Galactic Dark Matter Distribution

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by EBTX</i>
<br />I have yet to see any proposed mechanism by which dark matter could be sorted from baryonic matter. How is it that these two forms of matter, ostensibly evenly mixed to begin with, assume distributions that are topologically different, i.e. disk shaped and toroidal.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Dark matter has yet to be "discovered" or directly detected. Its existence is inferred through failures of the universal law of gravitation with respect to visible matter.

Of course, in the Meta Model, the universal law changes with scale for sound physical reasons, and there is then no need for dark matter to exist anywhere.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">why isn't there a big clump of dark matter in the center of the Milky Way as well?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Under Big Bang premises, the Milky Way is mostly (90%) made of dark matter, just as for most spiral galaxies. It is never "clumped in the center", but must be spread throughout the galaxy to give the observed star motions. -|Tom|-

The galaxy disk is mass in every area of the disk and not mass at the center of the disk. The use of Kepler's law to determine how the disk should act if the mass was centered is why the result doesn't agree with observations. The mass is not at the center.

Experts do not make obviously wrong "assumptions". They work such things out rigorously.

It is easy for anyone who knows calculus to prove rigorously that the force inside a uniform spherical shell of matter is exactly zero everywhere. So only the matter interior to one's position inside such a shell will contribute to the force one feels.

A uniform disk is not the same as a uniform spherical shell. However, an approximately similar rule applies. To a rough approximation, one can neglect matter exterior to one's position, and just consider the interior matter. However, the behavior of galaxy rotations is different from what even a rigorous calculation allows unless either
-- dark matter exists in ever increasing amounts away from the center; or
-- the law of gravitation changes on the scale of galaxies. -|Tom|-

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">-- dark matter exists in ever increasing amounts away from the center; or...<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

This is my point. The distribution of dark matter must be different than the distribution of visible matter. If it is other than "normal baryonic matter" ... what mechanism of separation is offered? I have heard of none.

The divergent distributions rule out all conceivable non-baryonic models (WIMPS,etc) where the "non-baryons" are present and uniformly distributed at the origin of the universe ... and leave only black holes, or brown "things" in quantity and yet the question would then still remain about why these normal things also have divergent distributions in elliptical galaxies.

Hence, MOND-type arguments are gaining favor with every year that dark matter goes undetected.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[tvf]
... either
-- dark matter exists in ever increasing amounts away from the center; or
-- the law of gravitation changes on the scale of galaxies. -|Tom|-
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">In the absense of any specific evidence to nudge us one way or the other, the latter seems (to me at least) to be the only reasonable choice. Given the existance of possibility number two, in fact, I find it difficult to understand why any grown up would give possibility number one a second thought.

I have heard that working astronomers, in their day-to-day activities, regularly assume an inverse linear acceleration law rather than the newtonian inverse square law simply because it "fits better". (Damn the theory, full speed ahead!)

You often have some historcal understanding of things like this, and I was wondering if you know how this odd state of affairs came to be?

Regards,
LB

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Larry Burford</i>
<br />I have heard that working astronomers, in their day-to-day activities, regularly assume an inverse linear acceleration law rather than the newtonian inverse square law simply because it "fits better". (Damn the theory, full speed ahead!) You often have some historcal understanding of things like this, and I was wondering if you know how this odd state of affairs came to be?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

A technical paper showing the superiority of the inverse linear law over inverse square on galactic scales appeared in the journal of the Australian Astronomical Society a decade ago, but has been largely ignored.

I think the working astronomers are simply responding to the observed fact that most galactic rotation curves are flat, and that it takes an inverse linear force law to give a flat rotation curve. Details such as whether the invisible dark matter has a spherical or planar distrubution are left for future experiments to resolve. It is sufficient for them that it is possible for dark matter to exist with a distribution that produces an inverse linear law, and that using an inverse linear law enables them to reproduce observations. Most of the astronomers consider that it is not their job to question why. -|Tom|-