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Question about Cosmological expansion?

  1. Jul 6, 2013 #1
    I've long wondered about an assumption that we have today and I've never found a direct answer to my question.

    Presently we can observe that there is a direct proportionality between an object distance and the factor by which its light is redshifted. We deduce that this observation implies that the universe is expanding.

    But is it really a scientific assumption to be made? Couldn't we equally extrapolate other plausible alternatives to explain an electromagnetic redshift without recurring to cosmological expansion?

    I've addressed this issue a few other astronomers and cosmology enthusiasts that I know, and the conversation always ends with the reference to the CMBR, but the way I see it, the CMBR is an entirely different phenomenon, all the CMBR really implies is that there is an almost uniform radiation being emitted from all directions, hence we infer that some time in the past the universe was a concentrated mass. Or in other words, the CMBR tell us that there was likely a big bang, but it doesn't tell us anything about cosmological expansion. Isn't this correct?
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  3. Jul 6, 2013 #2
    welcome to the forum.

    The redshift isn't an assumption, rather its a confirmed result of expansion. If you doubt expansion is occurring ask yourself the following question, why can we see farther than the radius that the speed of light could travel? The only answer to that is expansion. Scientists have also used various forms of parallax (triangulation techniques) to confirm distances that have been measured by redshift.
    So in truth we don't rely solely on the redshift measurements, rather each measurement made is confirmed through more than one technique. This allowed us to confirm the rate of change in the redshift formulas which is important as it also allows us to develop an apparent magnitude to luminosity relation. Which is another relation constantly under scrutiny. A while back several forum members assisted me in writing this article. In the section cosmological distance scale I briefly describe some of the other methods used in distance measures and the luminosity relation.

    Last edited: Jul 6, 2013
  4. Jul 6, 2013 #3
    You misinterpreted my question, I didn't say that the redshift is an assumption.

    The redshift is what we observe, but the redshift/distance observation is merely that, an observation, my question was why then do we assume that the redshift is an indicative of expansion? Can't there be other phenomenon that somehow naturally redshifts light over distance?

    The only thing that the redshift tell us is that the further an object is, the farther its light has traveled, the more redshifted the light will be.

    When we assume that the redshift implies that the universe is expanding, we are neglecting other factors that might influence electromagnetic radiation over long distances and naturally redshift it. Isn't it possible that other mechanism, other than dark energy, could stretch the wavelenght of light without recurring to universal expansion?
    Last edited: Jul 6, 2013
  5. Jul 6, 2013 #4


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    There have been some proposals. One was "tired light" which has been thoroughly discredited.
  6. Jul 6, 2013 #5
    I've read about it before, the case for tired light was that it would imply blurring of long distance objects.

    What I'm asking is, in a dynamic universe, there are an unfathomable amount of mechanisms that could cause a natural redshift of electromagnetic radiation, tired light was discredited, dark energy was created out of nothing to comply with predictions and observations, but surely there could be other potential alternatives. Isn't it right?
  7. Jul 6, 2013 #6
    Not really, there have been numerous attempts to explain cosmological redshift with other causes such as doppler shift, the article above notes that. I was getting the article when your reply was posted. Other attempts do not fit observational data, such attempts include forms of gravitational redshift, tired light, and forms of doppler shift. Even without redshift we can see expansion occurring visually by comparing distances measured at earlier times to measurements now. Though the change in the historical data is miniscule due to the length of time between those measurements they can be determined with reliable precision. As I stated above, scientist never rely on just one methodology. The redshift relation is a critical relation so is constantly under observational scrutiny and confirmation
  8. Jul 6, 2013 #7

    dark energy is a convenient popular pop media term for the cosmological constant, which is a vacuum pressure or stress energy tensor. You are correct that other factors can cause shifts, in truth separating one form of shift from another is a difficult task. Numerous attempts were made to define cosmological redshift as just another form of doppler shift. However the method used in one of the more reliable methods by A La Singe and Bunn and Hoggs, required numerous changes in observer and remeasure to work. The method does work but the cosmological redshift formula is far easier and flexible at the cosmological scale.
  9. Jul 6, 2013 #8
    But isn't it the case that all observational data that we collect from the cosmos is in essence electromagnetic radiation, if light, or interactions between light sources is all we can see, then it would stand to reason that any mechanism that would naturally redshift light over distance would be indistinguishable to the same observer (us) from that we're proposing as Einstein's cosmological constant?
    Last edited: Jul 6, 2013
  10. Jul 6, 2013 #9
    lets clear up one thing to start with on dark energy, Redshift allows us to measure expansion. Dark energy is a proposed cause of expansion. If expansion were caused by some other factor we would still measure the same redshift. The problem of what causes expansion plaqued scientists for years since it was discovered. In fact dark energy was proposed as early as the late 1900's and was only recently accepted. Same goes for dark matter. Both the dark energy and dark matter proposals were met with a general denial and attempts to find other causes. MOND for example of dark matter. Torsion proposal from Poplowskii, for dark energy as some examples.

    However all the alternate models that attempt to do away with dark energy fail when compared to observational data. You will find this article handy it discuses some of the alternative attempts.

  11. Jul 6, 2013 #10


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    It's not quite that simple. The main thing is the 1915 theory of GR, and the SOLUTION to the GR equation discovered in 1922 by a guy named Alex Friedman living in what is now St. Peterburg.

    GR predicts dynamic geometry, distances and angles between things can change, if expansion gets started somehow it will tend to continue, but can be slowed down or sped up by various things.
    GR has proven amazingly correct every time it has been tested at solar system scale and earth orbit scale, and where possible also at astronomical scale (with neutron stars and gravitational lensing of light, multiple images caused by light bending).

    So GR predicts expansion as one possible solution, and GR has proven so much better than Newton that people tend to trust it. Even before Hubble redshift was observed.

    And then Friedman came up with his solution to GR which assumes more or less even distribution of matter to make things simpler to calculate and that has given an amazingly good fit to a huge amount of data.
    So all working cosmologists use the Friedman equation (a simplified easier to use version of GR).
    And Friedman equation totally predicts expansion or else contraction but that case is obviously not happening. That feature is an essential part of the model, which is basically a formula for the expansion rate H (or alternatively the contraction rate, in a different case).

    Now 1922 was before Mr. Hubble observed the redshift pattern and way before people detected the CMB.

    So you could say that expansion is a very natural thing which some people were already coming to suspect might be happening. The observation of redshift in the 1930s CONFIRMED this suspicion. And indeed the detection of CMB around 1960s was already EXPECTED. It had been predicted on theoretical grounds using the Friedman equation model which had built-in expansion.

    So what you have is a really good theory (gr) of how gravity works---namely by geometry being a dynamic changing responsive field---and it has been checked hundreds of different ways not directly related to cosmological expansion. If you want to throw out GR then you should propose a better theory of gravity.

    And our theory of gravity (as dynamic geometry interacting with matter) teaches us to EXPECT expansion as one possibility, contraction would be another but it doesnt fit the data.

    and the particular solution to GR that cosmologists find gives the best fit, namely the Friedman model has been confirmed in different ways over and over. There is a list of independent types of observation that work together in support of expansion (which is what people were tending to expect anyway before some of those types of observations were made---like.eg. the abundance of various chemical elements, I don't remember all the details offhand.

    GR is in the process of being replaced by a more up-to-date (quantum) theory of geometry, of which there are several rival candidates But the replacement prospects share basic features like expansion distances and wavelengths and existence of black holes and the bending of light etc. Those are well-established features which are part of legacy of GR, not likely to go away even when the theory of how geometry&gravity work is modernized.
  12. Jul 6, 2013 #11
    So, in other words, cosmological expansion is proven, not directly from redshift itself but rather by the consistent verifications and implications of General Relativity?

    But isn't it equally true that General Relativity has as of yet failed to consistently explain phenomenon on large scale structures, such as galactic rotation and cluster filaments?

    General Relativity consistent explains all gravitational interactions in a localized scale, but there are observations that as of yet cannot be explained by General Relativity, not without the creation of special variables.
    Last edited: Jul 6, 2013
  13. Jul 6, 2013 #12


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    Sure! There are always puzzles and theories are always subject to revision. The Standard Model of particle physics is also enormously successful, but it also leaves some questions unanswered.
    GR predicts the existence of DM, and that's good! It's good for it to make yet another prediction. Now we can check various ways to see if DM exists.

    You don't throw a successful theory out (which so far has a track record of making predictions which were later confirmed) simply because it makes one more prediction.

    Keep an open mind and wait and see whether or not DM is confirmed.

    And if it is not, then what? There are proposed variant theories of geometry/gravity which don't involve DM, modifications that fit the galaxy rotation curves by other means but they also involve expansion.

    I think 20 years ago there were people seriously working on "tired light" and other ideas for mechanisms which might produce comparable redshift without distances expanding. But each idea they came up with failed. So what you are talking about went out of fashion. Now there are researchers challenging GR and working on variants or replacements, but they have given up on non-expansive explanations of redshift (that was tried a lot and failed). The ideas for alternatives tend to aim at other goals, like getting rid of the need for DM. (even that has not been going so well) or like explaining the slight acceleration in some other way besides a cosmological constant.
  14. Jul 6, 2013 #13


    Staff: Mentor

    Please stick to science. Not philosophy or religion.
  15. Jul 8, 2013 #14


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    The only physics that explains redshift is either kinematical, gravitational, or cosmologically based - all of which are strongly confirmed by observational evidence. No alternatives have observational support to date.
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