried to "dumb it down" a little.
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