Thursday, December 13, 2018

New experiment cannot reproduce long-standing dark matter anomaly

Close-up of the COSINE detector  [Credits: COSINE collaboration]
To correctly fit observations, physicists’ best current theory for the universe needs a new type of matter, the so-called “dark matter.” According to this theory, our galaxy – as most other galaxies – is contained in a spherical cloud of this dark stuff. Exactly what dark matter is made of, however, we still don’t know.

The more hopeful physicists believe that dark matter interacts with normal matter, albeit rarely. If they are right, we might get lucky and see one of those interactions by closely watching samples of normal matter for the occasional bump. Dozens of experiments have looked for such interactions with the putative dark matter particles. They found nothing.

The one exception is the DAMA experiment. DAMA is located below the Gran Sasso mountains in Italy, and it has detected something starting in 1995. Unfortunately, it has remained unclear just what that something is.

For many years, the collaboration has reported excess-hits to their detector. The signal has meanwhile reached a significance of 8.9σ, well above the 5σ standard for discovery. The number of those still unexplained events varies periodically during the year, which is consistent with the change that physicists expect due to our planet’s motion around the Sun and the Sun’s motion around the galactic center. The DAMA collaboration claims their measurements cannot be explained by interactions with already known particles.

DAMA data with best-bit modulation curve.
Figure 1 from arXiv:1301.6243

The problem with the DAMA experiment, however, is that the results are incompatible with the null-results of other dark matter searches. If what DAMA sees was really dark matter, then other experiments should also have seen it, which is not the case.

Most physicists seem to assume that what DAMA measures is really some normal particle, just that the collaboration does not correctly account for signals that come, eg, from radioactive decays in the surrounding mountains, cosmic rays, or neutrinos. An annual modulation could come about by other means than our motion through a dark matter halo. Many variables change throughout the year, such as the temperature and our distance to the sun. And while DAMA claims, of course, that they have taken into account all that, their results have been met with great skepticism.

I will admit I have always been fond of the DAMA anomaly. Not only because of its high significance, but because the peak of the annual modulation fits with the idea of us flying through dark matter. It’s not all that simple to find another signal that looks like that.

So far, there has been a loophole in the argument that the DAMA-signal cannot be a dark matter particle. The DAMA detector differs from all other experiments in one important point. DAMA uses thallium-doped sodium iodide crystals, while the conflicting results come from detectors using other targets, such as Xenon or Germanium. A dark matter particle which preferably couples to specific types of atoms could trigger the DAMA detector, but not trigger the other detectors. This is not a popular idea, but it would be compatible with observation.

To test whether this is what is going on, another experiment, COSINE, set out to repeat the measurement using the same material as DAMA. COSINE is located in South Korea and has begun operation in 2016. They just published the results from the first 60 days of their measurements. COSINE did not see excess events.

Figure 2 from Nature 564, 83–86 (2018)
Data is consistent with expected background


60 days of data is not enough to look for an annual modulation, and the annual modulation will greatly improve the statistical significance of the COSINE results. So it’s too early to entirely abandon hope. But that’s certainly a disappointment.

80 comments:

  1. Dear Sabine,

    you say:
    "The Xenon and Germanium atoms differ from Iodine in their spin, which comes from the particular combination of neutrons and protons in the atomic nuclei. A dark matter particle which preferably couples to non-zero spin could trigger the DAMA detector, but not trigger the other detectors"

    but this is not completely true. Some isotopes of Xenon and Germanium have spin and if I remember correctly they rule out the DAMA signal. Plus there's the PICO experiment which is designed specifically to look at Spin-Dependent interactions.


    To save DAMA result you have to choose a Spin-Dependent interaction which triggers only a restricted amount of targets. Quite hard.

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  2. Unknown,

    I know it's not quite correct in the sense that no one has managed to find a spin-dependent model that fits all existing data (at least not that I know of). But as long as you use a different target material, you'll not really be able to rule out that that's a possibility.

    In any case, I'll see if I can think of a better way to phrase that.

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  3. Belief in dark matter, string and SUSY theories seems to be so locked in that I wonder if that belief would persist even in light of definitive evidence that they're wrong.

    Many social scientists fear worldwide social disorientation if and when intelligent extraterrestrial life is discovered. Perhaps the same kind of mental disruption is already eating away at the minds of physicists with regard to the above theories. After all, there is no factual evidence to support religious belief, yet it persists. Are physicists really any different?

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    Replies
    1. Preconceptions are filters for perception and "mainstream" ideas(orthodoxy) are very strong filters for what could be accepted as possible by active professional scientists; their "universe of possibilities" is bounded by orthodoxy. Many times the reactions of active scientists to new ideas or facts had been essentially not different from the Church reaction to Galileo's claims; just remember Lavoisier on meteorites:  "Stones cannot fall from the sky, because there are no stones in the sky!", or S. Chandrasekhar's: "This exceeds my imagination". Attachment to ideas lead to dogmatism and the preemption of empirical evidence that contradicts these ideas/dogmas, at that point there is really no difference between these ideas and religion.

      Delete
  4. Baryogenesis, a Big Bang net matter universe, breaks conservation laws. Sakharov criteria include ~0.1 ppb vacuum chiral anisotropy toward hadrons. Physics postulates exact vacuum symmetries for exact Noetherian conservation laws - beauty! Forty years of derived quantized gravitation, SUSY, and "dark matter" are empirically sterile.

    ~0.1 ppb vacuum chiral anisotropy toward hadrons, ~0.1 ppb Noetherian non-conservation of angular momentum: universal Milgrom acceleration; Tully-Fisher relation in early galaxies, as observed (DOI:10.3847/1538-4357/aaedbf, arXiv:1811.01958).

    Extreme molecular opposite shoes embed within a vacuum left foot with different energies. A 3:1 right to left shoes cryogenic molecular beam has non-identical frequency low energy transitions. Use a crafted molecule in a published experiment (DOI:10.1002/anie.201704221). Heal physics’ defective postulate.

    http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/vibrot2.html
    … HCl, 35Cl (75.76%) and 37Cl (24.24%) shows line divergence below spectral resolution.

    Look at no dark matter.

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  5. Another negative result (maybe). How disappointing (if so).

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  6. Dear Dr H

    For the sake of us poor liberal arts majors, does this null result cast an doubt on the likely existence of dark matter? Or, is it just a failed attempt to detect it and with no greater implications?

    Thank you.

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  7. https://www.interactions.org/hub/dark-matter-hub
    … Bottom 2/3. An extraordinary collection of money, time, expertise, infrastructure, and theory in -log scale free fall. 24 hours in a microwave rotational spectrometer risks ugly success.
    https://phys.org/news/2018-12-cosine-dark-mystery.html
    … Yangyang Underground Laboratory (Y2L) in Korea, -700 meters, 38°N, 128°E
    https://news.yale.edu/2018/12/05/cosine-100-experiment-investigates-dark-matter-mystery
    … COSINE-100 is DAMA-100 (-3800 m, 42°N, 14°E) repeated at scale to retain fundable ambiguity.
    https://www.ibs.re.kr/cop/bbs/BBSMSTR_000000000739/selectBoardArticle.do?nttId=14844
    … COSINE-200, under Mt. Yemi. -1100 m. DAMA-200 repeated.
    http://andeslab.org/workshop5/talks/01_Dib.pdf
    … Agua Negra, -1750 m, 30°S 70°E. Do it in the southern hemisphere with opposite locally-induced modulations.

    Equatorial Earth moves ~20,000 mph faster at midnight (tangent spin with orbital velocity) than at noon (tangent spin against orbital velocity). So?

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  8. If dark matter really has no interaction with electromagnetism, if it has a low relative velocity to us, and if its mass is low so its momentum is low, why it would interact with some of these detectors sufficiently to give a detectable signal? Maybe it just does not interact with ordinary matter, other than with trivial impacts and through gravity.

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  9. It's well know that cold fusion only works in Italy (search for "Piantelli Chellini LENR" etc.) Perhaps the detector at Gran Sasso is being affected by the same mysterious force that enables cold fusion. You can moderate this through at your disgression. ;-)

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  10. Uncle al says beauty! Forty years of derived quantized gravitation, SUSY, and "dark matter" are empirically sterile.

    I like that term “empirically sterile” That is a very succinct description for the evidence of those things. For instance, of the evidence for Dark matter hinges on a narrative of the cosmos that is not empirical in nature. We should not be dismissing null results, as there is no positive results based on repeatable experiment.

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  11. Unknown,

    It doesn't change much about the current situation, that's the brief summary. The implication is that we're not any wiser :/

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  12. Ian,

    It's a possibility and one that cannot be ruled out. That's in a nutshell why it has been possible to amend the models for dark matter every time there was a null-result and to just make the particles more weakly interacting.

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  13. Sabine,

    Is there any theoretical reason why these particles might exist, but not interact with ordinary matter at all except gravitationally?

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  14. The WIMP theory has it that dark matter interacts with ordinary matter through neutral weak currents or the Z particle. This is starting to look dubious, and SUSY models such as the neutralino are now doubtful. They are not ruled out yet, but with the ZEPLIN III detector in a couple of years we may be able to rule this out. Axions are a possibility with a Klein-Gordon wave equation that has an inhomogeneous term κE∙B that couples the axion to QED. Atoms and nuclei have all sorts of complex physics involving arrangements of charges.

    The DAMA experiment is a bit different than the others in that it measures a variation in signals. This could mean a number of things. This annual cycle in the appearance of signal may just mean there is some change in the geomagnetic field in the northern hemisphere as it is impacted by the sun differently with its orientation relative to solar wind. I helped my daughter make a detector that found such variations in the Earth's magnetic field, and variations occur over the course of days and even hours. This due to differences in solar wind activity and charged particles interacting with the geomagnetic field. If I had to place a safe bet this is where I would put the money.

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  15. Here’s another sensationalist claim about dark matter: they’ve FOUND it in a galaxy billion of LY’s away. Strangely, they can’t find it on Earth ...

    phys.org/news/2018-12-astronomers-dark-dominates-cosmic.html

    Below the sensationalism is a link to the paper.

    sean s.

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  16. "Here’s another sensationalist claim about dark matter: they’ve FOUND it in a galaxy billion of LY’s away. Strangely, they can’t find it on Earth ..."

    No.

    Sabine's post and the comments are discussing direct detection of dark matter. What you link to is observational evidence which can be explained via dark matter in galaxies, something which has been around locally for decades---and why one is trying to detect dark matter directly in the first place.

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  17. @sean s: That is a disastrous paper. Massed dark matter is scavenged by black holes. An early universe with Tully-Fisher relation Goldilocks dark matter excludes the current universe having it. Goldilocks dark matter created as it disappears is fraud. If black holes evolve into white holes spewing dark matter, where are "stellar" spectra lacking atomic transition lines?

    LIGO is also a pair of Michelson interferometers: Compass roughly 221° and 311° (Hanford), 163° and 253° (Livingston). Given Fabry Perot cavities, then 1120 km long light paths, 144,000 times longer than than Michelson's instrument. Where are diurnal and annual phasing corrections for its substance differentially interacting with anything "interesting"?

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  18. Phillip Helbig;

    What you link to is observational evidence which can be explained via dark matter in galaxies,

    ... or explained by MOND. Or perhaps some third choice we don’t know about yet.

    Certainly, something is going on. All these new observations tell us is that, whatever is going on, it’s been going on a long, long time.

    But they did not find dark matter; no one has.

    Sabine's post and the comments are discussing direct detection of dark matter.

    Correct. But until we can directly detect it, or disprove all other explanations, dark matter is merely a S.W.A.G.

    sean s.

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  19. Helbig's point is that we know there is something that induces a gravitational field around galaxies that is not accountable with observable matter. The most reasonable explanation is there is some other form of matter outside of known elementary particle and gauge theory physics. If this is so and it interacts by some means then if we "kick it" with some interaction we know about it might just "kick back." Since this matter or field appears very weakly interacting the first and fairly reasonable hypothesis is that it might interact by the weak interaction. So attempts to measure a weak current interaction with lattices of atom in solids have been devised. The atom involved with the interaction will recoil and this will induce lattice vibrations or phonons.

    So far the results appear null. The neutralino is a condensate of the supersymmetric partner of the weak hypercharge and the weak neutral current called the bino and wino. With the weak angle rotation the weak hypercharge is rotated away. It may also contain the superpartner of the Higgs, called the higgsino. Since these all have the same quantum numbers they form a condensate. R-parity of the supersymmetric standard model (MSSM) computes this to be stable. This makes it a good dark matter candidate and it would interact by weak nuclear force. All appeared good, but LHC is not treating the MSSM model well and the WIMP idea of a weak neutral current interaction with dark matter is not yielding anything.

    We know there is matter or a field that produces a large curvature of spacetime around galaxies and clusters. The problem is that we do not know what it is. So far our attempts to "kick it" have failed to give a return signal.

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  20. This paper left me with more questions than answers.

    60 days of data vs over a decade of data at DAMA is a start, not a conclusion. Predicting expected noise in ultra-low background experiments is extremely hard to get right.

    DAMA results are about 0.01 counts per day per keV, and COSINE is 100kg, so its a difference of a few or 6 counts per day in the entire detector (2-6keV), so over 60 days, it's an extra/deficient ~300 counts on the measured ~50,000 that COSINE saw. Another way of looking at it is that they have ONE point on the DAMA main graph calculated.

    I don't think its surprising at all that the DAMA signal is not a classic WIMP - otherwise, the other experiments would not have seen it. Curiously, the data in the COSINE paper seems to be lower than the expected background by about 300 counts over 2 - 6 keV - which is consistent with DAMA results for the late fall (low signal, as DAMA peaks June 2) 60-day data gather. (result obtained by eyeballing figure 2 in COSINE).

    Why has it taken 2 years to analyze the collected data? Does COSINE have two more years of data?

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  21. As I mentioned in a comment on a different blog post, ANAIS-112 is also seeking to reproduce DAMA. They are not as far along as COSINE is; see https://arxiv.org/abs/1812.01377 for example.

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  22. @Bill, who wrote "Belief in dark matter, string and SUSY theories seems to be so locked in that ..." As others have already noted, you are conflating two very different things.

    Dark matter (DM), to an astrophysicist or astronomer (or some cosmologists), is something found in several kinds of observation, and is consistent across all of them. As I noted in a comment in a different thread, a shorthand list of these would include (not intended to be a complete list): the CMB (acoustic peak), galaxy rotation curves, lensing, and galaxy cluster dynamics. None of these observations sheds any light (ha!) on a microscopic theory of DM, except perhaps that DM does not "do" electromagnetism (at least, not like baryonic matter, to the power -3 or less).

    @Ian: to expand a bit on what Sabine wrote, the DM detectors/searches I am aware of - deep underground, heavily shielded, etc - all assume that the DM particles have certain properties at least somewhat similar to particles in the Standard Model (spin, able to react with ordinary matter via the weak force, etc). It's very hard - impossible? - to devise an experiment to detect DM particles whose properties are far outside the scope of the Standard Model (of particle physics, not cosmology).

    @David Bailey: It depends on what theory you are using ;-) For example, theories of neutrinos are several, and perhaps in one or more of those some sort of "DM as neutrino" might work.

    @sean s.: you wrote "All these new observations tell us is that, whatever is going on, it’s been going on a long, long time." But we already knew that ... the CMB is a lot older, and has an unmistakable, consistent-with-local-observations, DM feature.

    You also wrote: "But until we can directly detect it, or disprove all other explanations, dark matter is merely a S.W.A.G." You are making the same mistake Bill made, conflating two very different things.

    I like Lawrence Crowell's post! :)

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  23. @JeanTate: Dark matter (DM), to an astrophysicist or astronomer (or some cosmologists), is something found in several kinds of observation, and is consistent across all of them.

    Is that actually true?

    My understanding is that MOND is a good fit for galaxy rotation data when you properly handle error in nuisance parameters, but not for CMB.

    And particle dark matter models cannot effectively make predictions because of parameter degeneracy.

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  24. Technical question: As I understand it, without the DM effect (whatever it is) a galaxy would rotate rather like a solar system - i.e. stars near the middle would rotate a lot faster than those further out, and therefore the spiral arms of many galaxies would get smeared out over time. I guess the mass distribution of a galaxy isn't as peaked at the centre as a solar system, but that is a detail.

    I cannot see how DM can explain the existence of structure like spiral arms in galaxies, because the amount of DM and its distribute would have to be exactly right to achieve the effect - which seems really implausible. Isn't this a good reason to expect something like MOND - where the gravitational pull follows a definite mathematical law.

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  25. David,

    That's a good question, I haven't thought of this before. I'm afraid I can't offer a simple answer because I don't know enough about galaxy formation. Timing matters for the question how large the effect would be. I'll see if I can find an answer somewhere in the literature, or maybe someone else knows?

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  26. “… how DM can explain the existence of structure like spiral arms in galaxies …”
    “… maybe someone else knows?”
    As a layman in astrophysics I happened to sit in a talk by Volker Springel last week. In their simulation, including particle, cold DM (as Poisson-Vlasov system) they obviously can reproduce spiral galaxies. Here the eq. and explanation from a talk 3 years ago. It is a purely classical calculation – so, I also wondered why the SM mug is there ;-)

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  27. Resubmitted: link to a pdf file now removed - could have conflicted with your comment rule #3.

    “… how DM can explain the existence of structure like spiral arms in galaxies …”
    “… maybe someone else knows?”
    As a layman in astrophysics I happened to sit in a talk by Volker Springel last week. In their simulation, including particle, cold DM (as Poisson-Vlasov system) they obviously can reproduce spiral galaxies. Here the eq. and explanation from a talk 3 years ago. It is a purely classical calculation.

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  28. @Michael Musson: you wrote "Is that actually true?"

    Yes, it is true that CDM (Cold Dark Matter) is, from an astrophysics perspective, consistent across all relevant classes of observation. So far.

    For example, it's "cold", meaning that it's unlike (relict) neutrinos or photons. Also, when you estimate the proportion of mass that is CDM, you get similar answers (baryonic mass comprises only ~15% of all mass). If you assume the distribution of mass at the surface of last scattering (i.e. the CMB) - which will be dominated by CDM - and let gravity alone rip (i.e. you do a simple (ha!) simulation), you get large scale structures which closely resemble what you can see - clusters, filaments, and galaxies. At a wide range of redshifts (ages/time). It gets messy for smaller-scale things, hence the cuspy-core problem and the apparent lack of dwarf galaxies (simulation vs observation).

    "My understanding is that MOND is a good fit for galaxy rotation data when you properly handle error in nuisance parameters, but not for CMB." Indeed. But that has nothing to do with the consistency of CDM ... MOND (etc) is one alternative (non-mass) approach to understanding the nature of CDM, in this case a different theory of gravity.

    "And particle dark matter models cannot effectively make predictions because of parameter degeneracy." Indeed. But again, that has nothing to do with the consistency of CDM ... it's about finding a particle (mass) explanation.

    You could say that astronomers/astrophysicists are agnostic re the nature of CDM, but none of them doubts its (astronomical/astropysical) existence.

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  29. @David Bailey: you wrote " and therefore the spiral arms of many galaxies would get smeared out over time. I guess the mass distribution of a galaxy isn't as peaked at the centre as a solar system, but that is a detail."

    First, it's not a detail; the mass profile of most galaxies is indeed "peaked at the centre", but is quite unlike that of the solar system. The difference matters (ha!) a great deal.

    Second, spiral (and lenticular) galaxies are not the only morphological types; the dominant type (by mass) is ellipticals, and dwarf (of several morphologies, dominant by number); there are also the many kinds of irregulars.

    Third, it is pretty well established that the spiral arms are indeed transient; take a picture of a face-on spiral at many times in its life and you'll see quite dramatically different shapes. In fact, it seems that to get a nice "grand design spiral" (cf a "flocculent spiral", for example), you need a companion, generally lower mass, galaxy, and during a fairly narrow timeframe of the galaxy-galaxy interaction.

    Fourth, and finally (for now), the spiral arms of most spirals are quite small perturbations in the distribution of mass in the disk; to a good first order, the disks (two distinct ones are usually observed) of spiral galaxies have a smooth radial profile.

    "I cannot see how DM can explain the existence of structure like spiral arms in galaxies, because the amount of DM and its distribute would have to be exactly right to achieve the effect - which seems really implausible." Which is a good reason to not trust your intuition! Actual models of spiral galaxies, incorporating CDM and baryons (stars, plasma, gas, dust), work just fine in terms of being consistent with the observational data. Likewise elliptical galaxies. However, there are problems with (many) dwarf galaxies.

    "Isn't this a good reason to expect something like MOND - where the gravitational pull follows a definite mathematical law." No, if only because - as you describe it - it's a solution to a non-existent problem.

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  30. Spiral structure is not well enough understood to say what can - and more importantly, cannot - happen with dark matter. However, David's point that the mass distribution has to be exactly right to achieve the required effect is spot on. This is the chief objection to the dark matter interpretation in galaxies: why, out of all the endless possibilities that arise for a stellar disk embedded in a dark matter halo, does nature always pick the unique and rather bizarre option that looks just like MOND?

    David's question motivated a question on my blog, so I comment further on this in the response to that query: https://tritonstation.wordpress.com/2017/12/18/the-star-forming-main-sequence-dwarf-style/comment-page-1/#comment-1531

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  31. I brought David Bailey's excellent observations to the attention of Stacey McGaugh, over at TritonStation, and he responded: https://tritonstation.wordpress.com/2017/12/18/the-star-forming-main-sequence-dwarf-style/comment-page-1/#comment-1531

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  32. Hi Sabine,
    What do you think about this speculation of Rovelli: https://www.newscientist.com/article/mg24032080-100-if-you-think-black-holes-are-strange-white-holes-will-blow-your-mind/
    In it he mentioned the possibility of black matter are actually tiny white holes formed from small primordial black holes.

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  33. Oops … yesterday the links in here still worked. This official conference page obviously has the same problem. I just found almost the same talk. Best start here and spiral galaxies are reproduced 5 min later here. Since the eq. are missing in this talk best go here page 78.

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  34. First, I am pleased that my simple observation provoked such interesting and contrasting responses. This ranged from David Schroeder, who seemed to agree with me (both here and in his comment), to Jean Tate's response which was more or less to dismiss me as an outsider (which I most certainly am)!

    I suppose Jean Tate's concept of the galactic arms as transient structures, depends on time scales. I looked up the orbit time of the sun round the galaxy, and got 225 million years. I don't know how far in towards the centre the spiral arms remain distinct, but presumably before we reach that point the stars will be orbiting much faster. Thus if the spiral structures are indeed transient, it would have to be on a pretty small time scale - say 30 million years.

    Thus I suppose it might be reasonable to conclude that either these transient changes are fairly fast, or DM doesn't provide a good explanation because you need a precise distribution of DM (whatever the mass distribution within a galaxy) to make the explanation work?

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  35. Happy to see Stacy McGaugh has posted here! :)

    Lots to write about - be sure to read Stacy's entire blog post - but maybe in a day or two (I do have to spend time on my own research projects, one of which has to do with spiral galaxies). For now, this:

    @David Bailey: you wrote " Jean Tate's response which was more or less to dismiss me as an outsider (which I most certainly am)!" Sorry you read it that way. I was trying to emulate Sabine, to some extent, by indirectly saying that you were perpetuating myths, about the nature of spiral galaxies. Perhaps, if you are an outsider, it might be prudent to be a bit more humble? To not automatically assume that your understanding of spiral galaxies (etc) is correct?

    You also wrote: "Thus if the spiral structures are indeed transient, it would have to be on a pretty small time scale - say 30 million years." A conclusion you base on a BOTE (back of the envelope) calculation concerning just one spiral galaxy. Interesting.

    Finally, for now, you wrote: "Thus I suppose it might be reasonable to conclude that either these transient changes are fairly fast, or [...]" This reflects what seems to be very common in the posts here of those who like to speculate and who have an interest in non-particle explanations of CDM at local scales (i.e. individual galaxies), namely, unabashed acceptance of the logical fallacy, false dichotomy. Being commonly used does not make it any less of a logical fallacy.

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  36. Jean wrote:
    "This reflects what seems to be very common in the posts here of those who like to speculate and who have an interest in non-particle explanations of CDM at local scales (i.e. individual galaxies), namely, unabashed acceptance of the logical fallacy, false dichotomy. Being commonly used does not make it any less of a logical fallacy."
    What logical fallacy have I perpetuated? If the spiral arm structure lasts for a time that is long compared with the orbital motion of the stars - say the time to cover the distance from one arm to the next - then the force exerted by gravity has to have just the right value to make that possible, otherwise the orbital motion smears out the pattern.

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  37. David Bailey:

    I cannot see how DM can explain the existence of structure like spiral arms in galaxies, because the amount of DM and its distribute would have to be exactly right to achieve the effect - which seems really implausible.

    I'm not entirely sure what your argument here is. Perhaps you're thinking that spiral arms are fixed material structures (always with the same stars and gas clouds), like swirls of cream in a coffee cup being stirred, so that the inner part of the arm would move ahead and the outer part would lag behind, and so the arm would "smear out"? (This is often called the "winding problem", the idea being that the spiral arm would rapidly wind up and merge together.)

    But we have good evidence that many, probably most, spiral arms are *density waves*, which means that the arms are local "traffic jams", where stars are temporarily slowing down and bunching up as they move through the arm. The arm itself rotates more or less uniformly, with a particular angular speed; fast-moving stars at small radii approach and move through the spiral arm from behind, while slower-moving stars at large radii are overtaken by the spiral arm.

    In any case, simulations of galaxies with dark matter and without dark matter have, for decades, had little trouble forming spiral arms. And there are spiral arms in the very inner regions of bright spiral galaxies, where baryonic matter dominates over dark matter (or: where the accelerations are well above the MOND threshold). So in general you don't need dark matter to have spirals (and you don't need MOND, either).

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  38. Stacy McGaugh said: Spiral structure is not well enough understood to say what can - and more importantly, cannot - happen with dark matter. However, David's point that the mass distribution has to be exactly right to achieve the required effect is spot on. This is the chief objection to the dark matter interpretation in galaxies: why, out of all the endless possibilities that arise for a stellar disk embedded in a dark matter halo, does nature always pick the unique and rather bizarre option that looks just like MOND?


    What I do not at all understand about the supposed DM vs MOND being a scientific argument is that Dark Matter should be able to be falsified (proven wrong) without MOND being proved right or having even a small number of the answers of why the maths works. Why is Dark Matter being given such *huge* leeway in the burden of proof (DM is assumed with no burden of proof for its existence being accepted).

    For some reason MOND cannot be entertained as even part of the answer without a complete theory, while DM is what it is without any proof of its existence whatsoever except that we disbelieve that symmetry can be broken.

    regards
    Marco

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  39. "Dark Matter should be able to be falsified (proven wrong)"

    Be my guest. After it has been disproven, post a link to the paper.

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  40. I second Bailey's comments about galactic spiral arms. I am far from being an astrophysicist who studies galaxies. With my Celestron scope I can just make out spiral arms on some nearby galaxies. There are people who devote their lives to such studies, and as far as I know the structure of spiral galaxies and how they emerged in the early universe is not well known.

    Spiral arms in galaxies are density waves that orbiting stars run into. The larger density of gas and dust, which is most of the mass there, attracts stars by gravitation and temporarily holds them. While they look like the swirls of water going down the drain or tendrils of cream in coffee they generally rotate with a constant angular velocity along their length.

    Dark matter plays a secondary role in galaxy structure as I understand, and the ΛCDM gives little information on this. How these galaxy structures emerged from early density variations observed against the CMB is from my understanding unknown. It is rather odd, and one might ordinarily expect galaxies to be elliptical galaxies.

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  41. erratum: I meant Erwin who wrote that galaxies had density waves as spiral arms.

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  42. Lawrence Crowell:

    The larger density of gas and dust, which is most of the mass there, attracts stars by gravitation and temporarily holds them.

    A minor quibble: most massive spiral galaxies in the local universe (like ours) have a gas content that is only 5 or 10% that of the stars. Even in the middle of spiral arms the stellar mass density is usually higher than that of the gas. (The dust is a much smaller fraction of the mass, and so doesn't count for this.)


    How these galaxy structures emerged from early density variations observed against the CMB is from my understanding unknown. It is rather odd, and one might ordinarily expect galaxies to be elliptical galaxies.

    Actually, it's elliptical galaxies which are, in principle, a bit harder to explain. Because stars form out of dense gas, you have to have gas collapse to form regions of higher density, and on galactic scales, the combination of collisions between gas clouds and conservation of angular moment means that you will tend to end up with disks of dense gas, out of which most of the stars will form.

    Elliptical galaxies require an extra step: mergers between disk galaxies (or mergers between elliptical galaxies which themselves results from mergers between disk galaxies), which scramble the orbits of stars with different angular momenta. (Of course, since we see this sort of thing happening in simulations and in the real universe, it's not actually a problem.)

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  43. Peter Erwin, thanks for the hint with the “winding problem” - I was not aware of this at all. Just found animation 3 in here very descriptive.

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  44. @Marco Parigi: you wrote: "Why is Dark Matter being given such *huge* leeway in the burden of proof (DM is assumed with no burden of proof for its existence being accepted)."

    I'm a bit surprised to read this, from you.

    You do know, don't you, that CDM (Cold Dark Matter) is consistent with observations of the CMB (Cosmic Microwave Background), and that MOND is not? CDM is also consistent with galaxy cluster dynamics, lensing, and so on. So where does this "no burden of proof" claim of yours come from?

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  45. Some comments on spirals, and galaxies in general (if Bee moves to Facebook exclusively, this may be my last post here). These are rather disorganized.

    Are spiral arms leading, or trailing? As an outsider (those of you who are), how would you go about determining which? How would you assure yourself that, if you found a spiral galaxy that is, per good observational data, leading (say), that all spiral galaxies have leading arms? How does whether the arms are leading or trailing affect your expectation that differential rotation will "smear" the arms out?

    In terms of obvious morphological components, the bars which are seen in some spirals (and lenticulars, and irregulars) may be much more important as possible tests of things like MOND or CDM; they may even play a crucial role in the formation and long-term persistence of spiral arms! Ditto rings, which even some ellipticals have.

    If you look at the history of extra-galactic astronomy, you will see a common pattern: in the early days, just a few galaxies are examined, without much understanding of selection effects; a pattern is discerned, gets written up, and astronomers who come later accept the results. Sometimes without question, sometimes with more data (better telescopes/detectors, more galaxy) - confirming, partially confirming, not confirming - sometimes discarding the pattern, sometimes realizing that it's an epiphenomenon. In my own research, I have found rather too many of these "persistent patterns" which even modest research using today's huge, publicly available datasets show are myths (or at least that it's a great deal more complicated).

    A clue to how diverse "spiral arms" can be: ignoring redshift effects (e.g. limit yourself to a somewhat narrow range of z), how does a ~face-on spiral galaxy's appearance change with filter/band? In general the arms are more prominent in the UV and in [OIII] and H-alpha emission lines (say), becoming ~invisible in the NIR. Easily understood: arms are sites of greater (recent or on-going) star-formation, young stars (etc) are copious emitters of NUV, and the gas in star-formation regions, of [OIII], H-alpha, etc. But for some spiral galaxies, the wavelength dependent prominence is quite unlike this. Why?

    Homework question: the "spirals" are commonly described as "logarithmic spirals". Why? What does the data - from a large, modern dataset - say?

    Puzzle: in at least one NGC galaxy, there are two sets of spiral arms, "rotating" in opposite directions. How come?

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  46. Sabine,

    I do hope that you will not move to Facebook exclusively for this blog. I have found it extremely informative, but I am not on Facebook because I don't do social media - maybe it is an age thing!

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  47. @ Peter Erwin In looking at https://en.wikipedia.org/wiki/Galaxy There is the following quote: "Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[69] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[70] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[71]" If I assumed all the stars have the mass of the sun this would mean there is some 4x10^{11} solar masses not accounted for. Also the sun is a G-class star which make up about 13% of stars as I recall. Between 50 to 60% of stars are red dwarf stars with less than half the mass of the sun. This seems to suggest there is a lot of gas in the galaxy.

    This is as far as I got in my cursory look into this. I seem to remember learning that over 50% of the mass of a galaxy is gas.

    Spiral arms are important for star formation, for as gas runs in and out of them it is compressed which triggers star formation. Elliptical galaxies have low rates of star formation since they lack spiral arms.

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  48. JeanTate said You do know, don't you, that CDM (Cold Dark Matter) is consistent with observations of the CMB (Cosmic Microwave Background), and that MOND is not? CDM is also consistent with galaxy cluster dynamics, lensing, and so on. So where does this "no burden of proof" claim of yours come from?


    I have read widely both in the peer reviewed literature and elsewhere and I note, when I look for it in those papers, that many other theories are just as consistent with observations of the CMB as CDM. Similarly with Galaxy clusters and so forth. As we don't really observe them dynamically (we just see lots of different ones, presumably at different stages of an assumed dynamic), there is a lot of Ockham's Razor within CDM models as with others to presume that we know the dynamics that cause the clustering and so forth. MOND does not specifically say anything at all about the CMB or Galaxy clusters - They are too far away to check whether galaxy rotation relations extend to similar laws in clusters.

    But I am not saying anything about MOND's burden of proof at all. In fact, MOND is considered as an alternative when many other variants such as Quantised Inertia do not even list as a possibility even though the relations work out better, with no adjustable parameters.


    I am strictly questioning why we accept CDM without proof of its existence, regardless of lack of proof of its alternatives. Where is the supposed (scientific) scepticism and reasonable doubt of Newton's efficacy in the low acceleration domain? Or our reasonable doubt about what the CMB is? Or our reasonable doubt about how any particular galaxy cluster evolves?


    Dark Matter is not proven (and I would suggest galaxy rotation relations disprove it) but it is accepted until a theory for everything is proven based on an accepted alternative for the DM segment (like MOND).

    Regards
    Marco

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  49. "Are spiral arms leading, or trailing?"

    Trailing, i.e. they look like they are formed as a result of winding up. One can easily see the orientation, and with the Doppler effect measure the direction of rotation. See https://en.wikipedia.org/wiki/Spiral_galaxy", particularly the animation.

    Let me take this opportunity to plug a book on galaxies which I translated from German to English. It is mainly a photographic atlas of galaxies, with many nice pictures, but has a fair amount of text. The interesting thing is that the level is between a popular-science book and a university-level textbook, so is appropriate for non-experts who want more than a superficial explanation. (And it makes a great Christmas present!)

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  50. Stepped out on the back porch on this frigid -11 C. morning, and gazed upon the outer reaches of our Milky Way galaxy; a band of light just faintly visible arching across the sky. Somewhere out there an invisible boundary demarcates the region where stars deviate from Newton, whether it be from a vast halo of yet to be discovered particles, or a new law of nature whose essence is captured, at least for galactic systems, by MOND. Either way, the resolution of this mystery will expand our understanding of nature.

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  51. @ Lawrence Crowell:

    The "total mass" in the statement you quote is an estimate of all the mass within a certain radius, including dark matter. If you go to the Wikipedia article on the Milky Way itself, you can find the following in the "Size and Mass" subsection, which is a reasonable summary of what we know:

    The total mass of all the stars in the Milky Way is estimated to be between 4.6 x 10^10 M_sun and 6.43 x 10^10 M_sun. In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass, with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen. The mass of this gas is equal to between 10% and 15% of the total mass of the galaxy's stars.

    There are certainly galaxies where gas is 50% (or even 90%) of the total baryonic mass (i.e., excluding any dark matter). But for massive spirals like the Milky Way, 10% is a more typical value.

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  52. @ Marco Parigi:

    I'm sorry, but when you say things like this

    As we don't really observe them dynamically (we just see lots of different ones, presumably at different stages of an assumed dynamic), there is a lot of Ockham's Razor within CDM models as with others to presume that we know the dynamics that cause the clustering and so forth. MOND does not specifically say anything at all about the CMB or Galaxy clusters - They are too far away to check whether galaxy rotation relations extend to similar laws in clusters.

    it's clear to me that you don't really understand what's going on here. "We really don't observe them dynamically" applies just as much to the rotation curves of individual galaxies, which we are also seeing "at different stages of an assumed dynamic". Only in the Milky Way is it possible to see individual stars moving across the sky (proper motion); everything else is based on Doppler shift measurements, just as for the galaxies in groups and clusters.

    (There's also the evidence from the hot, X-ray emitting gas in some groups and most clusters: the temperature and pressure of this gas is too high for it to remain confined within the group/cluster unless there's some extra, unseen mass providing the gravity to hold it in. Otherwise, it would have dispersed into the surrounding space hundreds of millions or billions of years ago.)

    And of cours we can check "whether galaxy rotation relations extend to similar laws in clusters" -- proponents of MOND all agree that MOND doesn't work on scales of galaxy groups and clusters: you need extra mass of some kind (maybe massive neutrinos?). The very first evidence for dark matter came from Fritz Zwicky's measurements of the motions of galaxies in the Coma Cluster, way back in the 1930s.

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  53. @ Peter Erwin
    "But for massive spirals like the Milky Way, 10% is a more typical value."

    You don't agree with this paper: https://arxiv.org/abs/1412.3116v2 ?

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  54. @ Yves:
    The discussion was about the amount of gas in galaxy disks (e.g., whether there's more mass in the form of gas within a spiral arm than there is in the form of stars). Since the stellar disk of the Milky Way extends to roughly 15 or 20 kpc in radius, that's the scale of the problem.

    The paper you referenced is about gas in a very extended halo, mostly outside the main disk. Note that their estimate is 4 x 10^9 M_sun (about 10% of the stellar mass) within a radius of 50 kpc, and then 4 x 10^10 M_sun (comparable to the stellar mass) if you extend the radius to 250 kpc.

    I'm inclined to think it's likely that extra diffuse, hot halo gas is real, but it's not directly relevant to the question of how gas-rich or gas-poor disks (or, e.g., the main stellar bodies of ellipticals) are.

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  55. @David Bailey: please re-read what I wrote. Th logical fallacy is in the last sentence/paragraph, where you present a choice between two things, as an either/or. Even if one restricts oneself to just two, there are four logical possibilities, “both” and “neither” being the two you did not, apparently, consider.

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  56. @Marco Parigi: I echo what Peter Erwin wrote; what you wrote seems to reflect several misunderstandings of the relevant astrophysics. And cosmology. You also seem to misunderstand the scope of MOND; it is universal, so it is entirely reasonable to ask how well it can explain the CMB observations.

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  57. Jean Tate wrote

    "Th logical fallacy is in the last sentence/paragraph, where you present a choice between two things, as an either/or. Even if one restricts oneself to just two, there are four logical possibilities, “both” and “neither” being the two you did not, apparently, consider."

    I would argue that suggesting DM plus modified gravity would fail on the grounds of Occam's razor. Proposing neither leaves an unanswered question.

    I tend to find that folk engaged in constructive discussions rarely make appeals to the primitive axioms of logic, even though they are implicitly relevant. In this context, I would have thought a constructive response might be to simply answer my original question, explaining how you reconcile the perceived structure of spiral galaxies with GR or Newton's laws.

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  58. @Peter Erwin: I recall in the comment section of an earlier post here, (can't remember which post), it was mentioned that angular kinetic energy would be conserved in a Mondian interpretation of the anomalously high rotation rates of the outer stars of a galactic system, without a Dark Matter halo. That, naturally, puzzled me as I automatically assumed that extra kinetic energy can't just appear in a system without some additional kinetic energy source being provided, such as from a Dark Matter halo circulating around the galaxy. It's possible my memory is faulty, or I misinterpreted what was said.

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  59. "whether galaxy rotation relations extend to similar laws in clusters" -- proponents of MOND all agree that MOND doesn't work on scales of galaxy groups and clusters:

    I will go back to the issue of burden of proof. Dark Matter does not *explain* Galactic rotation relations. In fact, the galactic rotation relations mean that dark matter is in precisely the right locations within a galaxy to fit the maths of MOND with only one adjustable parameter a0, or to fit the maths of Quantised Inertia with no adjustable parameters at all, or a number of other plausible variants of acceleration dependent dynamics. I have little to no investment in these alternatives as foundational theories of everything, but that they must certainly be part of the picture, and Dark Matter cannot be for Galactic rotations.


    Dark Matter is generated ad hoc by the Newtonian equations of motion. The dark matter that is calculated does not then tell us anything at all except for the motion dynamic which calculated it in the first place. Dark Matter is dead in the water for the very galactic rotation relations it has been invoked for, and we must be cautious in assuming GR's Newtonian Dynamic aspects are perfectly correct for distant galaxy clusters and so forth based on the failure of Newtonian Dynamics to explain galactic rotations.


    That being said, what we think we know about galaxy clusters has to be mediated by the gaps in understanding of what is going on with Newtonian Dynamics. The arbitrary placement of Dark Matter to suit equations of motion has to be taken with a pinch of salt.


    Again - I'd be happy enough, like with Stacy McGaugh, to say that Dark Matter and MOND look disproven in different domains. Yet we keep studying Dark Matter as if it is proven to exist, and we allow MOND as only a minority alternative, and completely dismiss other theories with merit, such as Quantised Inertia.


    Why is Dark Matter so special that we accept it without proof, considerable evidence against it, and a lack of utility to boot?

    regards Marco

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  60. Peter Erwin says Only in the Milky Way is it possible to see individual stars moving across the sky (proper motion); everything else is based on Doppler shift measurements

    Only in and near our local group can we resolve individual stars for their Doppler Shift. We can calibrate doppler shift with proper motion and be quite certain of the relationship between star's velocity and redshift. However, individual stars cannot be resolved for the very clusters and superclusters to check whether individual star movements is related to acceleration and/or baryonic mass. We can only compare and contrast to the relative movements within our own local group. Dark Matter can be invented to suit any and all equations of motion, including MOND ones. Surely you can see that the only reason that we don't falsify Dark Matter is that we construct it *always* in an unfalsifiable manner. Of course there can be a consensus that Dark Matter is *required* even for MOND, but that only goes to demonstrate its relative lack of falsifiability - Not actual, repeatable experiment.

    regards
    Marco

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  61. David Bailey:

    Allow me, please, to try to describe how I see things, especially “MOND” vs CDM”.

    First, if we’re having a discussion about physics, particularly about fundamental physics, then MOND was DOA the day Milgrom’s paper was published in 1983. Why? Because even he knew (and likely said, I’ll see if I can find what he said on this), as he developed MOND, that it is inconsistent with both Special Relativity (SR) and General Relativity (GR). So, if there’s to be an alternative theory of gravity, it has to be one of the several “MOND extension” that are compatible with SR at least. And any such theory which seeks to extend, or replace, GR must pass all the same experimental and observational tests as GR has. I do not follow this, but recall that at least one - TeVeS? - does (I’ll be a bit cheeky and say that, unlike GR, it’s ugly :-)).

    Second, if we regard MOND as “Milgrom’s Fitting Formula”, then it can join the many similar which astronomers use, as a very good, empirical relationship! :) Some of these - such as the Hubble relationship - went on to get a very firm theorical explanation (in this case GR). Others remain, to this day at best weak in terms of ties to fundamental physics, e.g. the Fundamental Plane. In part because what’s being studied - galaxies - are incredibly messy, complicated things. Being cheeky again, it’s like why you can’t easily tie ecology’s predictor-prey relationships to what the quarks, gluons, etc are doing ;-).

    Third, Occam’s Razor: if you’re looking at an-SR compatible MOND extension, then it’s DOA. Any such alternative gravity theory MUST be consistent with all relevant observational and experimental results. From this perspective, the failure to match CMB observations means Occam is not relevant. As a fitting formula, MOND wins (so far, at least). Or does it? As Stacey McGragh said, in an earlier comment, spiral formation in galaxies is ... complicated (I’m paraphrasing). And MOND is of limited use. E.g. ASAIK, MOND is of no help in trying to understand the huge range of observations of AGNs (Active Galactic Nuclei, quasars are a subset of AGNs).

    Hope this clarifies things a bit.

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  62. Marco Parigi,

    I’ll turn the contrast up to beyond Spinal Tap’s 11.

    Is a relatistic MOND extension compatible with lung cancer oncogenesis? Is CDM? Silly question, right? But why is it silly?

    How about with the Permian-Triassic Great Extinction? Still silly, but perhaps there might be some indirect connection.

    The Orion Molecular Cloud Complex? No longer a silly question ... but what is the relevance, exactly?

    SgrA*, the G2 cloud, the Arches star cluster? Certainly relevant, but how, in detail?

    I’ll skip galaxy rotation curves, and even the apparent motions of galaxies and the hot IGM; clearly relevant.

    How about the formation and evolution of galaxy clusters and giant filaments? AFAIK, not much work has been done, from a MOND-like perspective. CDM does very well.

    Finally, the CMB. Perhaps you can tell us what a MOND perspective is, on the main parameters, and their values, derived from the many CMB datasets? Lambda, Omega_b, Omega_m, H0, redshift of the LSS, thickness of the LSS, effective number of neutrino species, ...

    OK, one more: burden of proof. I approach this from the perspective that experimental/observational results are king; your fave theory, hypothesis, model, even empirical relationship is not much good if is inconsistent with all relevant results, within its domain of applicability. Viewed this way, CDM is good to very good ... but MOND (and even its relativistic extensions) is bad to totally dead, except as an fitting formula for galaxy rotation curves. Why do I give CDM a pass wrt galaxy rotation curves? Because, despite many scientists’ wishes, galaxies are very complex, messy systems, whose evolution histories are, even today, poorly understood.

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  63. Erratum: Is not much good if it is inconsistent with all relevant results -> “is not much good unless it is consistent with all relevant results”

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  64. @ Marco,

    Let me see if I can explain what's going on with galactic and cluster kinematics and dynamics, and hopefully alleviate some of your confusion.

    No one measures galaxy rotation curves using individual stars. In most cases -- including the classic work by Vera Rubin and collaborators -- the measurements are Doppler shifts for *gas clouds*: either ionized clouds seen in the light of hydrogen emission lines or cool atomic gas seen in the 21-cm radio emission from hydrogen atoms (and sometimes radio emission lines from carbon monoxide molecules). This is for two reasons: 1) It's much easier to measure accurate Doppler shifts using gas emission lines than it is to measure it using the spectra of stars; and 2) When you're dealing with disks, the gas is usually going to be in close to circular orbits, which makes it much easier to deduce the gravitational (or other) forces involved. (This is because gas clouds on elliptical orbits will tend to intersect with other gas clouds until they settle onto non-intersecting circular orbits, while stars can have very elliptical orbits without intersecting other stars.)

    There *are* galaxies (e.g., lenticular and elliptical galaxies) where there is little or no gas, in which case you can measure mean stellar motions using the integrated light of millions or billions of stars at different regions within the galaxy. It requires higher-quality data, and it also requires that you account for the fact that stars don't have to follow circular orbits when you model the system. You can also sometimes measure the Doppler shifts for individual globular clusters, and sometimes for planetary nebulae (which have bright gas emission lines). This gives you velocity estimates for objects which are usually not in circular orbits, so the modeling is more complicated. But these basically always end up demonstrating the need for extra mass at large radii, just as we deduce from the Doppler shifts of gas clouds. (And this has been done using both approches for some galaxies; the results are consistent.)

    For groups of galaxies and clusters of galaxies, the measurements I'm talking about are not stellar or gas motions inside individual galaxies. Instead, people measure the mean velocity of each individual galaxy, so you end up with single velocity measurements for each of several dozen (or hundred, or thousand) galaxies moving around within the group or cluster. (This is what Zwicky did back in the 1930s.) Then the question is: is the motion of the ensemble of galaxies consistent with the mutual (Newtonian) gravity from the mass of all the galaxies themselves (and the mass of the hot intergalactic gas that often exists in large groups and clusters)? The answer is: no, the galaxies are moving too fast, and these groups and clusters should have flown apart long ago. You can also ask the same question using MOND instead of Newtonian gravity, and the answer is still: no, the galaxies are moving too fast, and these groups and clusters should have flown apart long ago (maybe a little more slowly than in the Newtonian-gravity case, but still).

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  65. Very nice, well-written post, Peter Erwin! :)

    To see just how messy/difficult it is to derive a galaxy rotation curve from data on stars alone, check out arXiv.1810.09466, “The Circular Velocity Curve of the Milky Way from 5 kpc to 25 kpc”.

    Also of note is that, with GAIA, there are papers on the rotation curves of some nearby galaxies, based in part on proper motions of stars.

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  66. JeanTate says: How about the formation and evolution of galaxy clusters and giant filaments? AFAIK, not much work has been done, from a MOND-like perspective. CDM does very well....

    Finally, the CMB...

    Viewed this way, CDM is good to very good ...

    Because, despite many scientists’ wishes, galaxies are very complex, messy systems,...



    Again, I don't want to be defending MOND, because my argument is clearly that Dark Matter has no utility and is either falsified or unfalsifiable via definition creep.

    Dark Matter, whether to explain the big bang in terms of evidence of the CMB or to explain the movements of galaxies within clusters etc. is invented to fit the prevailing hypothesis. Now, all scientific models invent things - you have to start somewhere. What gives a hypothesis utility is convergence. The closer you look at relevant evidence, if your hypothesis is correct, the more convergence you should get. Close observations give you ground truth at high resolution, and if there is concordance between ground truth and distant measurements then, as scientists, we can be confident we are on the right track. If there is instead divergence, than the higher the resolution of the observation, the greater the weight we have to place on the observation.

    What is happening with Dark Matter instead is divergence. Getting back to the original post we are commenting on, and perhaps Sabine would like to comment on this, is that the particle view on Dark Matter is coming up with consistently Nil results.


    As we go further out, Wide binary star systems in the Milky Way are orbiting their barycentre much faster than Newtonian Dynamics would suggest. Must there be dark matter at their barycentre? Our own Sun should be in a Halo of Dark Matter to explain the movement of stars around the edge of our Milky Way. We measure the doppler shift of the gases corotating on the edges of nearby galaxies, and the Dark Matter is in precisely the locations that follow galactic rotation rules rather than any matter clumping probabilities.

    So at the maximally non-local evidence, the CMB, we can invent precisely the amount of Dark Matter that can explain the mechanism of the early universe that... we invented. It is easy to make a self consistent theory when we fine tune the values that we need by making them up rather than observing them. That we can add just the right amount of Dark Matter to make our universe origins theories work hardly makes it scientific. There is serious divergence the closer we try to look at the very thing we are hinging our hypothesis on. What is the point of Dark Matter if it is more complex, more divergent, messier, not understood, invisible, beyond our power to detect it within our Galaxy, and more to the point, as a tangible object.

    What this divergence tells me and should be taken in to evidence, is that we are mistaking a non-local effect with an invisible local object. What we should really be doing is taking a broader look at our assumptions of the narrative of the universe, and look at why there is conservation of our conservation laws (why gravity, inertia, 4 forces etc) work the same here and now to the far reaches of the cosmos. Noether's theorem states that if something is conserved, that there should no HAS, to be a symmetry associated with that. There needs to be a symmetry between particles and the maximally non-local (CMB) for the conservation of conservation laws to hold true. A hypothesis along these lines is far more likely to explain non-local issues such as universal gravity and Hubble expansion than Dark Matter and Dark Energy.


    We are lost in Maths, as a species of Foundational theorists. We have to again, take a look at our basic assumptions of the universe and think outside the closely confined false dichotomy of DM vs MOND.


    regards

    Marco

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  67. Marco Parigi,

    Thanks for your post, I think I now have a better understanding of your perspective.

    Clearly, we view astrophysics differently, and - it seems to me - have very different approaches when it comes to evaluating observational results. Out of curiosity, wrt CDM, could you sketch an observational program that you think could advance our understanding of the relevant (astro)physics?

    Re the wide binary: can you cite a reference please? I don’t think I’ve read anything on this. Likewise the “Halo of Dark Matter ”.

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  68. "MOND (and even its relativistic extensions) is bad to totally dead, except as an fitting formula for galaxy rotation curves"

    Whatever the arguments for and against MOND and LambdaCDM, to claim that "MOND is just rotation curves" displays an ignorance of the literature.

    Whatever one thinks of MOND from a theoretical point of view, there can be no denying that MOND phenomenology (i.e. things which naturally fall out of MOND but are at least not obvious with CDM) is real, and it is much more than rotation curves.

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  69. Re the wide binary: can you cite a reference please? I don’t think I’ve read anything on this.

    I recently discussed this with someone who knows more about it than I do. If I understood correctly, it is an interesting test of MOND, because MOND makes a clear prediction here, while any CDM explanation would look very contrived. But I don't think that there are any observations supporting the claim. Nor are there any which contradict it. Observations are just not yet good enough. But perhaps in a few years GAIA data will allow one to make this test.

    "Likewise the “Halo of Dark Matter ”.

    There are literally thousands of papers on dark-matter halos. They have been one of the main objects of study in astrophysics in the last 30 years or so.

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  70. Phillip Helbig,

    Can you cite a summary paper (or two) on “MOND phenomenology ...” (sorry can’t copy/paste the rest), other than galaxy-scale things? Especially those “not obvious with CDM”? To clarify one thing: I am aware of some of “MOND phenomenology”, other than galaxy rotation curves, but no unexpected successes at scales greater than ~100 kpc.

    My question to Marco was on his very specific statement re “Halo of Dark Matter”, not the general topic. Yes, I am very well aware of the existence of those thousands of papers, and quite familiar with many of them. In this regard, I’d be very surprised to learn that you know exactly what Marco meant.

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  71. Problems with the "halo of dark matter". I suggest to read this piece by Stacy McGaugh and then to drill down to specific peer reviewed papers he has written on the subject. I recommend a really broad view *before* drilling down to the specific maths that demonstrates how arbitrary the use of Dark Matter is in terms of Galactic Haloes. There's no point spending hours understanding the mathematical argument for a particular reason for a halo without first looking at the broad picture of the galactic rotation relations for hundreds of galaxies, and the strategy of Dark Matter haloes to fit them. Don't take it from me. Take it from a physicist who is looking at the broad scale utility of DM and MOND.

    http://astroweb.case.edu/ssm/darkmatter/WIMPexperiments.html

    For the case of Wide Binaries - I suggest you get a broad view from the work of Mike McCulloch - see blog entry:

    http://physicsfromtheedge.blogspot.com/2018/09/wide-binaries-20.html


    There are very large error bars in these Wide Binary relative velocity measurements, but the point is that Newtonian Dynamics calculations are *Outside* even those large error bars, while MOND and Mike McCulloch's speculative alternatives are within the error bars.


    This would strengthen the case for MOND, especially if GAIA etc. can narrow down these error bars in the near future.

    Following link is of the preprint of the paper referenced for Wide Binaries (peer reviewed - link is for Arxiv archive)
    https://arxiv.org/abs/1401.7063

    Regards
    Marco Parigi

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  72. As I mentioned, I am a former chemist, and as a kid I read about the gas laws (PV=nRT) in a textbook. I was enthralled by the 'fact' that the outcome of so many experiments could be predicted (maybe to arbitrary accuracy - I didn't know) by such a simple formula. I didn't express this as mathematical beauty, but I guess that was what I felt. I still remember how let down I felt when I learned that this was only an approximation which tended to break down at low temperatures and/or high pressures. More accurate equations were far more messy and more like curve fitting - the beauty had vanished. Of course the reason is obvious - gasses are made of molecules which have structures and inter-molecular forces.

    This made me very aware of the fact that many physical laws only work for a range of values, and huge extrapolations can fail. I wonder if there is any evidence that matter on the scale of galaxies moves in accordance with Newtonian/GR predictions - and if it doesn't, would we ever know if the CDM hypothesis can fill the gap?

    It seems to me that physical laws should not be simply assumed to continue to hold over many orders of magnitude of variation of the parameters - scaling up from solar system distances to galactic distances involves a factor of about 10^8!

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  73. Jean Tate and Phillip Helbig, here's the paper on wide binaries that's mentioned by Marco Parigi above, which is linked on Mike McCulloch's site.

    https://arxiv.org/abs/1401.7063

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  74. Just a flash-in-the-pan thought. Gravitomagnetism (GM), which is incredibly feeble, would, at first blush, hardly be considered a factor in the anomalous rotation curves seen in galactic systems. From a quick Google search the idea of GM influencing galactic rotation curves seems to be limited to the GM field emanating from the massive black holes at the center of most disc galaxies. Even if the strength of such a field, emanating from a galaxies central black hole, was the right magnitude/direction to account for Mondian behavior, the variation in size of each galaxies black hole would seem to require a custom fit to reproduce the Tully-Fisher relation, and other relations, inherent in the MOND paradigm. On the other hand, if central black holes are somehow implicated in Mondian behavior, the failure of MOND to work in galactic clusters could stem from the fact that the black holes are distributed throughout the cluster and randomly oriented. Similarly, MOND would fail in systems that lack a central black hole.

    Something else to consider regarding GM fields. Back in 1989 Jean Tate, and her colleagues, discovered a discrepancy (going from memory, paper behind paywall) in the apparent mass of Cooper-pairs, during experiments involving the high speed rotation of silicon balls with a thin niobium ring etched around the circumference of the ball, and which were brought down to liquid helium temps. These balls were being used in the Gravity Probe B experiment to detect the Earth's GM field. In the period between 2003-2006 Martin Tajmar's group at the Austrian Research Center (ARC) repeated Tate's experiment with a small ring of Niobium, that was spun-up in a cryogenic chamber. Hundreds of runs were conducted. They found that an acceleration field emanated from the ring, picked up by accelerometers in the ring's equatorial plane, which was 10 times larger than could be accounted for by General Relativity.

    Still, such a GM field is quite feeble. But galaxies have great clouds of hydrogen gas that are near absolute zero, so in principle could form Bose-Einstein condensates (BEC's). Now BEC's have been invoked for particulate Dark Matter, as written up by Sabine in earlier posts, and in formal papers by Khoury et. al., Sabine with her grad student Tobias Mistele, and others. But these hypothetical Dark Matter BEC's at the cusp of galaxy's would be quite massive, maybe as much as all the galaxy's baryonic matter combined (didn't completely read the relevant papers). Unfortunately, the amount of hydrogen gas in disc galaxy's is only so maybe not enough to produce a GM field sufficient to affect the dynamics of the galaxy, even if enhanced by being in a BEC state. But something else to consider is that MOND fails where the gas content is much larger than in disc galaxies. So, who knows, maybe something is going on there with respect to GM field from a hydrogen gas BEC's?



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  75. Erratum, the name of the chief investigator of the Cooper-pair mass anomaly is Janet Tate, not Jean Tate, as I mistakenly wrote above. She is now at Oregon State University.

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  76. @ David Schroeder:

    But galaxies have great clouds of hydrogen gas that are near absolute zero, so in principle could form Bose-Einstein condensates (BEC's).

    The coldest gas in galaxies has temperatures of about 10 K, which isn't exactly near the microkelvin levels required for BECs. (Also we see the same dark-matter-dominated dynamics in galaxies without any cold gas, so that's a non-starter.)

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  77. Out of curiosity, has the possibility that seasonal variations in volume and/or distribution of the Gran Sasso mountain acquifers might in some way affect the frequency of detected "interaction events" ever been investigated? I mean, granted the exquisite sensitivity of the DAMA detectors, is that also just as exquisitely selective to definitely rule out the detection of other types of interaction events ? Coming back to the aquifer seasonal variations, for one thing the glacier at the mountain top would supply more water starting late spring, and so would the snow cover. Average precipitations are also season dependent. Could that affect in any way some as yet unidentified background "source of events"? in its path through the mountain, water might dissolve, or pick up, some radioactive substances/elements and distribute those around differently through the year. Moreover, a seasonally affected aquifers distribution might also result in different shielding effects (increasing it or decreasing it, according to the particular variation of said distribution) with respect to some less variable background source. But I have no idea whether the DAMA detectors are indeed so extremely sensitive that they could be affected in any way even by such tiny seasonal variations, if any ...

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  78. by the way, rather than just plotting the multi-decade average seasonal variation in the frequency of detected interaction events, what about a quick cross-check looking at variations between exceptionally dry and exceptionally wet years ? sure some must have occurred through the decades the experiment has been running.

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