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Originally Posted by dgruss23
As explained in my last post - given the collisionless property of the hypothesized CDM particles, the distribution of baryonic matter (which produces the galaxies light) and the CDM should not be coupled. References are provided in this post. I'm going to try and select just a handful from the numerous papers in my files.
Over 20 years ago now Bahcall and Casertano (1985) noted that one fact of the observed rotation curves is that there is no feature to mark a transition between the inner region of the spiral galaxy where the visible matter dominates the gravitational dynamics and the outer region where the DM halo dominates the dynamics.
This image may help clarify the problem. The observed rotation curve is represented by the plotted data points with error bars. The contribution to the rotation curve derived from the observed disk is represented by the long dashed lines. The short dashed line represents the portion of the curve that comes from the hydrogen (mostly) gas in the galaxy. These two lines comprise the baryonic mass contribution. The halo contribution is added in to make up the difference between the baryonic mass contribution and the observed rotation. Note that at small radii the baryonic matter dominates the rotation curve, but at larger radii the DM halo becomes more important.
However, what is missing in this rotation curve is some feature to indicate a transition between the zones in which the normal matter dominates to the zone in which the DM dominates. The DM and normal matter must be precisely distributed in such a way that they "conspire" to keep the rotation curve flat. But this is an odd conspiracy (known as the "disk-halo conspiracy") if the DM is collisionless CDM. As Bahcall and Casertano noted in 1985:
Palunas&Williams (2000) demonstrated with the maximum disk model that the distribution of the DM must be tightly coupled to the distribution of normal matter:
They go on to note that this result is in conflict with simulations which predict that DM should dominate the inner dynamics of even the most massive (fastest rotating) spiral galaxies.
Renzo Sancisi (2004) takes this coupling even further by noting that even on the scale of wiggles and bumps in the light profile of a spiral galaxy, the rotation and the wiggles/bumps are correlated - again at odds with what would be expected from a collisionless particle such as CDM:
Sancisi provides an example of this phenomenon in Figure 2 of the paper.
Donato, Gentile and Salucci (2004) demonstrated that the core radius for the DM halo is correlated with the disk scale length which is derived from the luminous mass:
It is worth noting that in version 1 of this paper on arXiv the words "very challenging task" were simply a single word: "impossible".
And that is the issue with all of these cases I've noted and it constitutes a falsification of the expectations for CDM particles - and any collisionless DM particles. If DM is collisionless it should not be coupled to the distribution of luminous matter. On the scale of individual galaxies CDM is a failure. And in this regard there are plenty more references where these came from. I could go into the Tully-Fisher relation and how CDM fails to explain it. I could go into the fact that the mass discrepancy from which DM is inferred only appears at a specific acceleration scale. I could go into the references that discuss the failure of CDM simulations to predict the correct shape for the DM cores (cuspy core problem). And there are still more issues.
I've explained one problem on galaxy scales in this post - the distribution of DM is coupled to the distribution of luminous matter - a contradiction with the properties of collisionless particles such as CDM. [/SIZE][/FONT][/LEFT] |
Very interesting collection of papers
dgruss23, thanks.
As luck would have it, my last post overlapped with this one of yours (that I'm quoting), so the 'assumption too far', or 'extrapolation too far' that leaps out at a reader is entirely coincidental.
But, to repeat, in brief:
There is nothing of any significance in the collective body of all astronomical observations that constrains the extent of non-baryonic CDM "collisions" (self or with baryonic matter; other than gravitational) to any extent worth keeping ... unless assumptions about the particulate nature of this non-baryonic CDM and/or the (low energy) EMR nature of such collisions are included; assumptions which are, IMHO, entirely unwarranted^
Also, and again just briefly, there are some other issues in the content of
dgruss23's post; for example:
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I could go into the references that discuss the failure of CDM simulations to predict the correct shape for the DM cores (cuspy core problem)
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At least part of this "problem" is due to a misunderstanding of the relevant physics (just the sort of thing
Ken G referred to in his posts in
this thread the General Science section?) - the conflating of 'here's what we got when we did this (admittedly restrictive) model of such non-baryonic collisionless CDM profiles' with what an unconstrained (by assumptions made to simplify calculation) model would show.
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On the scale of individual galaxies CDM is a failure.
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It's a huge stretch from
rotation curves of some spiral galaxies and various (necessarily restrictive) collisionless non-baryonic CDM models to
all (individual) galaxies, and all forms of non-baryonic CDM, and ...
More generally, and pertinent to points several folk have made earlier:
* what a burden of good observations is dropped onto the backs of alternative explanations: not only the wide range of rotation curve ones (stars, gas, dust, plasma, ...), but also the gravitational lensing ones (of spiral galaxies)
* and what about the gravitational lensing observations (as well as all the others) of elliptical galaxies?
* in the total scheme of things, the estimated mass of non-baryonic CDM in galaxies, whether spiral or not, is essentially trivial compared with that inferred to exist in clusters and groups of galaxies - even if, somehow, a good, consistent, robust explanation for all the relevant observations of individual (and collective) galaxies were to be proposed, there's no reason (
a priori) to think that it would account equally well for all the group/cluster/cosmological observations.
In addition, there's a 'philosophy of science' issue:
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it constitutes a falsification of the expectations for CDM particles
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Now 'falsificationism' may be satisfying to many philosophers, and to many who are not actively working at the coalface of science, but I don't think I'm exaggerating when I state that it is an easily falsified view of how science - and in particular physics, astronomy, astrophysics, and cosmology - are actually done. Simply put, unless and until a new (or several new) alternative explanations come along that are better at generating testable hypotheses than the current paradigm (not to mention accounting for all good, relevant observational and experimental results), simply 'falsifying' something gets you little traction. After all (for example), the anomalous advance of the perihelion of Mercury very clearly 'falsified' Newtonian gravity for what, many decades? Yet it was not until Einstein and GR that that theory was 'retired' as being merely a good, limited-domain explanation.
^
of course I recognise that this represents a decoupling of particle physics and astronomy, in the sense that particle physics has good grounds for expecting a huge zoo of particles hithertofore undetected ... but why should these two be so closely coupled? After all, we already know that the universe is perfectly capable of working in energy (etc) regimes that are far, far, far, far {add as many more 'fars' as you wish} beyond what we can produce in our Earthly labs today ...