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CMB Prediction or Post-diction?

  1. Oct 1, 2014 #1
    One of my friends majoring in physics with me was telling me that the CMB radiation was not "predicted," by the big bang cosmology, but was in fact a "post-diction," and that the range of values for temp. obtained by Penzias were in fact way off, 5K to 50K and that other "cosmologies," can more accurately assess the temp. 2.7 K.

    He couldn't tell me where he remembered reading about this but the claim seems to be coming from the following,

    Mentor note: The "following" is a reference to an article in an unacceptable journal. AdkinsJr you want to learn physics, the journal Apeiron is one of the very last places one should go. I have deleted the reference and the summary thereof.


    I can't find much discussion on this topic so I thought it'd be interesting to post since I don't know much about cosmology. I knew that the CMB radiation is considered a turning point in the history of the model, but I am not familiar with the details, is this information correct?
     
    Last edited by a moderator: Oct 2, 2014
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  3. Oct 1, 2014 #2

    Chronos

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    The important thing is not so much the temperature of the CMB, rather that steady state [which was the main contender at the time] failed to explain the CMB at any temperature.. This proved to be the fatal blow to steady state theory.
     
  4. Oct 1, 2014 #3

    PeterDonis

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    It seems to me that this paper neglects to consider a key distinction between two notions of "temperature":

    (1) The temperature at which an object that can absorb incoming radiation from the sky is in thermal equilibrium with that radiation;

    (2) The temperature of the radiation itself, as determined from its spectrum and where it peaks (i.e., at what wavelength its amplitude is highest).

    To see the distinction, suppose we put a perfect black body out in space at the same distance from the Sun as the Earth. This black body (i.e., perfect absorber of radiation from the Sun) will be in thermal equilibrium at something like -18 degrees Celsius, or 255 Kelvin. However, the spectrum of the radiation coming in to the black body from the Sun peaks in the visible light range (that's why our eyes evolved to see light in that range), consistent with the Sun's surface temperature of 6000 K--i.e., the "spectrum temperature" (not sure of the correct technical term) of the Sun's radiation is 6000 K, even though the black body absorbing that radiation at the distance of the Earth from the Sun is in thermal equilibrium at 255 K. (In fact, the main reason we know the temperature of the surface of the Sun is by measuring its spectrum.)

    The paper claims that models of the universe in which it is not expanding, and in which all parts of it are in thermal equilibrium, on average, with one another, explains the CMB observations better than the standard Big Bang, expanding universe model. They base this claim on the fact that, if you calculate temperature #1 above for a black body sitting out in interstellar or intergalactic space, and absorbing incoming radiation from all parts of its sky, you get something like 3 K--i.e., the temperature that is always quoted for the CMB. (This is what most of the previous estimates of the "temperature of the universe" described in the paper--all except Gamow's AFAICT--were doing.)

    However, if you ask what temperature #2 of this radiation will be--i.e., what its spectrum will look like--in the "equilibrium universe" model, you get something like the answer we saw above for the spectrum of the Sun. After all, this incoming radiation is largely composed of starlight--very, very faint starlight, yes, but that doesn't change its wavelength, only its intensity, just as for sunlight. So on the "equilibrium universe" model, we would expect to see a "CMB" whose temperature #1 was about 3 K, but whose temperature #2 was much higher. We might even expect to see multiple temperature #2-type peaks in its spectrum, since it is likely to be a mixture of radiation from sources at very different surface temperatures.

    But when we look at the actual CMB, the one Penzias and Wilson detected, that is not what we see. We see radiation whose temperature #2 is 3 K (actually 2.7 K from the latest measurements). That is, this is radiation coming from an "object" whose apparent surface temperature, as determined from its spectrum is 3 K. I put "object" in quotes because, of course, we don't know of any such object; the only way we know of that the entire universe could be filled with radiation whose temperature #2 is 2.7 K is if that radiation is from an early hot, dense phase that the universe went through, and it has been redshifted by a very large factor due to the expansion of the universe since that phase. That is why we say the CMB is evidence for the Big Bang.
     
    Last edited: Oct 3, 2014
  5. Oct 2, 2014 #4

    Drakkith

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    From wiki: http://en.wikipedia.org/wiki/History_of_the_Big_Bang_theory#1950_to_1980s

    In addition, the discovery of the cosmic microwave background radiation in 1965 was considered the death knell of the Steady State, although this prediction was only qualitative, and failed to predict the exact temperature of the CMB. (The key big bang prediction is the black-body spectrum of the CMB, which was not measured with high accuracy until COBE in 1990).

    As has been said, the key here is that the Big Bang predicts that the CMB has an emission spectrum of a near-perfect blackbody, not that it has a temperature of 2.7k. Also, to paraphrase PeterDonis, the CMB has a spectrum exactly like we would see from an object at 2.7k. The relative brightness/intensity of each wavelength fits this perfectly. Other models use "temperature" in a different manner and none that I know of predict that the background radiation should be effectively identical to the radiation emitted by a perfect black body at 2.7k.
     
  6. Oct 2, 2014 #5
    This is a good description of what the paper lacks, wich is a thermalisation mechanism that substitutes expansion. Absent that alternative mechanism, the BB model is the best one available to explain the CMB.
     
  7. Oct 3, 2014 #6
    Q: "One of my friends majoring in physics with me was telling me that the CMB radiation was not "predicted," by the big bang cosmology, ... is this information correct?".

    The description of "the" radiation is very narrow, besides the other problems discussed. The Hot Big Bang, without inflation, makes a much more constrained prediction. In fact, the CMB is by itself sufficient to predict the HBB!

    "Still, at the small angular scales, the polarisation data can be trusted and in this data Planck have one of their most impressive figures. The figure below shows how both the temperature multiplied by the polarisation (pixel by pixel on the sky) and how the polarisation itself varies with angular scale. The blue dots are the measured signal. Now, the red curve is not the best fit curve to this data. That is worth pausing and reflecting on. If it isn't the best fit curve, then what is it?

    That curve is the unique prediction from analysing Planck's temperature data. There are no free parameters in defining those red lines. Once the temperature data is analysed, we can make an unchangeable prediction for what the polarisation should look like. The fact that the red line goes straight through the blue data points is absolutely remarkable. However, if one believes in the big bang and standard cosmological model, this is all that could have happened. If one doesn't believe in the big bang, then not only is there no reason to suspect that the CMB exists, or that it is polarised, but certainly not that the way the polarisation averages on particular angular scales should look like that.

    I think it is worth pausing for one second longer on this. I'm about to start describing a few "features" and anomalies that might be present in the Planck data. It is tempting for a person cynical about natural science to pick up on these anomalies and say "scientists don't understand what they're doing, look at all these anomalies". The thing is, scientists are trying to understand everything. It isn't enough that the model explains almost everything, every possible failure is looked for and analysed. If someone wants to replace the big bang, or any other aspect of cosmology (or well-established science) it isn't enough just to explain how to create these anomalies. Any alternative model must also reproduce everything that works. Without the big bang the prediction for that red line would be a horizontal line through zero. That wouldn't be called an anomaly, that would be called a completely failed model."

    Also, to keep in mind besides that alternative models now fail across the line, is that these unacceptable journals by promoting such despite the overwhelming evidence to the contrary will eventually tar humanity and nature both:

    "These curves reflect some of the best of humanity. These are the tiny fluctuations in the polarisation of a field of radiation, left over from a hydrogen plasma that permeated the entire universe, 14 billion years ago. The oscillations in the curves come from sound waves in this hydrogen plasma. The curve is our prediction for this data, with no free parameters to play with at all. Just reflect on that. I'm unable to describe how incredible this is. We don't even know whether Shakespeare wrote Shakespeare's plays, but we can predict exactly what the polarisation in the CMB should look like."

    [ http://trenchesofdiscovery.blogspot.se/2013/04/the-universe-as-seen-by-planck-day-one.html ; my bold.]
     
  8. Oct 3, 2014 #7

    Drakkith

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    Nice link, Torbjorn. It looks like an excellent description of the Planck data.
     
  9. Oct 3, 2014 #8

    CKH

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    Is there only one possible interpretation of the CMB or is this possibly due to a lack of imagination? Does it really predict?

    I'm no expert but I have read that predictions about or resulting from the CMB involve some six adjustable parameters. These parameters are tuned to provide fits with other observations and with the theory of nucleosynthesis (which has a decades old "tension" with Lithium abundance). The theory requires large amounts of an as yet unidentified type of matter with special properties. Every time new data is available, the parameters are tweaked to match. Can this truly be claimed as prediction? It's hard to see where this "no free parameters" claim comes from. I assume you are talking about the E polarization, since B measurements are already in conflict.

    The theory seems nothing like SR (for example) that makes specific quantitative predictions with no adjustable parameters.

    There are anomalies in CMB measurements that were not predicted. There is much debate about their cause and reality but they did not disappear with the substantial improvements in Planck over WMAP. Even supposing that these anomalies are merely the result of improper analysis or local (e.g. Galactic) noise, what does that say about the CMB data as a whole?

    The theory may well be perfectly correct, but given its constant adjustments, yet to be proved assumptions and the unresolved conflicts with local observations, it seems premature to write QED at the bottom.

    While keeping things in mind, keep in mind that this theory is seriously challenged on several fronts in explaining the local universe and more generally galaxies. Isn't it a good thing to hang onto some scrap of skepticism about our theories? Advocates want to bully out all other ideas with the claim that our theory explains more things than yours can. That does not constitute a proof of it's validity nor is it a sufficient reason to exclude exploration of alternative possibilities.
     
  10. Oct 3, 2014 #9

    Drakkith

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    You seem to be under the mistaken impression that science is about "proof". It is not. The current cosmological model is the one that best explains our observations using the fewest/simplest assumptions and changes to known laws. It's a simple as that. Until another model is developed that explains things better it will remain the dominant one and scientists will try to explain future observations within the framework of the model before seeking alternative models or theories.

    One cannot interpret the CMB or any other observation without some sort of model or theory to explain its relation to other observations. The dominant competing theories at the time when the CMB was discovered was the steady state theory and the big bang theory. The steady state theory could not explain why the CMB had a thermal spectrum, while the BBT could. The fact that the BBT could explain the spectrum of the CMB while the SST couldn't was powerful evidence that the steady state theory was not accurate. This and other evidence caused scientists to mostly abandon the steady state theory in a short amount of time. New evidence gathered since then has continued to support the BBT over anything else, and it will remain the dominant theory until another theory is developed that does a better job at explaining things.

    None of this means that the BBT is complete. There are still many things that we can't fully explain yet. The key here is that nothing can fully explain all of our observations at this time. Plenty of new ideas have been proposed over the decades since the CMB was discovered, and most of them have failed to pass observational and theoretical tests, leaving the BBT the dominant theory. Since the BBT is the dominant theory, the CMB is going to be explained and interpreted according that that theory as long as it remains the dominant one. It is certainly possible to interpret the CMB according to another theory, but since there are no other theories that explain as well as the BBT does, there's little reason to do so.

    Since the BBT is the dominant theory, let's look at what it has to say. The BBT says that the universe has expanded from a much denser, hotter state to a cooler, lower density state over time. If we take the current conditions of the universe and extrapolate backwards, we reach a point where all matter in the universe consists of high-density plasma that is opaque to EM radiation. As the universe expands, this plasma cools and becomes less dense until the particles can combine into neutral atoms. At this point the matter becomes transparent and the EM radiation that was bouncing around the plasma constantly getting absorbed and re-emitted is suddenly able to propagate freely. This EM radiation is the CMB.

    The fact that we can remove any observational data of the CMB from the BBT and still come up with this burst of EM radiation is what is meant by a "prediction". It may be better to think about it as that the CMB "Can Be Predicted" from the theory, not that "it was predicted".

    The exact parameters of the CMB cannot all be predicted, as we don't know enough about the conditions of the early universe to do so, but that is very common in science.
     
  11. Oct 4, 2014 #10

    I believe it is not correct. The CMB was predicted by Alphard and Gamow, though I don't think they got the exact value of the temperature.
     
  12. Oct 4, 2014 #11

    CKH

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    I have no expectation that LCDM cosmology is proved or can ever be proved. Proofs arise in mathematics, not in physics. Proofs can be closely approached in experimental sciences because we can control conditions. In cosmology we are not so lucky. We make hypotheses and see if they fit observations, then we conclude from fits that perhaps they are correct.

    I admit that I'm not an expert but I do understand the basic reasoning behind the existence of the CMB. However, the claims of exactness associated with this hypothesis seem overstated to me. I have some awareness, however vague, that many assumptions and adjustable parameters are used to get these convincing fits. There are peaks in the power spectrum that are claimed to be precisely predicted. Really? How many adjustable parameters go into that match? In fact that match has been wrong in the past and has been adjusted to make a fit.

    It would be helpful if someone could explain some of the specific quantitative predictions made for the CMB and how they demonstrate that the associated theory is verified by observation with great precision. I'm guessing, that the claim that the CMB is a BB relic, comes from the fit to a black body radiation profile.
    ------
    On the other hand any SS theory, particularly without expansion, has huge problems explaining how the universe could remain stable and accounting for the redshift. The universe is far easier to explain if it has a finite history and is evolving.

    It's fine for LCDM to be the dominant model at large scales considering successes in interpreting the CMB and large scale structure. But, to exclude all other possibilities, with the claim that this theory explains the most things, is not justified. This is what I see happening, when the theory is forced upon observations of the local universe and the observations of galaxies. Here, in my view, the theory (CDM) has run into serious problems. The approach taken is to hammer the DM models into these observations. Effort is focused on keeping the theory viable and searching for ideas to reduce the many "tensions" and unexplained empirical relationships. In fact, fixing tensions is the subject of the majority of papers in this area.

    Our theory of galaxies is not built bottom up from observation of galaxies, but top down by imposing a cosmological theory on observations and trying to make it work. So, simpler ideas, for example that explain rotation curves without non-baryonic matter, are simply ignored (I've done enough research of publications to back this up). It is taken for granted that the explanation must be DM halos, even though such a fit already appears unlikely. I doubt, in the absence this dominating cosmological theory, that astronomers would be expecting galaxies to consist of 80% undiscovered non-baryonic particles distributed in a more or less spherical halo.

    You can claim that it's the best or dominating theory because nobody has a better one. But, what do you do when it runs into problems? Do you say it must be correct because it has been successful and simply conjecture that a solution based on the theory will eventually be found, or do you admit a problem and open the door to the possibility of alternative explanations that may in fact conflict with your theory? From what I see, alternative theories are anathema, not to be taken seriously, ignored or ridiculed.
     
  13. Oct 4, 2014 #12

    Drakkith

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    And you find something wrong with the fact that we can't predict every detail of the CMB without relying on adjustable parameters?

    Not only is excluding most other possibilities justified, it's the only realistic option there is. Otherwise we'd be trying to make the data fit a hundred different theories, 99% of which are not accurate without getting so unnecessarily complicated that they border on nonsense. You literally cannot make realistic scientific progress without excluding all but the most accurate theories. In any case, the LCDM model is not the only one under consideration, which is why research into things like MOND is ongoing. I don't know why you believe that all possibilities have been excluded.

    Of course. We are at a time where new discoveries cannot be adequately explained by current theory. A prime time for science to do its job. I have little doubt that the LCDM model will undergo significant changes in the next few decades, and may be replaced by something else entirely.

    This is just wrong. Various theories have been proposed to explain galaxy rotational curves without dark matter, including multiple variants on MOND. It doesn't look like you've done much research at all if you claim that they've been ignored.

    Again, this is wrong. Nothing is taken for granted. Dark Matter halos have good success in explaining galactic rotation curves, as does MOND. And remember that the LCDM model was developed after dark matter was theorized and in conjunction with other alternatives. It became dominant because it was the one that worked the best.

    Being the dominant theory does not make it "correct". In fact, the failings of the LCDM model are well known. The problem is that alternatives that explain some of these failings simply don't work very well when they are incorporated into a larger theory. For example, the changes to gravity that MOND proposes work for galactic rotational curves but don't work on a larger scale where they should also work, and attempts to modify the theories so that they do work has thus far failed. The very fact that alternatives have been explored and considered directly counters your claim that they haven't.

    The real problem here is that you just don't like the LCDM model and don't want to understand why it's considered the dominant model.
     
  14. Oct 5, 2014 #13
    It is quite easy to browse the Planck archive, here the paper about extracting cosmological parameters. They show that 6 parameters, which was best for WMAP, is still best. I.e. the penalty of adding more free parameters is more severe than the success of a better fit.

    Now, this is for inflationary LCDM, i.e. it includes inflationary predictions. What I was referencing was that without the HBB, the CMB can't be predicted. Or reversely, you can conclude from the CMB that were was a HBB.

    Yes, that is my understanding of that description of the CMB.

    I'm repeating myself to you from another thread, but it can be said again.

    What Drakkith says is correct, the best theory is the one supported. And I gave a reference to why there are no more contenders to predict CMB today, those are all failed theories. (There are theories that include a HBB that are contenders, albeit not so well quantified, to inflation. But that is discussed on another thread.)

    But one can take a more basic approach.

    Physics isn't math, because there is physical dimensions of reality and hence uncertainty involved. This shows up in that you can't use mathematical "proof", mutually agreed on procedures, in physics as axiomatic procedure can't cope with quantization (say). Instead we have to use testing to mutually agreed on quality standards (3 and 5 sigma, say).

    That takes us to measurement theory, which is what I had to study under basic physics at the university. It tells us that everything empirical can be described by hypothesis testing, whether observations (a hypothesis on observed value and its uncertainty), hypotheses (a hypothesis on a mechanism) or theories (sets of interrelated hypotheses; a hypothesis on a process).

    Most poignant there is that when we test an observation or a mechanism, we also fix free parameters in range and test the constraints of the experiment. No "assumptions", no "adjustments". The result is that we have to look at robustness and tension with other experiments. If an observation or a theory survives repeated testing, it is robust. Sure, the constraints vary a little between WMAP and Planck, there is some tension at 1-2 sigma on some parameters, but the result is robust.*

    Another test of robustness and ease of tension is if a theory is self-consistent, so it is more likely to survive internal and external challenges. LCDM is such, the first self-consistent cosmology.

    Further test of robustness is usefulness. Is the theory surviving long and productively? LCDM is such, it has launched 2 space experiments (WMAP that made its case, Planck that strengthened it) and many land based ones, and many theory variants and useful cosmological methods (BAO as distance rulers, weak lensing to probe structures, ...).

    That measurement theory goes beyond usefulness to what may one day approach "proof", mutually agreed on procedure, was clinched by LHC finding a standard Higgs:

    "The Laws Underlying The Physics of Everyday Life Are Completely Understood

    ... A hundred years ago it would have been easy to ask a basic question to which physics couldn’t provide a satisfying answer. “What keeps this table from collapsing?” “Why are there different elements?” “What kind of signal travels from the brain to your muscles?” But now we understand all that stuff. (Again, not the detailed way in which everything plays out, but the underlying principles.) Fifty years ago we more or less had it figured out, depending on how picky you want to be about the nuclear forces. But there’s no question that the human goal of figuring out the basic rules by which the easily observable world works was one that was achieved once and for all in the twentieth century.

    You might question the “once and for all” part of that formulation, but it’s solid. Of course revolutions can always happen, but there’s every reason to believe that our current understanding is complete within the everyday realm. ..."

    [ http://blogs.discovermagazine.com/c...s-of-everyday-life-are-completely-understood/ ; my bold.]

    The process of competition under testing works generally, for everyday physics as well as the exotic physics of LCDM, eventually there are no more possible contenders.**

    [So maybe some day this will be accepted as "proof", in the sense of mutually agreed on procedures moving beyond reasonable doubt, in the same way as "testing", in the sense of mutually agreed on quality levels moving beyond reasonable doubt. But that is a philosophical issue, and I don't want to do those.]

    * So what happens if the fixed parameter ranges fail, and we have to change them? Why, we have an old, dead theory and new, alive theory of course! The old parameter ranges can't be resurrected, unless there is something wrong with the observations or the treatment of the theory when testing. The "theory" under test is the constrained one, not the whole set of theories that the unconstrained free parameter range describe, which can still be viable with some other parameter choice.

    Then again, if we test and fail many times, if we add parameters under penalty, eventually the area must be abandoned as unproductive.

    ** Why that works is an open issue, which goes beyond the simple observation.
     
    Last edited: Oct 5, 2014
  15. Oct 6, 2014 #14

    CKH

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    Is not exactly that. In physics there are constants that we cannot account for, but we can measure them by experiment. My concern is that by adjusting these otherwise unknown parameters and assuming the existence of an undiscovered dominate form of matter with conjectured properties, we might convince ourselves of something that isn't true.

    I don't know of any other theory that work as well at the cosmological level. However when LCDM fails to explain the local universe, such an alternative theory must be possible.

    MOND is pursued by a somewhat fringe group in the community. Mainstream BBT is based on GR. The mainstream rejects MOND as a theory of gravity because it fails for clusters (which is fine). But while rejecting it as a theory, the implications of MOND's tight fits in disk galaxies tend to be undervalued, with the false claim that DM does as well. The seriousness of this challenge is under acknowledged by the mainstream apparently in the belief that it will all work out somehow.

    OK, then we agree. It is important to keep an open mind and retain some skepticism about what we think we know.

    I have researched this issue because it interests me. The very simplest explanations which do not change gravity but add some baryonic dark matter are ignored, as far as I can tell.

    LCDM works at the cosmological level including the interpretation of the CMB, BBT, nucleosynthesis and large scale structure. It's predictions on small scales are wrong. Theorists are trying to fix this problem, but it does not look good.

    Alternatives are explored by those who deviate from the mainstream. They have not successfully challenged LCDM on large scales. On the other hand the mainstream is rather desperately defending LCDM in the small scale realm where it is an awkward explanation at best.

    It's not farfetched to claim that LCDM is falsified on small scales as much as MOND is at large scales. So what do we do? Why not study the universe bottom up with a fresh view that is not chained to LCDM?

    Consider the specific areas in which this model is dominate (already mentioned above). In galaxy formation, the model is weak. Why do we have large, bulgeless spiral disk galaxies? In recent years, the genesis of large galaxies, SMBHs and high metallicity have been pushed back to less than 1 GY post BB. Large quiescent galaxies also appear early. LCDM is running out of time for the evolution of galaxies. Where are the population III stars? Why do galaxies cira z=2 appear so massive but extremely compact? How are star formation rates of 3000/y possible? But most important, in the local universe, where are the dwarf galaxies predicted, where are the DM cusps, why do spiral galaxies follow simple relationships across a vast range of masses, how does baryonic matter rule non-interacting CDM and why do galaxies that should be stripped of CDM behave just like the others?

    The spectacular successes of LCDM are limited to the broad picture of the early universe.
     
  16. Oct 6, 2014 #15

    Drakkith

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    Who says we aren't looking for alternatives in just the manner you're suggesting? The problem is that no new "fresh" approaches have worked yet. Don't mistake the acceptance of a theory as dominant as a sign that it is totally accepted and all research has stopped on alternatives. See this article, especially page 165: http://bccp.berkeley.edu/beach_program/presentations11/COTB11Matos.pdf
    Not only are there a dozen different possible types of dark matter listed, there are a half dozen other alternatives.

    I really don't understand your objections. It seems like you're saying that since LCDM doesn't explain everything, we should just completely abandon it. All you've done so far is post about where LCDM fails. What about where it works? Why does that not seem to count for anything?
     
  17. Oct 6, 2014 #16
    This is seriously wrong.

    - LCDM is spectacularly successful, gives very high confidence by being understood on mechanisms and by conforming with many observations, on every scale from structure formation down to galaxies on one side and nucleosynthesis on the other; and on every time from right after HBB to the far future due to its inclusion of DE.

    "Which Parts of the Big Bang Theory are Reliable, and Why?"

    universehistoryreliability2.png
    [ http://profmattstrassler.com/2014/03/26/which-parts-of-the-big-bang-theory-are-reliable/ ]

    - There is no "false claim" that DM does well on galaxies including spiral, and the remaining challenge (which is a tension with observation*) has grown smaller over the years as simulations has been able to include more effects.

    That is a sign of a healthy, useful science. And then it is unlikely such a trend will change.

    Today's simulations don't only predict galaxies, but their emergence and their features like spirals, from first principles despite that galaxies are the most complicated and smallest structures the theory should cover. [ http://www.illustris-project.org/ ] To point to remaining tension with observations would be like arguing over if quantum mechanics is a valid theory because we have problems predicting its most complicated systems, macroscale objects, all the way up.

    As for MOND mentioned here, it is no longer a suitable alternative. But it was always ad hoc. It can predict galaxy behavior superficially naturally since it is a curve fit. But it doesn't make sense, since then it can't predict galaxy collisions (or other gravity effects). There is no physics continuity, except an extraordinary claim without evidence - "just because". Morally, it fails before it starts. (Sure, it is always easy to say in retrospect.:rolleyes:)

    *And of course it would be nice to constrain it more. We know of neutrinos, which is a minute part...
     
    Last edited: Oct 6, 2014
  18. Oct 6, 2014 #17

    CKH

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    Where did I say we should completely abandon it? We have it. We have its cosmological fits. We've invested deeply in cosmological observations that support LCDM. Most cosmologists obviously have great confidence in it as a theory of the early universe and an explanation for large scale structure.

    Should we insist that it is absolutely correct and that every tenet of the theory is verified and correct? When we do so, we have problems explaining the small scale universe. Either we find a way to get it to fit, or we deny the correctness of some part of the theory and try to explain the local universe in some other way.

    Mainstream scientists are trying to get it to fit the local universe. Some ideas have been put forward such as supernovas to eliminate DM cusps (other scientists deny that this solution is viable). Although various solutions have been proposed for problems, no complete, consistent solutions have been found.

    Galaxy rotation curves can be explained with DM halos, but the fits are different in each case. Various distributions are tried. No galactic DM distribution can as yet be directly measured. On the other hand, we have a formula that reliably yields rotation curves based only on baryonic matter distribution. This means that galactic rotation curves are tightly correlated with baryonic matter. How can that be if DM is responsible for rotation curves? That is a problem to be solved. If it can be solved in the context of CDM, great, but it really doesn't look likely.

    What good does it do to endlessly repeat the cosmological successes of LCDM (which have not been denied), if the theory doesn't fit the surrounding universe? That just creates a paradox.

    All I'm suggesting is that it may be necessary to contradict some tenets of LCDM to explain the local universe (and some aspects of galactic evolution). If we insist that cannot be done, but we cannot explain the local universe, then where are we?

    Trobjorn_L,

    Your illustration does a good job of showing what LCDM explains so well.

    Once again, MOND (just as an empirical formula, not as a theory of gravity) derives rotation curves from the distribution of baryonic matter alone. There is no comparable predictor for the DM model. There cannot be anyway because we have no direct way of knowing precisely how much DM there is and how it is distributed in any given galaxy. But somehow, miraculously, the MOND formula only needs the distribution of baryonic matter to dictate the rotation curve. Do you understand the significance now? This is what scientists like Stacy McGaugh and Pavel Kroupa are trying to get other scientists to recognize. (Kroupa goes so far as to assert that DM (as envisioned) does not exist.)

    There are several problems with DM as a theory when applied to observations of the local universe. Denying them doesn't help. Pointing to the successes of LCDM doesn't help.

    You can conjecture all sorts of DM particles and DM properties and even interactions to fix things, but what's the point if you cannot prove they even exist? It becomes more or less of a game of conjectures without physical validation. If you conjecture some form of dark matter that gives you a fit, does that mean it actually exists? This is part of what is going on now to solve problems on small scales. Mainstream theorists are trying different sorts of dark matter (WDM, self-interacting DM, multiple species of DM) to fix the problems. However, they exclude the possibility that no appropriate form of non-baryonic matter actually exists because that is inconsistent with LCDM which they believe to be correct.

    Do you know what priciples are actually used to achieve this impressive simulation (http://www.illustris-project.org/)? I have no idea and that is usually the case when I read papers about the results of simulations. No one actually does simulations from first principles, we don't even know exactly how stars form from first principles.
     
  19. Oct 6, 2014 #18

    Drakkith

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    That is exactly what I've been getting out of your arguments this entire time.

    You're suggesting that it might be necessary to contradict LCDM? Sorry to be blunt, but scientists already know this. Of course it might be necessary. It might be necessary to do any number of things in order to explain our observations, so if this is your suggestion, it's a very poor one.

    That's assuming that future research won't alleviate current difficulties with the model. There's no way to know at this time.

    You're assuming that we will never be able to prove they exist. Again, there's no way to know at this time, which is why research is ongoing.

    I don't believe anyone's been endlessly repeating the successes of the LCDM model. The difficulties of the model have been posted numerous times. I even posted a link that detailed some of these difficulties.

    I'm getting very tired of you claiming that people are insisting that LCDM is absolutely correct. Multiple members have posted numerous times explaining that LCDM is simply the model that best fits the available evidence at this time and this could easily change in the future. If you continue to ignore what we've posted and keep making incorrect claims and logical inconsistencies then I will take moderation action. Consider this your only warning.
     
  20. Oct 7, 2014 #19

    CKH

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    I've tried to point out why observations of the local universe, in particular galaxies, are inconsistent with DM. This is not some idea that I invented, it is the view of some well known scientists in the field who have published many papers on the subject and debated the subject with other well known scientists. My motivation for discussing this is that these views are often dismissed for the wrong reasons. In particular they are dismissed with the argument that MOND is an incorrect theory of gravity. I agree with that but that's not the point, as I repeatedly stated.

    It remains to be seen whether these issues can be addressed with non-baryonic dark matter. However, conjectures about the nature of dark matter and it's interactions are insufficient, even if such conjectures provide explanations. The issue cannot be resolved until we have measurements that can distinguish between models of galaxies. Perhaps the new Gaia mission (which will make high precision measurement of stars in the Galaxy) can shed some light on the distribution of dark matter. If such measurements prove the existence of dark matter halos, then we can make some progress in understanding dark matter and have more confidence that it will eventually be directly detected.
     
  21. Oct 7, 2014 #20

    bapowell

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    Science Advisor

    You need to elaborate on this comment. The temperature power spectrum is based on some model with tunable parameters. Six parameters are the minimum to describe LCDM with a power law spectrum of perturbations.
     
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