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Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel Send a noteboard - 25/05/2011 12:20:10 AM
OK, read it again; he doesn't seem to address MACHOs directly, if at all. The closest he comes seems to be referencing the "the large majority (about 90%) of ordinary matter in a cluster... not in the galaxies themselves, but in hot X-ray emitting intergalactic gas". It's an interesting statement, because he's basically saying that 90% of the matter in the galaxies isn't really "in" them at all, but around them, which, whether we can (normally) see it or not, is exactly what MACHO theories claim; however unlikely the large masses of low density hydrogen I referenced above are, they appear to be real. That doesn't account for the greater gravitational lensing observed beyond the gases in the pictures, but an intervening highly MACHO(s) might.
Lensing happens when there's a concentrated source of matter between the observer and the object; for dark matter that's distributed fairly homogeneously within the galaxy, that's very unlikely to happen. When we look at lensing from entire clusters of far-away galaxies, we get results that match exotic matter expectations, but not ordinary matter expectations.

In other words,
there's enough dark matter to significantly affect galactic rotation but not enough to create nearby gravitational lensing,
and those propositions aren't contradictory because
there's a BIG difference between the amount of dark matter necessary for the former and the latter.
Again, if that's what the evidence and you are saying, fine, but the blog post about the Bullet Cluster does not, in itself, rule out MACHOs; the cluster only rules out MACHOs on the basis that theories about them say they should be homogeneous. Heterogeneous MACHOs could explain the Bullet Cluster, too, but if that's inherently contradictory I don't object to ruling out MACHOs.

Read the last three paragraphs again, but especially in the second-to-last paragraph:

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).

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
What really gives me pause though is statements like the articles first paragraph
Dark Matter Exists

by Sean
The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components....

Above emphases mine. Does that last sentence sound at all familiar? Maybe like the notion a century ago that all of physics had been mapped out and nothing remained except to fill in the details, the idea so often ridiculed and that "simply doesn't occur as often" now? Is Dr. Carroll one of those oversensationalizing undereducated media hacks you mentioned? If so, should I recognize the sweeping statements with which he leads off his article as authoritative? Or do statements like those and his later one that "We would all love to out-Einstein Einstein" underscore my valid concerns?

Sean says "we can tell a story that fits all the known data." He does not say that there are no new phenomena out there, merely that we have a basic understanding of the phenomena we have seen so far.

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.
This is a qualitative question, or should be; we're debating whether something exists, not the frequency of its occurrence. If you want to put that in quantitative terms that exotic dark matter is 99% likely, OK, but a lot of your statements (and almost all of Dr. Carrolls) aren't just quantitative, there without qualifiers. If you don't want me objecting that you're being too definitive where too much remains undefined, be less definitive.

At some point, probability becomes sufficiently high enough to use definitive language. From an empirical standpoint, "X is true" always means "we should act as though X were true, because given the evidence, X has a probability nearing 100%."

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.
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Exciting video about the universe - 28/04/2011 10:14:55 AM 1095 Views
Cool, and true *NM* - 28/04/2011 11:46:29 AM 333 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 823 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 753 Views
I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 01:52:49 AM 685 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 02:56:32 AM 798 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 05:02:49 PM 753 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 08:56:35 PM 624 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 02/05/2011 01:28:30 AM 658 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 04/05/2011 04:18:18 AM 763 Views
There's such a thing as knowing when you're licked, and I believe I am. - 07/05/2011 02:04:53 AM 834 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 678 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 623 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 712 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 632 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 710 Views
The Pati-Salam model was the one I had in mind. - 24/05/2011 10:34:04 PM 653 Views
Re: The Pati-Salam model was the one I had in mind. - 24/05/2011 11:08:01 PM 860 Views
Re: The Pati-Salam model was the one I had in mind. - 25/05/2011 01:27:10 AM 672 Views
Re: The Pati-Salam model was the one I had in mind. - 31/05/2011 09:16:18 AM 741 Views
Also, re: lensing from ordinary matter: - 29/04/2011 05:18:47 AM 687 Views
This seems like another example of what confuses the issue. - 30/04/2011 05:25:04 PM 805 Views
Re: This seems like another example of what confuses the issue. - 30/04/2011 08:56:40 PM 776 Views
That discussion seems to reduce to "as little new and exotic physics as possible". - 02/05/2011 01:29:03 AM 769 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 04/05/2011 04:18:24 AM 731 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 07/05/2011 02:05:02 AM 907 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 09/05/2011 11:29:36 PM 673 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 14/05/2011 05:35:56 AM 954 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 17/05/2011 02:09:55 AM 579 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 19/05/2011 02:47:25 AM 919 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 24/05/2011 09:46:30 PM 704 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 25/05/2011 12:20:10 AM 986 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 1002 Views
Seems to happen to me a lot; sorry. - 29/04/2011 12:56:14 AM 702 Views
None of this reflects on the actual facts of dark matter. - 29/04/2011 01:32:52 AM 667 Views
I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 04:30:28 PM 785 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 08:56:44 PM 619 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 02/05/2011 01:28:58 AM 1136 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 04/05/2011 04:18:27 AM 663 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 854 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 780 Views

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