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Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel Send a noteboard - 10/06/2011 12:04:06 AM
OK, done, but I'm expanding the quote because I think it has additional relevance:
The point is that, independently of any specific model of modified gravity, we now know that there definitely is dark matter out there. It will always be possible that some sort of modification of gravity lurks just below our threshold of detection; but now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.

That’s huge news for physicists. Theorists now know what to think about (particle-physics models of dark matter) and experimentalists know what to look for (direct and indirect detection of dark matter particles, production of dark matter candidates at accelerators). The dark matter isn’t just ordinary matter that’s not shining; limits from primordial nucleosynthesis and the cosmic microwave background imply a strict upper bound on the amount of ordinary matter, and it’s not nearly enough to account for all the matter we need. This new result doesn’t tell us which particle the new dark matter is, but it confirms that there is such a particle. We’re definitely making progress on the crucial project of understanding the inventory of the universe.

I see nothing in there that, in itself, rules out MACHOs causing the observed gravitational lensing of the Bullet Cluster; the limits he references only rule them out in because MACHOs are expected to be homogeneously distributed at large scales, which he treats as implicit, without stating. Now we can pinpoint the other 75% of the universe, the dark energy that should be homogeneously distributed at large scales. It's necessary for observed expansion acceleration at great distances because large masses can't be causing it since the universe should be homogeneously distributed at large scales. Can you spot the established principle all of that logic deliberately avoids re-evaluating? ;) Again, I'm not saying that principle is invalid, just that there might be cause to re-examine it. That's not my main point here (though it's a consequence of my overarching one in this thread).

The universe being homogeneously distributed at large scales and MACHOs being homogeneously distributed at large scales sound like similar concepts, but they come from different principles (and the "large scales" in each case are quantitatively different). The distribution of the universe is due to the early inflation period, and MACHOs would not have existed at that point. MACHOs would be homogeneous within galaxies, and it's galactic clusters that are homogeneous within the universe. This is orders and orders of magnitude in difference.

In that case I humbly suggest avoiding such confusion by using distinct terms for distinct conceps. Regardless, it still seems premature to say MACHOs don't cause faster than predicted galactic rotation because they don't create the local gravitational lensing they would if homongeneously distributed at the "large scales" relevant to them. A heterogeneous distribution ought to be on the table, particularly if what every GUT but one predicts about conservation of baryon number proves valid. Regardless of THAT, homongeneous dark matter distribution at galactic scales, the only basis on which the article can be taken to rule out MACHOs, is never addressed BY the article, so it doesn't so much make as accept the case against MACHOs.
That the article doesn't tackle MACHOs directly isn't my main point here either, but what it DOES tackle instead. The sections I bolded above, as the others I bolded below, unequivocally state exotic dark matters existence as a fact observations of the Bullet Cluster prove. Dr. Carroll takes pains to make clear he's saying that; he explicitly says some form of MOND might still be valid, but that exotic dark matter is a fact and reality regardless. His use of words like "definitely" and "confirms" isn't figurative and doesn't come with attached qualifiers; when he says, "we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics" that's just what he means. In my very humble opinion, I don't think the case has been proven beyond a reasonable doubt, only by a preponderance of evidence. MACHOs may have to sell their Heisman and give exotic dark matter the profit, but no death penalty. :P

Exotic dark matter has been established beyond a reasonable doubt. That doesn't mean that many people don't still have doubts about it, obviously, but a full consideration of the evidence and theoretical concerns shows those doubts to be unreasonable.
I wish I were as confident that's all he's saying, but his language reads more like those hypersensational publicists you (rightly) disparage than a cautious methodical researcher. In fact, it reads like the very thing about which I remain concerned despite your assurances that it's now rare to the point of being negligible: Someone who, rather than diligently collecting and analyzing all the data to determine what it says, has relentless sought out the specific data that "confirms... beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics". That sounds like fitting data, or in this case datum, to ones curve; a single point makes a lousy curve. He doesn't say there remain things we don't know, explicitly says the opposite, but he DOES say exotic dark matter is something we DO "know" as certainly as anything can be known. I simply can't agree with that; exotic dark matter is not, in my admittedly non-professional opinion, established on the same level as, say, the Laws of Thermodynamics, or wave/particle duality. It's probably valid, and you HAVE convinced me that's the way I'd bet if pressed, but I don't consider it the fact that he does.

You have no evidence that he has "relentless (sic) sought out the specific data that 'confirms...'". The Bullet Cluster is far from the only data point on which that conclusion is based; there are other cluster collisions that have been observed (link below), and other forms of evidence beside that.

He explicitly says "The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components." That is admitting there are things we don't know.

Indeed, rigidly defined areas of doubt and uncertainty. Somehow I doubt you're as indulgent with handwaving what's "still undetermined" about "known" supernatural phenomena. ;) He explicitly makes a statement very reminiscent of the conventional wisdom a century ago naught remained for physics save filling in the details of the comprehensively known science. That's the kind of hubris you keep insisting science has outgrown, but the article is rife with it, IMHO because until humanity outgrows such presumption it will persist in all our disciplines. THAT'S my evidence that he relentlessly sought out specific data to confirm his suppositions: He explicitly treats the composition of the universe itself as a settled matter, leaving room for no uncertainty except the nature of those detailed nature of those comprehensively known components. Each persons standard of "reasonable" doubt is unique (hence we have juries); I wouldn't want to say you and Dr. Carroll are "wrong" about a probability (particularly not of something you've largely convinced me is true), but I can't share the level of confidence ya'll apparently do, nor do I envy it.
That is indeed the rub; I don't think the probability is near enough to 100% yet for me to treat it as equal. If you (or Dr. Carroll) want to say exotic dark matter is 95±5% certain, I won't argue with that, but 100% is too much for me at present.

100% is not really a probability, in the same way that infinity is not really a number; that's why I said "nearing 100%." I would estimate the probability of exotic dark matter being the correct class of hypotheses to be at least 99.9%, i.e. I feel that I could make 1000 such statements and only 1 of them would be wrong. (No, I am not going to make 1000 such statements in a reply in this thread.)

Each persons standard of "reasonable" doubt is unique (hence we have juries); I wouldn't say you and Dr. Carroll are "wrong" about a probability (particularly not of something you've largely convinced me is true), but can't share the level of confidence ya'll apparently do, nor do I envy it. Much of what separates rigorous disciplined philosophies like science and law from fantasy is that they deal with what can be PROVEN rather than what is known. I agree that exotic dark matter is proven by a preponderance of evidence (though much of it is circumstantial), but can't go as far as proven beyond a reasonable doubt, and think it dangerous to strain at the edge of our leash on certainty only until unearthing a long and eagerly sought smoking gun proving the case we always knew to be valid. Investigators already convinced of something producing proof they "knew" existed CREATES reasonable doubt even among those otherwise inclined to believe them and the evidence, and I wouldn't want the Bullet Cluster to become dark matters bloody glove. Look what minor irregularities in Climategate did to Global Warming.
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This message last edited by Joel on 10/06/2011 at 12:08:55 AM
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Exciting video about the universe - 28/04/2011 10:14:55 AM 1090 Views
Cool, and true *NM* - 28/04/2011 11:46:29 AM 331 Views
I still think dark matter's just non-luminous matter without a convenient light source to reflect. - 28/04/2011 10:34:21 PM 817 Views
We've just about ruled out the idea that dark matter is just non-luminous "ordinary" matter. - 28/04/2011 11:44:34 PM 748 Views
I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 01:52:49 AM 679 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 02:56:32 AM 792 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 05:02:49 PM 747 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 08:56:35 PM 618 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 02/05/2011 01:28:30 AM 653 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 04/05/2011 04:18:18 AM 757 Views
There's such a thing as knowing when you're licked, and I believe I am. - 07/05/2011 02:04:53 AM 828 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. - 09/05/2011 11:28:48 PM 673 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. - 14/05/2011 05:36:45 AM 617 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. - 17/05/2011 02:09:40 AM 706 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. - 19/05/2011 04:55:21 AM 627 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. - 24/05/2011 09:32:27 PM 704 Views
The Pati-Salam model was the one I had in mind. - 24/05/2011 10:34:04 PM 647 Views
Re: The Pati-Salam model was the one I had in mind. - 24/05/2011 11:08:01 PM 854 Views
Re: The Pati-Salam model was the one I had in mind. - 25/05/2011 01:27:10 AM 666 Views
Re: The Pati-Salam model was the one I had in mind. - 31/05/2011 09:16:18 AM 735 Views
Also, re: lensing from ordinary matter: - 29/04/2011 05:18:47 AM 681 Views
This seems like another example of what confuses the issue. - 30/04/2011 05:25:04 PM 799 Views
Re: This seems like another example of what confuses the issue. - 30/04/2011 08:56:40 PM 770 Views
That discussion seems to reduce to "as little new and exotic physics as possible". - 02/05/2011 01:29:03 AM 763 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 04/05/2011 04:18:24 AM 725 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 07/05/2011 02:05:02 AM 901 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 09/05/2011 11:29:36 PM 667 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 14/05/2011 05:35:56 AM 949 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 17/05/2011 02:09:55 AM 573 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 19/05/2011 02:47:25 AM 913 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 24/05/2011 09:46:30 PM 698 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 25/05/2011 12:20:10 AM 981 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 31/05/2011 09:16:22 AM 797 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 10/06/2011 12:04:06 AM 1038 Views
Re: I still think... (apparently, there is a 100 character limit on subjects, and yours was 99) - 28/04/2011 11:57:15 PM 997 Views
Seems to happen to me a lot; sorry. - 29/04/2011 12:56:14 AM 696 Views
None of this reflects on the actual facts of dark matter. - 29/04/2011 01:32:52 AM 662 Views
I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 04:30:28 PM 779 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 08:56:44 PM 613 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 02/05/2011 01:28:58 AM 1130 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 04/05/2011 04:18:27 AM 657 Views
I don't object to changing my mind, but can take more convincing than I really should. - 07/05/2011 02:05:09 AM 849 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. - 09/05/2011 11:32:17 PM 774 Views

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