Active Users:1053 Time:22/11/2024 07:16:08 AM
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly Send a noteboard - 24/05/2011 09:46:30 PM
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:

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.

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.

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%."
Reply to message
Exciting video about the universe - 28/04/2011 10:14:55 AM 1091 Views
Cool, and true *NM* - 28/04/2011 11:46:29 AM 332 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 750 Views
I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 01:52:49 AM 681 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 29/04/2011 02:56:32 AM 793 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 05:02:49 PM 750 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 30/04/2011 08:56:35 PM 621 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 02/05/2011 01:28:30 AM 654 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. - 04/05/2011 04:18:18 AM 760 Views
There's such a thing as knowing when you're licked, and I believe I am. - 07/05/2011 02:04:53 AM 830 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 674 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 620 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 708 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 629 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 706 Views
The Pati-Salam model was the one I had in mind. - 24/05/2011 10:34:04 PM 648 Views
Re: The Pati-Salam model was the one I had in mind. - 24/05/2011 11:08:01 PM 857 Views
Re: The Pati-Salam model was the one I had in mind. - 25/05/2011 01:27:10 AM 667 Views
Re: The Pati-Salam model was the one I had in mind. - 31/05/2011 09:16:18 AM 738 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 800 Views
Re: This seems like another example of what confuses the issue. - 30/04/2011 08:56:40 PM 773 Views
That discussion seems to reduce to "as little new and exotic physics as possible". - 02/05/2011 01:29:03 AM 764 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 04/05/2011 04:18:24 AM 726 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 07/05/2011 02:05:02 AM 902 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 09/05/2011 11:29:36 PM 668 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 14/05/2011 05:35:56 AM 950 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 17/05/2011 02:09:55 AM 576 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 19/05/2011 02:47:25 AM 916 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". - 24/05/2011 09:46:30 PM 701 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 999 Views
Seems to happen to me a lot; sorry. - 29/04/2011 12:56:14 AM 699 Views
None of this reflects on the actual facts of dark matter. - 29/04/2011 01:32:52 AM 664 Views
I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 04:30:28 PM 781 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 30/04/2011 08:56:44 PM 614 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 02/05/2011 01:28:58 AM 1132 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - 04/05/2011 04:18:27 AM 658 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 850 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 776 Views

Reply to Message