> > Dr. Brin, in one of your recent blog posts you mention the article about > dark energy but not the paper about dark matter. This is confusing, > because the paper on dark matter is the much more interesting of the > two, seeing as it doesn't rely on any might-be's or other supposition's, > but rather takes existing data and matches it against the current MOG > theory, and shows how the theory matches the data, as is. MOND/TeVeS > may be dead theories, but it appears MOG is still a viable alternative > to Dark Matter.
I'm a mere plumber, but let me try to explain the difficulties with the ideas presented in this paper. FWIW, I know these guys are not crackpots; they have come up with a theory that's a bit out there, but that's what boffins do. It's up to plumbers like me to show that they are wrong, and every now and then to show that they are right. There are two real problems with their theories. First, they have to modify the theory every time new data comes in. There is still enough room in parameter space for them to do that, but their room to maneuver in that space is shrinking. We cannot explain the rotation of galaxies with only the matter that is seen and GR. So, we either modify GR or postulate matter that follows GR, but does not emit detectable radiation, like stars, or illuminated galactic dust, or hides emitted radiation like galactic dust that covers stars, or is otherwise directly observable. Dark matter seems to be a simple way to explain this. There is no fundamental reason for not having dark matter; but it is a free parameter because it is not seen. The other explanation we are considering is modifying GR. But, it's not a simple modification. For example, we can modify gravity to explain the bullet cluster. But, there has to be a rather specific modification that requires very unique preconditions....indeed as one of the sources I refer to indicates, we have not been able to walk through the whole collision process with this modified theory. Second, as new colliding clusters are seen; different behavior is seen. For dark matter, we have to postulate that different clusters have different ratios of dark/regular matter, different ratios of different types of dark matter, and so on. These are, in a sense, free parameters. But, in the same sense, to explain why some stars are blue and some are red; why some supernova and others don't, we have to postulate different amounts of matter in the stars. The two references given below discuss these difficulties. http://scienceblogs.com/startswithabang/2009/09/dark_matter_part_iii_dark_ma tt.php http://www.physicsforums.com/showthread.php?p=2404693 Having a theory that is changed with each new observation is problematic. In a real sense, the modified gravity folks are being painted into a corner. As this happens, physicists tend to lower the probability that this is the correct theory. But, time will tell. If, all of a sudden, the shrinkage slows, and we have a narrow range of possible modifications that work with everything, then that is evidence for modified gravity. Most people are betting, however, that plumbers will find enough evidence for there to be no room left to stand...there will no longer be any possible set of parameters for the modified gravity theory that allows it to fit the data. The second blow is that we have independent strong evident for dark matter: neutrinos. There is very strong evidence that neutrinos have mass: http://www.hep.anl.gov/ndk/hypertext/solar_experiments.html http://seesaw25.in2p3.fr/trans/heeger.pdf Now neutrinos alone will probably not provide enough dark matter by themselves to explain the behavior of galaxies, galactic collisions, etc. However, they are dark matter as long as they have rest mass....and the evidence keeps on piling up that neutrinos are massive. I hope this helps. Dan M. _______________________________________________ http://mccmedia.com/mailman/listinfo/brin-l_mccmedia.com
