>> 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.
I don't dispute anything you write, except for this. Forgive me if I am
mistaken, but my understanding was that they did not in fact have to
modify the MOG theory in order to explain the data in the case of the
bullet cluster. The paper seemed quite clear on this point. In fact
the whole reason I pointed out the paper in the first place, and what
makes it so interesting, is that they didn't have to change any of the
parameters of the theory in order to explain the Bullet cluster, whereas
Modified Newtonian Dynamics (MOND) was apparently eviscerated by the
bullet cluster data.
>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_matt.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.
Seems to me that if neutrino's oscillate between having mass and not
having mass, that they would oscillate between being bound by gravity
and moving in a straight line, and depending on the magnitude of the
gravitational attraction, that the particles would eventually escape
from clustering. So you would have to look at some sort of differential
between neutrino creation, and neutrino 'escape'.
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