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Originally Posted by Nereid
Very interesting collection of papers dgruss23, thanks.  |
Thanks for saying so!
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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^
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It is in the realm of those proposing the various CDM candidates to specify the properties or range of properties of each individual candidate (for example to what extent CDM particles will collide with other CDM particles or perhaps rarely with normal baryonic matter). But the point here is global enough to be applicable to all CDM particles as CDM has been defined for the last 20 years - CDM is a collisionless particle. The exact nature of this collisionless particle has been unknown and is still unknown. However, an expectation - or prediction - is that collisionless particles should not couple in their distribution to the luminous matter distribution.
The fact that observations reveal that light traces mass and the DM must couple to the distribution of normal matter contradicts this expectation. The options would seem to be (1) Come up with some mechanism that can account for this coupling between normal matter and a collisionless particle (no-one has found a solution via this route yet). (2) Propose a viable particle that has the property of collisional interaction with normal matter (but then we're no longer talking about CDM). (3)Abandon non-baryonic DM altogether and look for other solutions such as MOND.
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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 ...
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I understand what you're trying to say, but I disagree for this reason - There are no successes for CDM on galaxy scales upon which to have any confidence that a collisionless particle can explain the observations. Observational Result after observational result after observational result points to the same conclusion - the inferred DM distribution is tightly coupled to the luminous matter distribution. Refining the properties of the CDM candidates will not fix this. You must be willing to change the properties of CDM candidates - thus making them something else besides CDM.
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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?
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Do you have specific references in mind for lensing? I'd be looking for results that compel a conclusion that the DM must be a collisionless particle - independent of cosmological considerations as the results I've discussed are (for the most part).
CDM has not fared too well in local elliptical galaxies either. There are quite a few galaxies for which DM does not seem to be needed at all, at least one of the papers in my files discusses the fact that the luminous DM coupling must also be present in ellipticals because of how tight the fundamental plane is.
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* 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.
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This is quite true - and it is goes back to what I was saying about research specialization. Researchers that look at CDM on cluster scales and larger don't pay a lot of attention to the significant observational contradictions CDM faces on the scale of individual galaxies. So it might be possible to argue that CDM is the solution on galaxy cluster scales, but on individual galaxy scales there is some other explanation - such as MOND. But then if CDM researchers are constantly poking at MOND because it doesn't fully account for the mass discrepancy on galaxy cluster scales, they should be equally concerned that their favored explanation fails completely on galaxy scales.
And I don't say "fails completely" lightly. Can someone find an example of an individual galaxy where the coupling between luminous and DM is not present?
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In addition, there's a 'philosophy of science' issue: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.
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Sure that may not be the way science is done. But it is the way science should be done. I'm just stating an opinion here, but since we're talking philosophy... collisionless CDM is a failure on galaxy scales. It fails with the observed coupling between luminous and DM. The predictions from computer models fail to live up to the realty provided by observations. It fails in the realm of detection of the hypothesized particles. How much is progress in our understanding of the universe being limited by this type of thinking?
I know you're right Nereid - that is how researchers think - but it is one of the most illogical ways of thinking that an intelligent group of people engages in. Seriously just step back and think about it - multiple CDM failures have been demonstrated for over 20 years now. And there are people that will continue to explore CDM because that is the "best" there is to offer?! Where galaxy scales are concerned it is not the "best" - MOND is actually better.
Thanks for your comments Nereid. I'm enjoying this discussion. dgruss