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Cosmological Observations Conundrum

  1. Jan 24, 2013 #1
    Perhaps I am missing something obvious, but here it is anyway.

    Observation of an astronomically distant object is essentially observation of a past condition of the object, due to the fixed speed of light. And IIRC, the Hubble constant seems to say that all sufficiently distant objects recede from the observer (any observer, supposedly anywhere in the universe) at a rate of about 77.3 Km/sec per mega-parsec of distance, or 77.3 Km/sec per ~3.28 million light-years.

    So if we look at a galaxy 3.28 million light-years distant, we see it receding at 77.3 Km/second. But what we are really seeing is that it was receding at 77.3 Km/second 3.28 million years ago. If we see a galaxy many times that distance receding many times faster, we are (of course) really seeing it much earlier in time. Although Hubble's constant is a d(v)/d(s) value, acceleration is not defined as d(v)/d(s), it is defined as d(v)/d(t).

    If the observed recession velocities of galaxies are plotted as a function of time, not distance, the galaxies were decreasing their recession velocities as time progressed.

    What does that say about the expansion of the universe "accelerating?"
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  3. Jan 24, 2013 #2


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    Hi wstrohm. Ned Wright's Cosmology Tutorial is one of the most authentic, respected, and well-written sources for information on this subject. You may find a satisfactory explanation at his website. See this FAQ: “Why do we think that the expansion of the Universe is accelerating?”


    (edit) If you do not get satisfied using Ned Wright's site, do return here with your doubts and questions. Some members here are highly qualified to answer your doubts/questions.
  4. Jan 25, 2013 #3


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    Hubble law is a statement about today's rate of expansion of today's distances. Where did you get the 77.3 figure? I don't recall ever having seen it before. Nowadays some people are using numbers around 70 and others using 71 and some higher but nothing as high as 77, that I know of.

    Hubble law relates the instantaneous distance (if you could stop expansion TODAY to get time to measure it) to the rate that distance is expanding right now, this very day.

    The Hubble rate actually evolves over time according to a form of the Einstein equation. The rate at some time t in the past (call it H(t) ) is the corresponding thing for that time. the distance THEN related to the rate it was increasing THEN.

    So the cosmology model give you a whole past history of the instantaneous expansion rate. that is what determines the predicted redshifts. And that is what is required to fit the observational data.
    Last edited: Jan 25, 2013
  5. Jan 25, 2013 #4


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    What it says is that we have to be a bit careful when understanding our observational data. When the light from a far-away galaxy was first emitted, the expansion rate was faster but things were closer together. We have to incorporate the rate of expansion over the entire period from the time the light was emitted to now to determine how the distance we measure is related to the object's redshift.
  6. Jan 25, 2013 #5
    Thank you all for your thoughtful answers and references. I have looked at Ned Wright's paragraph in the link above, which for me raises more questions. I was thinking of our basic observations, assuming only speed of light and recession velocity (due to red shift), not even considering whether distance has been determined accurately.

    Faraway objects are seen moving away faster than close objects, which inherently means objects were at more ancient times moving away faster than objects at more recent times.

    No matter whether the Hubble dv/ds value is a constant over time or not, no matter whether it is correct or not, the curve of recession velocity with time has a negative slope. The curve may be linear or concave upward or even convex upward, but the slope is always negative, if plotted (between all the "snapshots" of our astronomical observations) versus time. I am leaving out altogether the question of accurate distance.

    If my text is unclear, I apologize. Think of what I am writing in this way: Consider the red shift of a galaxy as a dot on an x-y plot. Time flows in the +x direction and distance in the -x direction. On this graph, two sets of units or x-axes can be overlaid... light-years for distance increasing to the left, years for time progressing to the right. And because of the choice of units (years and light-years), they are numerically equal at any point. Recession velocity increases in the +y direction. The plot will show the highest red shifts/velocities on the upper left and the lowest red shifts/velocities on the lower right. The curve's y-values increase to the left (per Hubble) and decrease to the right (with time).

    The following is all IMHO and subject to change if convinced otherwise:

    The logical conclusion from the above is that recession velocities were decreasing as time flowed forward.

    Also I question Ned Wright's premise that we can ever know a galaxie's distance "NOW," by any means whatsoever. We can never know anything about an object in space at any more recent time than at that time at which light left it.
  7. Jan 25, 2013 #6


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    The problem with that statement is that there is no unique definition of recession velocity, because there is no unique definition for the velocity of a far-away object.
  8. Jan 25, 2013 #7
    Isnt t=0 the moment the scale factor goes to zero and Gr gives infinite answers for pressure, curvature, density etc?
  9. Jan 28, 2013 #8
    You are right for the first 5bn years when galaxies were being formed and you are wrong for the last 5bn years when expansive dark energy has become much stronger than gravity in intergalactic space. The "cosmic jerk" took place between 5bn and 8bn years ago. Read about it here:


  10. Jan 28, 2013 #9

    Fascinating article, mostly over my head. But note that in the range of -5 billion years (light-years) to present, objects still appear to be slowing their recession velocity from us as time flows forward. I think I have a basic lack of understanding of time itself, maybe...

    Thanks for your response!
  11. Jan 28, 2013 #10


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    I think the source of your confusion could be that you equate measuring an object's redshift with measuring its recession speed at the time it emitted the light.

    That would give a decent approximation for comparatively nearby objects like those studied by Hubble in 1930s and 1940s but overall it does not work. The redshift is NOT determined by the recession speed at the time of emission. So someone who equates those two would inevitably get into a terrible mess of confusion.

    I am only guessing. I can't be confident I understand what you have been saying or that I understand your process of reasoning.

    However if that is what you are doing, if that is the basis of the problem, then people can easily help by explaining what the observed redshift actually represents mathematically!

    1+z = received wavelength/emitted wavelength

    1+z = distance now/distance then
    (now = when light is received, back then = when light was emitted)

    The 1+z ratio is the proportion or ratio by which both wavelengths and distances have increased while the light has been in transit. The convention is to subtract 1 from this ratio and call that "z" the redshift number.

    Cosmology is about fitting a math model (of the expansion history) to the observational data and getting the simplest model with the best fit. In that sense it is holistic. You have to make the model fit ALL the data. That is how, for instance, distances of things at various times now and in the past are determined. They are what they have to be in order to get the simplest best fit to the data which we can observe.

    Here is a picture of one object's recession history as you vary the model parameters
    the dashed curves and the thin-line curves are what you get from adjusting the parameters differently.
    The dark heavy curve is what you get when you adjust the two key parameters how we think is right. The model is derived from 1915 Einstein GR equation, which is our law of gravity (and geometry). There is a lot of confidence in GR because it has been checked and rechecked in many ways at many scales and gives amazingly accurate predictions. The model, once it is derived from GR, has very few adjustable parameters. You can see in the Figure 14 that there are basically just two main numbers being adjusted.

    What you see there is the two-decimal-place best fit to ALL the observations as of 2003 when the article was published. At that time the central values of the confidence intervals were .71 and (.27, .73). In the figure you can see the .71 down at the bottom and the (.27, .73) in the legend at the upper left corner labeling the dark split curve.
    Last edited: Jan 28, 2013
  12. Jan 28, 2013 #11
    Last edited: Jan 28, 2013
  13. Jan 28, 2013 #12


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    Computing the eventual positions is, for the forseeable future, impossible. Basically what you'd be asking for here is to run a massive N-body simulation without knowing the initial velocities.
  14. Jan 28, 2013 #13
    Thanks Chalnoth, good to know. Its a bit like seeing a picture map of Gondwana land and trying to work out from this where your house is relative to NYC. They should make this clear more I think.
    A very nice pic though:


    Can we at least produce a rough version of the above which takes into account the hubble expansion of space? It should be just based on how distant objects are away from us.
    Last edited: Jan 28, 2013
  15. Feb 1, 2013 #14
    Marcus, you always seem to be focused on the heart of the issues at hand. This post is no different. Fig 14, comparing the recession history of an object using different values for the model's parameters is instructive.

    (The following has been corrected from the original post)

    The ΛCDM model, the one employed to interprete the data in the paper, "Baryon Acoustic Oscillations in the Ly- forest of BOSS quasars", by members of the SDSS III group referenced in Johninch'sr post, is based on GR.

    In the model employed by SDSSIII, Λ (Lambda) stands for the cosmological constant which is currently associated with a vacuum energy or dark energy inherent in empty space. This is the plug-in is used to make the model fit the parameters of the current accelerating expansion of space against the attractive (collapsing) effects of gravity from matter. In that model, the cosmological constant is interpreted as the fraction of the total mass-energy density of a flat universe that is attributed to dark energy. Currently, about 73% of the energy density of the present universe is estimated to be dark energy.

    That is, according to the ΛCDM model, the amount of dark energy is a direct function of the metrics governing the accelerating expansion of the universe, and therefore, our understanding of the need for/role of dark energy/matter in the model is intimately dependent on our measure of the expansion based on the cosmological redshift.

    In essence, everything we think we know about dark matter, dark energy and the expansion of the universe is entirely dependent on our determination of the value of the Hubble constant and the behavior of EM across cosmological distances.

    The paper by the SDSS III group is very interesting for the unique techniques used to measure the absorption of light from quasars by the intervening Ly-α gas clouds.
    Last edited: Feb 1, 2013
  16. Feb 1, 2013 #15


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    Sorta kinda. Our measurements of all of the parameters are interrelated, and any significant change in one of them impacts all of the others.

    However, that said, we now have quite good measurements of the expansion rate.
  17. Feb 1, 2013 #16
    It's good to see the return of phlogiston... (j/k)
  18. Feb 1, 2013 #17


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    We don't seem to be communicating. I did not reference a SDSSIII paper. I did not refer to "Baryon Acoustic Oscillations in the Ly-forest of BOSS quasars" which you say I referenced.

    I also did not make any reference to "vacuum energy" or "dark energy" which I find is an unnecessary idea. I treat the cosmological constant Lambda simply as a curvature constant that appears naturally in the Einstein GR equation.

    There is no evidence AFAIK that this curvature arises from some type of "energy". That way of thinking of it was very popular for a time but now seems to be going out of style--as evidence piles up that we are simply dealing with a constant that appears in the equation which is our law of gravity/geometry.

    If you want to pursue the subject I would recommend the article you get by googling "rovelli prejudices". It's written for wide audience, is quite accessible and "down to earth". Worth a serious examination.
    I don't know what you are talking about when you refer to "the paper by the SDSS III group" maybe you should give a reference, but probably it does not matter. Dark matter and the cosmological constant are very different topics. I don't see the sense in lumping them together. Dark matter can be observed (by weak lensing) and regions of higher and lower density can be mapped---one has contour maps of DM concentration. This does NOT depend critically on precise determination of the Hubble expansion rate! (Contrary to what you suggest.)

    For historical reasons people CUSTOMARILY quantify the cosmological constant as an energy density equivalent.
    That is a convention, maybe they should change the convention. But in physics that kind of thing is done all the time--energy equivalents are used to express (lengths, inertias) quantities which are not energies.

    As I see it, your joke is about 10 years out of date. Back in 2003 various "dark energy" ideas were getting a lot of attention. People were talking about "quintessence" and "big rip". Some mysterious energy field was causing expansion to accelerate. That, I think would have been a witty time to mention "phlogiston". Now after that fad has begun to subside and one sees less and less speculation along those lines, it falls kind of flat.

    What I've found is that, for me, a handy way to quantify Lambda is to remember what it's square root is:
    the reciprocal of a certain distance. Namely 16.3 Gly.
    Last edited: Feb 1, 2013
  19. Feb 2, 2013 #18
    Quote by ConformalGrpOp

    In essence, everything we think we know about dark matter, dark energy and the expansion of the universe is entirely dependent on our determination of the value of the Hubble constant and the behavior of EM across cosmological distances.

    Quote by Chalnoth

    Sorta kinda. Our measurements of all of the parameters are interrelated, and any significant change in one of them impacts all of the others.

    However, that said, we now have quite good measurements of the expansion rate.

    This is perpetually at the back of my mind also. Over the past 2000 years we have had several instances where scientific theories have been rewritten in the light of new evidence and understanding. Here we have built an entire intricate cosmology pretty much on the value of the Hubble constant and the behavior of EM across cosmological distances.

    I realise that it is rare or perhaps never that we can say anything with absolute certainty, but always at the back of my mind I feel a nagging doubt of a finite element of risk that our observations are somehow deceiving us.

    Perhaps if I had taken these measurements myself with instruments that I fully understand I then might reduce these occasional doubts, but we have to rely on others for this and also others for the interpretation of these measurements.

    I hope that Cosmologists will keep their minds open to the admittedly remote possibility of alternatives and that means welcoming new ideas and working professionally through they pros and cons without being overly defensive.
    Last edited: Feb 2, 2013
  20. Feb 2, 2013 #19


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    Tan, a moment's reflection will remind you that cosmology is not based simply on redshift measurements. It is built as well on angle measurements, on counts of various types of objects, signal timing, and study of different types of radiation.

    I follow the literature as a cosmology watcher (not myself an expert!) and I am constantly seeing papers that explore ALTERNATIVE theories of gravitation/geometry that offer alternative explanations for the data. But not, I think, in ways you would imagine.

    I can't think of any field of mathematical science where the specialists are MORE OPEN to considering alternative theory and explanation.

    So I kinda had to chuckle when I read your "hope that Cosmologists will keep their minds open to the admittedly remote possibility of alternatives..."


    There is a lot more at stake besides the distance-redshift relation. What cosmologists are, in effect, testing is our geometric law of gravity.

    Our law of gravity is (as you know) a law of geometry and the passage of time, as well as gravitational force. It can be tested in many different ways and at many different scales (earth clocks, earth satellites, solar system scale, light-bending, compact objects, galactic scale, intergalactic, lensing by clusters, background radiation...)

    The fact that there is a relation between distance and redshift is just ONE OF MANY things that our law of gravity/geometry predicts and explains.

    You shouldn't get obsessed by the distance-redshift relation. That is not the basis, it is just one feature. Cosmology is not "built" on that. It is built on the GR equation as its theoretical basis and it is GR itself that cosmologists are so often considering alternatives to, and challenging and checking.
  21. Feb 2, 2013 #20
    Thanks Marcus, your reply is reassuring.

    Incidentally, regarding your statement, "Our law of gravity is (as you know) a law of geometry and the passage of time, as well as gravitational force."

    I have asked several times now whether the observed passage of time i.e. the length of time taken for events near the BB from t=0 to say t=300K years are given in the time frame of reference then in the presence of very large gravitational fields, or the time frame as observed from here and now? Perhaps I have not worded the question properly or perhaps the question does not make sense. No one replies anyway so I was not sure which applies!

    The kind of chronological events I am talking about are here:
    Last edited: Feb 2, 2013
  22. Feb 2, 2013 #21
    Gee, sorry. My interest in cosmology is very recent. I'll try to keep more up-to-date. Should have said "cosmological fluid," I guess. BTW, I believe the value of 77.3 KM/sec/mega-parsec for the Hubble constant came from a recent issue of "Astronomy," to which my stepson subscribes. (But I could be wrong... could have been from an article in the L.A. Times, or maybe an online blog.)

    I also read the first three pages of this article to try & understand Einstein's equation of General Relativity... I am totally lost. (I think I will go up on my roof and rake off the autumn leaves.)

    I have another question about the Cosmic Microwave Background Radiation, but that probably belongs in another forum, or at least another sub-forum... maybe you could direct me? Thanks!
    Last edited: Feb 2, 2013
  23. Feb 2, 2013 #22


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    At this point we have too many independent, corroborating pieces of evidence for there to be much of a chance that our models are substantially incorrect.
  24. Feb 2, 2013 #23
    Thank you for the rovelli prejudices search. I had previously come across the paper, but its always good to reacquaint oneself with this material.

    As for dark matter, I reviewed the paper by Taylor et al investigating the gravitational lensing effects of Abell 1689 (arxiv:astro-ph/9801158v1), cited in the wikipedia discussion. One might have hoped that they had studied a better known cluster group. Though one can admire the effort, its hard to have a comfort level with the conclusions, which, though having some merit, seem to lie somewhere to the right of the mean on the speculative spectrum. See also Jorg, et als work on investigating an apparent DM filament between Abell 222 and 223 using shear distortion analysis (arXiv:1207.0809v1). As they note in their paper, "a reliable direct detection of the underlying Dark Matter skeleton, which should contain more than half of all matter, remained elusive, as earlier candidates for such detections were either falsified or suffered from low signal-to-noise ratios and unphysical misalignements of dark and luminous matter."

    As for the relatedness of DM to the cosmological constant, one might suppose that, to the extent the Hubble relation informs the value of Lamda, it does not seem possible to disambiguate the results obtained by Dr. Rubin as well as van Albada (see, eg Sofue and Rubin 2001 Rotation Curves for Spiral Galaxies (arXiv:astro-ph/0010594v2) and van Albada and Sancisi (http://links.jstor.org/sici?sici=0080-4614(19861217)320:1556<447:DMISG[>2.0.CO;2-O), for edge on spiral galaxies from the strict interpretation of the Hubble relation as a doppler effect (as opposed to, e.g., a non Minkowskian propagation metric; See, Marmet 2013 On the Interpretation of Red-Shifts for a survey of various theories relating to redshift at http://www.marmet.org/cosmology/redshift/mechanisms.pdf)

    Marmet also has an interesting paper up on arXiv "Rotation Dynamics of a Galaxy with a Double Mass Distribution", arXiv:1210.1998v1, which addresses whether the observed motions require a nonbaryonic dark matter for their explanation).

    Not for nothing, but it is interesting that the discovery of the Hubble relation and the theory of dunkle Materie coincided, though of course, the non keplerian motion of the outer regions of various galactic complexes were noted earlier than Zwicky's paper on the subject.
  25. Feb 2, 2013 #24


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    Well, nothing has really changed about our understanding of dark energy, except that we're now more confident than we were previously that the observed acceleration is real and that it's a bit less likely to be some form of modified gravity.

    The main problem is that detailed experiments that allow us to nail down the nature of dark energy are very hard to do. So people generally just assume a cosmological constant because it's the easy thing to do, or a simplified, heuristic model of varying dark energy (using [itex]w[/itex] or [itex]w_0[/itex] and [itex]w_a[/itex]).

    And I don't see any problem in equating the cosmological constant with an energy density. For all intents and purposes, that's exactly what it is.
  26. Feb 2, 2013 #25
    Tan, I have had the same question. Marcus has a greater degree of confidence about some of these issues. To my understanding, (I am always ready to be further enlightened), all of the measurements, etc., he references are all the result of assumptions, which have never really been tested, about the behavior of EM radiation across cosmologically relevant distances. I have asked on this forum for any reference to any data from any experiment which undertook to test the behavior of EM waves across distances where the Hubble relation becomes observable. I haven't received any responses.

    Essentially, from day one after the GR folks, starting with Eddington, got a hold of Hubble's data, at least as far as I can ascertain, no one has given any thought to conducting an experiment which would confirm that EM radiation propagates in metric that is Minkowskian at cosmological scales. You'd think that astronomers, astrophysicists and cosmologists would be vitally interested in such an experiment. Apparently, there has been little motivation for such an experiment because, as Geller and Peeble's observed, there has been a "lack of a reasonable alternative physical basis for the redshift". Geller and Peebles, Test of the Expanding Universe Postulate, AstroJ, 174:1-5, 1972.

    Clearly, in the context of the time when Hubble's work was popularized, there were several GR models that indicated that we must be existing in an expanding universe. In the absence of any reasonable alternative explanation for the Hubble relation, the impossibility of conducting any experiments to verify the behavior of EM radiation propagating across great distances with the technology which existed at the time, and only lab experiment data on EM available, it is difficult to see how cosmologists could come to any conclusion other than the universe must be expanding in a manner suggested by the Hubble recession data. But why there has never been an impetus to send a space craft out to test the behavior of light with instruments up to the task continues to amaze me. It seems like such a basic thing to do given our near complete dependence on the interpretation of EM radiation for virtually all the information we obtain about the universe and its contents.

    I guess I am naive to think that such an experiment would appeal to just about everyone interested in learning about our universe. But, I always come back to Richard Feynman and his quip about common sense thinking which he wrote in connection with his service on the commission charged with investigating the Challenger Disaster; "reality must take precedence over public relations, for nature cannot be fooled." Despite this, it is incredible how many excellent minds are working in these fields and undertaking absolutely extraordinary research projects.
    Last edited: Feb 2, 2013
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