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Thread: No Dark Matter in our part of the galaxy?

  1. #1
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    No Dark Matter in our part of the galaxy?

    This is should be a shock for DM theorists.

    Press version here.
    Preprint of paper here.

  2. #2
    The final piece of the paper states,

    It is clear that the local surface density measured in our work, extrapolated to the rest of the Galaxy, cannot retain the Sun in a circular orbit at a speed of ∼220 km s−1. A deep missing mass problem is therefore evidenced by our observations. Indeed, we believe that our results do not solve any problem, but pose important, new ones.
    I look forward to seeing how this can be reconciled with the extensive body of other measurements for Galactic dynamics.

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    It's 35 pages, and I'm going through fairly carefully. At the bottom of page 6 and top of 7, the authors note a lack of radial velocity measurements, and in general a look forward to having data from the ESA's Gaia probe (ten years from now)... and that radial measurements are critical to their method of trying to assess the DM density at our part of the galaxy.

    I'll keep reading to see if the simplifying assumptions are the cause of their alarming conclusion.
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    So, I looked ahead... and am still plowing through. Someone correct me if I am wrong, but it looks like they are looking at other people's data and interpretations on the motion of red giants in the inner part of the galaxy, not more than half way from the center to where we are, and then guessing how the matter density curves extrapolate once they get to us.

    Is there some critical thing I'm missing here? (I haven't gotten to "Assumption X" yet in my detailed read). It is a long paper with several authors from the ESO in Chile, so I have to assume there's more here than what I note above, but I haven't found it yet.
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    Abstract: "Only the presence of a highly prolate (flattening q >2) DM halo can be reconciled with the observations, but this is highly unlikely in LCDM models."
    With the dynamical rotation of our galaxy over the last ~10 billion years, why is this so highly unlikely?
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    With the dynamical rotation of our galaxy over the last ~10 billion years, why is this so highly unlikely?
    I'm not sure... I'm not looking at any simulations, but they would probably have the answer. Without numbers and just handwaving, I can imagine that between past interactions with M31, and with the merging with numerous smaller galaxies, if they had any planer orientation on joining, that could also result in a flattened phase space for the halo.
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    From the top of Page 24...
    In fact, the calculation does not measure the mass directly, but
    a variation of the gravitational potential, from which the mass density is derived. Moni Bidin et al.
    (2010) noted that, if the DM halo was too uniform, extended, and poorly concentrated, it could
    cause a negligible change of the potential in the volume under analysis, thus resulting undetectable.
    This matches some of my prejudices about this topic... If the density of the DM is too uniform, this calculation is not valid. So (if I read this correctly), this study could be more about the uniformity of density of DM, and not about its existence.
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    Quote Originally Posted by antoniseb View Post
    I'm not sure... I'm not looking at any simulations, but they would probably have the answer. Without numbers and just handwaving, I can imagine that between past interactions with M31, and with the merging with numerous smaller galaxies, if they had any planer orientation on joining, that could also result in a flattened phase space for the halo.
    Is it safe to say that theorists weren't predicting a prolate-spheroidal distribution of DM with q>2, kind of like a football aligned with the galactic axis?

    Is Dark Matter more affected by encounters with small galaxies than visible matter? Seems like a stretch. 10GYs is also plenty of time for things to settle down. Besides which the MW is a rather typical spiral, not unusually disturbed.

    The properties of DM are highly speculative. The only substantive evidence for DM is gravitational effects that imply more mass (presuming GR is the correct theory of gravity) than has been directly detected.

    A more direct conclusion is that there is no non-baryonic dark matter and the rotation curves have another explanation.

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    Quote Originally Posted by TooMany View Post
    Is it safe to say that theorists weren't predicting a prolate-spheroidal distribution of DM with q>2, kind of like a football aligned with the galactic axis? ...
    Ummm, my read is not that it's shaped like a football, but that q>2 means that it is the short axis is perpendicular to the plane of the galaxy. Also, I don't know what you mean by safe to say. Some theorists don't expect that, but looking at weak lensing studies it is clear that not all dark matter halos are spheres.
    Is Dark Matter more affected by encounters with small galaxies than visible matter? Seems like a stretch. 10GYs is also plenty of time for things to settle down. Besides which the MW is a rather typical spiral, not unusually disturbed.
    More affected? No, I wouldn't think so... but we're not seeing a more affected case here, are we?
    The properties of DM are highly speculative. The only substantive evidence for DM is gravitational effects that imply more mass (presuming GR is the correct theory of gravity) than has been directly detected.
    highly speculative? No, not in the large. We have some observations that narrow down what the behavior must be pretty tightly. It must be in the form of small, neutral masses. WIMPs make a lot of sense. Something else small and plentiful would also work. Not much else can.
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    Among the final conclusions in the paper:
    if our results are interpreted as evidence of a highly prolate cold DM halo with q ≥2, this would have a local density lower than 2 mM⊙ pc−3, i.e. more than a factor of four lower than what usually assumed
    Less Dark Matter is not No Dark Matter. Still other issues listed in posts above probably still apply to weaken the statement in the title of this thread.

    All that being said, this is an interesting study, and one of the points that it makes is that we have some additional interesting/important things to use Gaia data to look at.
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    Quote Originally Posted by TooMany View Post
    A more direct conclusion is that there is no non-baryonic dark matter and the rotation curves have another explanation.
    That conclusion would not explain why, when you use one amount of dark matter for the rotation curve, you also explain galactic cluster movements, lensing observations, and effects seen in the CMB. It's just not rotation curves.

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    Quote Originally Posted by Tensor View Post
    That conclusion would not explain why, when you use one amount of dark matter for the rotation curve, you also explain galactic cluster movements, lensing observations, and effects seen in the CMB. It's just not rotation curves.
    Rotation curves, etc.

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    Quote Originally Posted by antoniseb View Post
    Ummm, my read is not that it's shaped like a football, but that q>2 means that it is the short axis is perpendicular to the plane of the galaxy. Also, I don't know what you mean by safe to say. Some theorists don't expect that, but looking at weak lensing studies it is clear that not all dark matter halos are spheres.
    OK, like a jelly bean with long axis as galactic axis. Footballs are a bit pointed. I wasn't aware that there were weak lensing studies that claimed conclusions about the shape of a galactic halo. Can you site one?

    "We have observations that it must be in the form of small neutral masses" - what observations are you referring to? I have never heard the suggestion that DM might not be electrically neutral.

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    Quote Originally Posted by TooMany View Post
    OK, like a jelly bean with long axis as galactic axis. Footballs are a bit pointed.
    There are three coordinates. One short, two long, so no, not like a jelly bean.
    I wasn't aware that there were weak lensing studies that claimed conclusions about the shape of a galactic halo. Can you site one?
    Not without taking more time to look it up than I currently have free. Aspherical cluster halo images are easy to find.
    ... what observations are you referring to? I have never heard the suggestion that DM might not be electrically neutral.
    If dark matter were not electrically neutral it would interfere with and emit photons... and hence would not be Dark.
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    Quote Originally Posted by ngc3314 View Post
    The final piece of the paper states,



    I look forward to seeing how this can be reconciled with the extensive body of other measurements for Galactic dynamics.
    I found it a bit funny how they stuck this bit on the end. I mean, it's not even true. The apparent discrepancy between observed mass and the galactic rotation at the sun's galactocentric radius is not large. It's probably within realistic uncertainties, and you would not need to invent new forms of matter if this were the only issue.

    On the other hand, the other thing I find strange is that they do not even reference the recent re-measurement of the circular velocity as being higher than 220 km/s (M J Reid et al and later papers by others). But I don't think this number is actually central to their argument.

    My third comment is that the sun is between spiral arms, so the locally-observed stellar density is lower than the average for the galactocentric radius. They use an equation which uses the scale height and the scale length to calculate mass density, using the local density as the base density to calculate from. This is just an observation, because I don't think it affects their argument, which is based on the locally-observed density versus the movement of stars perpendicularly above the disc at the same location.

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    Quote Originally Posted by kzb View Post
    The apparent discrepancy between observed mass and the galactic rotation at the sun's galactocentric radius is not large. It's probably within realistic uncertainties, and you would not need to invent new forms of matter if this were the only issue.
    Interesting - then that is not a good argument for unseen matter in the luminous part of the galaxy. It would seem to make a prolate DM distribution even less appealing.

    BTW Antoniseb, you seem eager to contradict every statement I make. The paper does not even mention triaxial distributions, so as far as I can tell the author means a prolate spheroid (which indeed looks like a jelly bean) when he says prolate. Pointing to cluster mass distributions doesn't address measurement of intragalactic distributions using weak lensing. What surprised me about this claim of distribution measurements is that the distribution is completely open to debate AFAIK.

    A troubling aspect of the whole "DM problem" is the implicit assumption that if ordinary matter isn't glowing or absorbing enough background to be detectable by current instruments, then it simply cannot exist. That assumption may be entirely wrong. This possibility seems to be dismissed in order to find consistency with a speculative creation theory that doesn't allow enough ordinary matter to exist as DM. It is rather similar to using the undetectability of distant galaxies similar to local ones to argue for galactic evolution.

    There is likely dark matter in our own solar system that we cannot directly detect. There is even a debate about the existence of very old dim stars in the galactic halo which may currently be impossible to detect, even though they do glow. The claim that we know how much ordinary matter is present is weak, driven more by theory than observation.

  17. #17
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    Quote Originally Posted by TooMany View Post
    ... BTW Antoniseb, you seem eager to contradict every statement I make. ...
    No just dedicated to correct ATM ideas presented in the mainstream part of the forum. You are trying to build a case against Dark Matter using faulty premises and misreads of papers. So, in this case, almost every statement you make turns out to something that needs challenging. It is not a matter of who you are, or who said it.
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    Quote Originally Posted by TooMany View Post
    A troubling aspect of the whole "DM problem" is the implicit assumption that if ordinary matter isn't glowing or absorbing enough background to be detectable by current instruments, then it simply cannot exist.... The claim that we know how much ordinary matter is present is weak, driven more by theory than observation.
    You seem to be ignoring the extraordinarily numerous searches for this "missing" ordinary baryonic matter that have been undertaken, searches that approach the problem from every conceivable method of detection. These are all observations that have essentially come up empty, and that's what is driving the theories. Certainly there have been alternative theories based on the assumption that maybe the law of gravity as we know it is wrong. But none of these alternatives have been able to fit all the data. Of course, the more recent weak lensing studies strongly support a weakly interacting non-baryonic solution. So there we are.
    Everyone is entitled to his own opinion, but not his own facts.

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    TooMany wrote:
    A troubling aspect of the whole "DM problem" is the implicit assumption that if ordinary matter isn't glowing or absorbing enough background to be detectable by current instruments, then it simply cannot exist. That assumption may be entirely wrong. This possibility seems to be dismissed in order to find consistency with a speculative creation theory that doesn't allow enough ordinary matter to exist as DM.

    But there's been the MACHO microlensing studies, which concluded there is not enough dark baryonic matter. Your dim old stars in the halo are excluded by these studies. However, the paper under discussion seems to be saying there's no requirement for a massive dark halo whether it's baryonic or not.

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    Matters of faith

    Studies that turn up null results are handicapped by the assumptions that must be made about what attributes Dark Matter should have, and how they might be investigated.

    Like the human soul, Dark Matter has escaped every scientific test of its existence.

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    Quote Originally Posted by Jerry View Post
    Like the human soul, Dark Matter has escaped every scientific test of its existence.
    Apart from those that confirm its (1) existence.

    (1) Whatever "it" turns out to be.

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    Quote Originally Posted by Strange View Post
    Apart from those that confirm its (1) existence.

    (1) Whatever "it" turns out to be.
    Strange. Yep. Non-Keplerian rotation curves puzzled Zwicky long before others, and require a sensible answer. I'm hoping new searches for molecular hydrogen being more abundant than previous instrumentation could detect, will yield a rather mundane exoplanation, similar to the recent super-luminal- neutrino-OPERA- puzzle, but the hunt for "it" will be interesting. pete

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    Quote Originally Posted by trinitree88 View Post
    Strange. Yep. Non-Keplerian rotation curves puzzled Zwicky long before others, and require a sensible answer. I'm hoping new searches for molecular hydrogen being more abundant than previous instrumentation could detect, will yield a rather mundane exoplanation, similar to the recent super-luminal- neutrino-OPERA- puzzle, but the hunt for "it" will be interesting. pete
    I still wonder about the Extreme Scattering Events. Phenomena that seem to be glossed over by most people.

  24. #24
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    antoniseb,

    I've only glanced at the paper, not read it at all.

    The term "prolate" is clearly used in the abstract, the introduction,
    page 26, and page 30 (the conclusion). Its use there shows that
    it is not a typo for "oblate". A prolate spheroid is indeed shaped
    like a jelly bean or an American football.

    That dark matter might have such a distribution is very surprising,
    whether it is in the plane of the galaxy or perpendicular to it. My
    vague impression from the paper is that they mean perpendicular.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Just so we are all on the same wavelength:

    A prolate spheroid is a sphere stretched along its polar axis

    An oblate spheroid is a sphere deformed so that its equatorial axis is the longest axis (i.e it's a squashed sphere).

    As well as what Jeff says above, I think they truly mean prolate, and not oblate, for another reason.

    They are saying there is no room in their gravitational potential model at our galactocentric locality for a significant amount of DM (of any persuasion). This model is based on gravitation and movement in the z direction, i.e perpendicular to the galactic disk.

    They then go on to say that if we have to have a DM halo, the only shape of that halo allowed in their model would be highly prolate, that is, you have an american football-shape of DM, centred on the galaxy centre, but with its long axis perpendicular to the disk. This model could indeed contribute so little DM at our radius that they could not detect it.

    They are not saying this is the shape of the DM halo. That's not the message at all. For one thing, this prolate halo wouldn't explain non-Keplerian orbital velocities in the outer disk. You'd be back to square one on that. Also, the DM distribution is precisely opposite to the distinctly oblate distribution of luminous matter in the galaxy.

    No, they are saying that only an extremely unlikely distribution of DM is consistent with their model, and (almost), therefore, it does not exist at all.

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    At p.24:

    In fact, the calculation does not measure the mass directly, but a variation of the gravitational potential, from which the mass density is derived. Moni Bidin et al. (2010) noted that, if the DM halo was too uniform, extended, and poorly concentrated, it could cause a negligible change of the potential in the volume under analysis, thus resulting undetectable.
    Well, the usual model has the dark matter extend well beyond the disk, perhaps out to 300,000 ly from the center, so its density in the relatively small region between 5,000 and 15,000 ly south of the Sun's location is going to be pretty uniform.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by kzb View Post
    ...the only shape of that halo allowed in their model would be highly prolate, that is, you have an american football-shape of DM, centred on the galaxy centre, but with its long axis perpendicular to the disk.
    Well, yes, except for the pointy ends. Better to envision rotating an ellipse around its long axis. They say their long to short axis is 2:1. I'm surprised they're not embarrassed to report such a goofy thing, even if it fits their calculations.
    Everyone is entitled to his own opinion, but not his own facts.

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    Here is a paper covering very related topics. It reports on a 3D simulation with plausible initial conditions, and allowing for the gravitational interaction between Baryonic matter and Dark Matter... and results in flattened cusps and only a few satellite dwarf galaxies... and a triaxially prolate/oblate halo that is plausibly within the error bars of what the OP's paper shows.
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    Quote Originally Posted by Cougar View Post
    Well, yes, except for the pointy ends. Better to envision rotating an ellipse around its long axis. They say their long to short axis is 2:1. I'm surprised they're not embarrassed to report such a goofy thing, even if it fits their calculations.
    OK I accept your point about the pointy ends. When you say the long to short axis ratio is 2:1, let's point out that this is the minimum ratio allowed. This coincides with the 2-sigma error bars. If you look at their diagrams you will see the central estimate is probably closer to 4:1. More like a straight sausage than an american football.

    I'm afraid with your last sentence you show that you completely misinterpreted the whole point. This is their arguement: only goofy DM distributions would fit their observations. That is what they are saying. Common models of the DM distribution shapes are excluded with high confidence (although Antoniseb has posted a link that apparently contradicts this, which I've not looked at yet).

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    Quote Originally Posted by antoniseb View Post
    Here is a paper covering very related topics. It reports on a 3D simulation with plausible initial conditions, and allowing for the gravitational interaction between Baryonic matter and Dark Matter... and results in flattened cusps and only a few satellite dwarf galaxies... and a triaxially prolate/oblate halo that is plausibly within the error bars of what the OP's paper shows.
    I find it hard to connect the two papers myself. The paper you link to presumably uses none of the data that the OP paper is based on, and has not taken it into account.

    To sum up, it is saying that one difficulty with theoretically-modelled DM halos is there is always a very steeply-rising density of DM towards the centre, which is not observed (the cusp problem). They say that, if you start with one of these cusped halos, over billion-year times scales, the interaction of baryonic matter with the DM will act to soften the distribution of DM, and basically get rid of the cusp.

    I'm not sure about your last statement:

    and a triaxially prolate/oblate halo that is plausibly within the error bars of what the OP's paper shows

    It actually says that an initially triaxial DM halo will be made more spherical over time. Also a prolate distribution with its axis perpendicular to the disk and q>2 is not considered.

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