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forrest noble
2010-Jul-16, 07:37 PM
Tommac,

As has been explained above, disc stars of spiral galaxies have been shown to be orbiting faster than GR permits. The outermost stars of our spiral galaxy orbit faster than a factor of 60 times what GR allows. Accordingly there would need to be roughly 15 times more matter orbiting outside the galaxy than what has been observed within the galaxy. There are primarily three choices which have been presented concerning an explanation for these observations. One is that General Relativity needs to be altered, the second is that baryonic matter of some sort or combination thereof exists in vast quantities outside spiral galaxies, and thirdly that a new type of yet-to-be-discovered matter makes up maybe 90% of all matter.

The idea of "dark matter" (matter that does not seem to radiate EM radiation) is certainly logical since there are many known possibilities. The problem with baryonic matter being one of those is that observations to date have not found any indications of the quantities needed . It also presently seems that there is no measurable magnetic influence of this dark matter so it would seemingly need to be an unknown type of neutral particle. Hence many believe there is a presently unknown particle field involved which they have dubbed dark matter. This is not a new idea.

The idea of a background particle field of some kind is about 2,400 years old which was collectively given the name aether as a hypothetical atmosphere of an unknown material. This was a major theory group with many versions of it in physics up until the last century. One of the problems with an omni-present aether theory of some sort being an explanation of dark matter is that accordingly much of the dark matter seems to be concentrated outside the plane of the galaxy and must progressively decrease its concentrations inward to explain observed orbital velocities. Matter of any known baryonic type would seemingly in time gravitate toward the galaxy's core. Such a distribution of matter that causes great gravitational influences on a galaxies periphery and at the same time does not seem to react to baryonic gravity, is thought to be a quandary by many. One possibility is that the vortex-like motion of a spiral galaxy could multiply dark matter's gravitational influence many fold by its inertial mass influences at the outside of the galaxy where the density of dark matter could remain relatively uniform throughout the galaxy as well as intergalactic space.

There are probably other possibilities one of which is that both GR needs to be revised and that dark matter of some kind is real. This is probably not discussed because the idea does not seem to be consistent with the principle that the simplest answer has a better chance of being correct than a more complicated one, all else being equal.

Cougar
2010-Jul-16, 07:48 PM
Accordingly there would need to be roughly 15 times more matter orbiting outside the galaxy than what is visible within the galaxy.

If it was just "outside" the disk of the galaxy, it would have no gravitational effect on anything in the disk of the galaxy. (See Newton's Principia (http://en.wikipedia.org/wiki/Philosophiae_Naturalis_Principia_Mathematica), page ___.) It must be throughout the disk of the galaxy, AND it apparently extends well beyond the edge of the disk. But it must be interior to an orbit to gravitationally affect that orbit. Some assumptions apply.

Cougar
2010-Jul-16, 08:01 PM
High resolution Milky Way dark matter halos simulated on NASA's Columbia and ORNL's Jaguar supercomputers. (http://www.ucolick.org/~diemand/vl/images.html)

Grey
2010-Jul-16, 09:54 PM
One of the problems with an omni-present aether theory of some sort being an explanation of dark matter is that accordingly much of the dark matter seems to be concentrated outside the plane of the galaxy and must progressively decrease its concentrations inward to explain observed orbital velocities. Matter of any known baryonic type would seemingly in time gravitate toward the galaxy's core. Such a distribution of matter that causes great gravitational influences on a galaxies periphery and at the same time does not seem to react to baryonic gravity, is thought to be a quandary by many.This is not correct. The distribution of dark matter needed to account for the roughly flat rotation curves seen in galaxies* turns out to be pretty much spherical, with a mass distribution that falls off like the inverse square of the distance from the center. Conveniently, it also turns out that if you do some basic kinematic analysis on a collection of particles that are gravitationally bound, but don't otherwise interact much, an inverse square mass distribution is exactly what you end up with. So that means that if there are some hypothetical particles that interact only through gravity (and maybe the weak force), it is reasonable for them to behave in a manner that would account for what we see.

* Note that it's the flatness of the rotation curve that's an issue, as much as the absolute speed of rotation seen. It's not just that there has to be more matter there than we can see, it's that it has to have a particular distribution.

forrest noble
2010-Jul-16, 10:58 PM
Grey,


This is not correct. The distribution of dark matter needed to account for the roughly flat rotation curves seen in galaxies* turns out to be pretty much spherical, with a mass distribution that falls off like the inverse square of the distance from the center. Conveniently, it also turns out that if you do some basic kinematic analysis on a collection of particles that are gravitationally bound, but don't otherwise interact much, an inverse square mass distribution is exactly what you end up with. So that means that if there are some hypothetical particles that interact only through gravity (and maybe the weak force), it is reasonable for them to behave in a manner that would account for what we see.

Your quotation of a part of my posting was only one of the possibilities which I discussed in my posting. One of the problems with the uniform density of dark matter in a spiral galaxy is that the halo would seemingly need to be many times the volume of the interior galaxy to possibly create the observed orbital velocities of the outer-most stars which would seemingly make complete sense if dark matter represents roughly 90% of the galaxy's mass.

One of the key problems with the dark matter hypothesis, which I discussed above, is that if dark matter is the cause of the vast gravitational influences believed, the dark matter should in time gravitate toward the center and of the galaxy which accordingly should become more dense concerning its gravitational influence. This is not the case for any dark matter models that I am aware of.

I cannot conceive of any way that an inverse square distribution of dark matter starting from the highest density in the center of the galaxy and decreasing outwardly, could create the flat stellar velocities observed in the Milky Way. The opposite decreasing density from the halo toward the core, it seems to me, could work however. Do you have a link where I might read up on how decreasing densities of dark matter starting from the core outward could produce the flat orbital curves observed without a humungous halo, or am I misunderstanding somehow what you are saying?

Grey
2010-Jul-17, 02:07 AM
Your quotation of a part of my posting was only one of the possibilities which I discussed in my posting. One of the problems with the uniform density of dark matter in a spiral galaxy is that the halo would seemingly need to be many times the volume of the interior galaxy to possibly create the observed orbital velocities of the outer-most stars which would seemingly make complete sense if dark matter represents roughly 90% of the galaxy's mass.Judging by the motion of globular clusters well outside the galaxy, the evidence suggests that the dark matter halo of a galaxy does indeed extend well beyond the visible portion of the galaxy.


One of the key problems with the dark matter hypothesis, which I discussed above, is that if dark matter is the cause of the vast gravitational influences believed, the dark matter should in time gravitate toward the center and of the galaxy which accordingly should become more dense concerning its gravitational influence. This is not the case for any dark matter models that I am aware of.I'm afraid that you are not familiar with any of the mainstream models of dark matter, then. All of them assume a higher density at the center. You're correct that, if it's interacting only through gravity, a group of particles should end up with the mass concentrated toward the center. In fact, as I pointed out, you'd expect the density to be roughly proportional to the inverse square of the distance from the center. So, highest at the center, dropping off slowly as you move away. Note that the visible matter in a galaxy drops off in density much more quickly than this.


I cannot conceive of any way that an inverse square distribution of dark matter starting from the highest density in the center of the galaxy and decreasing outwardly, could create the flat stellar velocities observed in the Milky Way. The opposite decreasing density from the halo toward the core, it seems to me, could work however. Do you have a link where I might read up on how decreasing densities of dark matter starting from the core outward could produce the flat orbital curves observed without a humungous halo, or am I misunderstanding somehow what you are saying?Whether you can conceive of it or not, that's the way the math works out. It's a pretty straightforward calculation, actually. In fact, it was one of the problems I had in an introductory cosmology class: assume a flat rotation curve (actual rotation curves aren't always exactly flat, but most of them are pretty close to that), and determine what the mass distribution needs to be. I can try to find a link, or if you'd like, I can just rework the problem here.

forrest noble
2010-Jul-17, 02:51 AM
Grey,

Since you and I have had interesting discussions before in this forum I realize that you are well educated in related matters, but you may not know that I've also done a lot of research on this subject myself and have come up with my own equations as an alternative to GR (admittedly still incomplete but they seem to work well at galactic distances and for galaxy clusters without the use of dark matter) maybe 15 years ago after Vera Rubin announced her discovery concerning the flatness of orbital velocities of disc stars. Also have formulated an addendum to the Hubble distance formula based upon my cosmological model to explain the observations of type 1a supernova, which would eliminate the dark energy hypothesis. Nereid has called my book on alternative cosmology and theoretical physics a tome. Some of my related theories are now more than 50 years old.

If you have the time I would prefer a link, if you could find one, concerning simply a spiral galaxy model where it specifically states that a denser core of dark matter decreasing outwardly leads to flat rotation velocities or something you think is similar. In this way others might be able to consider the possibilities and we could remain on topic.

Grey
2010-Jul-17, 04:01 PM
Since you and I have had interesting discussions before in this forum I realize that you are well educated in related matters, but you may not know that I've also done a lot of research on this subject myself and have come up with my own equations as an alternative to GR (admittedly still incomplete but they seem to work well at galactic distances and for galaxy clusters without the use of dark matter) maybe 15 years ago after Vera Rubin announced her discovery concerning the flatness of orbital velocities of disc stars.I've seen you claim that your equations work as well, but I haven't seen you actually show that. For example, you could take a typical galactic rotation curve, and show that you could match it using your own equations.


If you have the time I would prefer a link, if you could find one, concerning simply a spiral galaxy model where it specifically states that a denser core of dark matter decreasing outwardly leads to flat rotation velocities or something you think is similar. In this way others might be able to consider the possibilities and we could remain on topic.Sure. I'll give you more than one, since they're easy to find. Here (http://www.hectorparr.freeuk.com/hcp/galaxy.htm)'s a link where someone shows how to match a typical galactic rotation curve. Note how the curves that match assume a dark matter density that goes like 1/r2 once you get outside the very center (with a relatively constant high density at the very center, since otherwise the density would go to infinity at the very center if you keep the 1/r2 dependency). Here (http://www.naoj.org/Pressrelease/2010/04/26/index.html)'s a link to a press release about recent research on the shape of dark matter halos. Take a look at figure 2, and you can see that the models they're using all have a much higher density toward the center than further out; all they're doing is comparing things like what kind of microlensing effects you'd see in an elliptical distribution as opposed to a spherical one. Here (http://www.ucolick.org/~diemand/vl/images.html)'s the Via Lactea project, where you can see nice pictures of simulations of the dark matter halo of the Milky Way. Notice that all of them are much more dense toward the center than on the edge. I agree that the term "halo" is a bad one, because it seems to imply a high density in some kind of ring or hollow sphere, but that's not what astronomers really mean when they talk about dark matter halos.

grant hutchison
2010-Jul-17, 05:09 PM
I cannot conceive of any way that an inverse square distribution of dark matter starting from the highest density in the center of the galaxy and decreasing outwardly, could create the flat stellar velocities observed in the Milky Way.Circular orbital velocity is given by:

vc = sqrt(GM/r)

For a spherical distribution, the mass that determines the orbital velocity is that of the sphere contained within the orbital radius; the spherical shell outside the orbit has no net effect.

So we can recast the above according to the mean density of the matter contained within the orbit, since M = ρ4πr/3:

vc = sqrt(Gρ4πr/3r) = sqrt(Gρ4πr/3)

The right side of the equation contains only two variables, ρ and r. For vc to remain constant, ρ must vary as 1/r.

Grant Hutchison

forrest noble
2010-Jul-17, 05:31 PM
Grey,

You are correct in that these are valid examples of what you said.

Quote from the fist link, the last sentence of the conclusion:

In particular there seems to be no escape from the conclusion that the visible material in many galaxies is accompanied by a "halo" of dark matter which extends to several times the diameter of the visible galaxy and whose mass is at least 10 times that of the material we can see.

To me the emphasis should be on the words "at least 10 times." 20 times or more would seemingly even model out better.

In their model, the first link, they modeled a dark-matter halo 3 times the radius (diameter) of the visible galaxy which is 27 times its volume. This is not quite humungous (my idea of humungous was about one hundred times the volume) but it is still a pretty big halo whereby the visible galaxy would accordingly be less than 4% of the total volume. In such a case, it seems to me, one could model just about any distribution of dark matter within the visible galaxy and get a similar flat rotation curve. I have seen different dark matter distributions modeled within galaxies but now understand that the dark matter distribution within the galaxy does not seem to matter since the stellar rotation curves would accordingly be controlled by the halo where at least 90% of the mass is accordingly contained. Thanks for your effort in providing these relevant links.

regards, forrest

grant hutchison
2010-Jul-17, 05:35 PM
I have seen different dark matter distributions modeled within galaxies but now understand that the dark matter distribution within the galaxy does not seem to matter since the stellar rotation curves would accordingly be controlled by the halo where at least 90% of the mass is accordingly contained.This isn't so, because any spherical shell outside the galaxy will have negligible effect on orbits within the galaxy, no matter what its mass is. We could surround the galaxy in a spherical shell of lead thousands of light years thick, and orbits within the galaxy wouldn't change. So it's the internal matter distribution that matters.

Grant Hutchison

forrest noble
2010-Jul-17, 07:13 PM
grant hutchison,


Circular orbital velocity is given by:
vc = sqrt(GM/r)
For a spherical distribution, the mass that determines the orbital velocity is that of the sphere contained within the orbital radius; the spherical shell outside the orbit has no net effect.
So we can recast the above according to the mean density of the matter contained within the orbit, since M = ρ4πr/3:
vc = sqrt(Gρ4πr/3r) = sqrt(Gρ4πr/3)
The right side of the equation contains only two variables, ρ and r. For vc to remain constant, ρ must vary as 1/r.

As I'm sure you know, the mass distribution of the visible galaxy is not accordingly the controlling factor concerning the orbital momentum of the galactic stars. For the dark matter hypotheses this would accordingly be controlled by the orbital momentum of the halo. Without that the stars would slowly lose their angular momentum and the galaxy would contract. I think the question should be: what is it that could perpetuate the angular momentum of a dark-matter halo? It can't be the visible galaxy or the central black hole or core since that would be like the tail wagging the dog. The angular momentum of the individual dark matter constituents would also need to fall off in velocity as its radius increases according to present gravitational theory. I believe the dark-matter hypothesis in its present form has many problems with it as you probably know.

My statements are not intended to be for or against this hypothesis. The intent is/ was to show some pros and cons concerning the OP question.

Here is a link to some of the perceived problems with the dark-matter idea. http://nedwww.ipac.caltech.edu/level5/Bosma2/frames.html

Swift
2010-Jul-17, 09:54 PM
This discussion has been split off from this Q&A thread (http://www.bautforum.com/showthread.php/105978-Why-is-dark-matter-needed).

Tensor
2010-Jul-18, 04:05 AM
My statements are not intended to be for or against this hypothesis. The intent is/ was to show some pros and cons concerning the OP question.

Here is a link to some of the perceived problems with the dark-matter idea. http://nedwww.ipac.caltech.edu/level5/Bosma2/frames.html

For 2010, that's a very weak link forrest. It's from 1998 almost all of the arguments are outdated. It has none of the Weak lensing evidence (First tentative evidence was in 1996 and first definite statistical evidence was in 2000), does not provide the evidence from the Bullet cluster, does not explain that MOND also requires dark matter to explain cluster movements, and has an entire section on microlensing and the search for MACHOs, which have been pretty much eliminated as a possibility for dark matter. As for this:


One of the problems with an omni-present aether theory of some sort being an explanation of dark matter is that accordingly much of the dark matter seems to be concentrated outside the plane of the galaxy and must progressively decrease its concentrations inward to explain observed orbital velocities.

And, I know of no one who ever brought up the discredited aether theory as an explanation for dark matter. Do you have any kind of link that show this? And even if you did. Q and A is not the place to be bringing it up as that is definitely not mainstream.

forrest noble
2010-Jul-18, 05:20 AM
Tensor,


I know of no one who ever brought up the discredited aether theory as an explanation for dark matter. Do you have any kind of link that show this? And even if you did. Q and A is not the place to be bringing it up as that is definitely not mainstream

Thanks Tensor. I would appreciate your additional opinions on this matter if you have time. All the other answers have been great in my opinion. You've read my opinions concerning dark matter. What's yours? The rest of this discussion has been redirected to this thread and believe that all of the answers/ statements given on this thread so far have been good. The paper was republished in 2002; maybe opinions since then have changed somewhat but most problems listed I think are still relevant.

your friend forrest

Nereid
2010-Jul-18, 10:38 AM
grant hutchison,



As I'm sure you know, the mass distribution of the visible galaxy is not accordingly the controlling factor concerning the orbital momentum of the galactic stars. For the dark matter hypotheses this would accordingly be controlled by the orbital momentum of the halo. Without that the stars would slowly lose their angular momentum and the galaxy would contract. I think the question should be: what is it that could perpetuate the angular momentum of a dark-matter halo? It can't be the visible galaxy or the central black hole or core since that would be like the tail wagging the dog. The angular momentum of the individual dark matter constituents would also need to fall off in velocity as its radius increases according to present gravitational theory. I believe the dark-matter hypothesis in its present form has many problems with it as you probably know.

My statements are not intended to be for or against this hypothesis. The intent is/ was to show some pros and cons concerning the OP question.

Here is a link to some of the perceived problems with the dark-matter idea. http://nedwww.ipac.caltech.edu/level5/Bosma2/frames.html
The question of CDM distributions, and how they came to be what they seem to be, has been studied extensively in the past decade or so.

One of these studies is the Millennium simulation (http://www.mpa-garching.mpg.de/galform/virgo/millennium/) which started with a distribution of matter consistent with the CMB data and looked at how structure evolved subsequently. In simulations like these, galaxies form quite often, as 'dark matter halos'.

One of the biggest open questions concerns the evolution of galaxies - how to explain the plethora of weird shapes etc in deep images taken with the Hubble Space Telescope (in projects such as AEGIS, GEMS, and GOODS), and how those strange objects came to be the ellipticals (a.k.a. early-type galaxies) and spirals (a.k.a. late-type galaxies) we see in the local universe. Interestingly, this research is being done, in part, by citizen scientists, in the Hubble Zoo project. (http://www.galaxyzoo.org/)

Grey
2010-Jul-18, 03:46 PM
In their model, the first link, they modeled a dark-matter halo 3 times the radius (diameter) of the visible galaxy which is 27 times its volume. This is not quite humungous (my idea of humungous was about one hundred times the volume) but it is still a pretty big halo whereby the visible galaxy would accordingly be less than 4% of the total volume. In such a case, it seems to me, one could model just about any distribution of dark matter within the visible galaxy and get a similar flat rotation curve. I have seen different dark matter distributions modeled within galaxies but now understand that the dark matter distribution within the galaxy does not seem to matter since the stellar rotation curves would accordingly be controlled by the halo where at least 90% of the mass is accordingly contained.Forrest, you seem to have a number of misconceptions about dark matter models, and these seem to be the primary source of your criticism of those models. For example, you seemed under the impression that the orbital velocity of objects is affected by the matter further away from the center (so that having a large halo could influence these). As grant has pointed out, that's not the case. For an object orbiting around the center of a galaxy, only the matter in the sphere inside that orbit matters. The reason we know that dark matter halos extend well beyond the visible size of the galaxy is that we can measure the orbital velocties of globular clusters far outside the galaxy, and they still have the same roughly flat rotation curves. You've claimed that you have an alternate theory that can explain the flat rotation curves just as well, but I confess that I'm now much more skeptical about that, since it seems like you don't understand standard orbital mechanics, or how to determine what the orbital velocity of an object would be, given some specific mass distribution.


As I'm sure you know, the mass distribution of the visible galaxy is not accordingly the controlling factor concerning the orbital momentum of the galactic stars. For the dark matter hypotheses this would accordingly be controlled by the orbital momentum of the halo. Without that the stars would slowly lose their angular momentum and the galaxy would contract. I think the question should be: what is it that could perpetuate the angular momentum of a dark-matter halo? It can't be the visible galaxy or the central black hole or core since that would be like the tail wagging the dog. The angular momentum of the individual dark matter constituents would also need to fall off in velocity as its radius increases according to present gravitational theory. I believe the dark-matter hypothesis in its present form has many problems with it as you probably know.Your question here about the dark matter angular momentum doesn't really make much sense. There's no need for the dark matter halo to have any particular angular momentum as a whole for this model to work. The dark matter could be a completely static mass distribution, and you'd still get the orbital behavior of the visible matter that we see. Certainly, the individual particles of whatever kind that comprise the dark matter halo will need to be orbiting the galaxy, but as I pointed out above, the 1/r2 density dependency is exactly what you get if you have a cloud of particles that only interact through gravity.

forrest noble
2010-Jul-18, 04:51 PM
Nereid,

Thanks for the links. As time progresses I think insights will continue to improve concerning the reasons for galaxy formations, stellar velocities, and resulting galaxy forms after interactions and mergers.

forrest noble
2010-Jul-18, 05:07 PM
Grey,


Your question here about the dark matter angular momentum doesn't really make much sense. There's no need for the dark matter halo to have any particular angular momentum as a whole for this model to work. The dark matter could be a completely static mass distribution

Let's take a static sphere of 10 times the mass of the interior galaxy (your link) and place it as a hollow globe with the visible galaxy stars orbiting inside it with an inverse square mass distribution. Don't you think in time that those outer stars of the visible galaxy that are closer to this giant static mass would in time start to lose their angular momentum eventually moving outward (or inward) causing a domino effect on the visible galaxy?

nutant gene 71
2010-Jul-18, 05:23 PM
I've seen you claim that your equations work as well, but I haven't seen you actually show that. For example, you could take a typical galactic rotation curve, and show that you could match it using your own equations.

Sure. I'll give you more than one, since they're easy to find. Here (http://www.hectorparr.freeuk.com/hcp/galaxy.htm)'s a link where someone shows how to match a typical galactic rotation curve. Note how the curves that match assume a dark matter density that goes like 1/r2 once you get outside the very center (with a relatively constant high density at the very center, since otherwise the density would go to infinity at the very center if you keep the 1/r2 dependency). Here (http://www.naoj.org/Pressrelease/2010/04/26/index.html)'s a link to a press release about recent research on the shape of dark matter halos. Take a look at figure 2, and you can see that the models they're using all have a much higher density toward the center than further out; all they're doing is comparing things like what kind of microlensing effects you'd see in an elliptical distribution as opposed to a spherical one. Here (http://www.ucolick.org/~diemand/vl/images.html)'s the Via Lactea project, where you can see nice pictures of simulations of the dark matter halo of the Milky Way. Notice that all of them are much more dense toward the center than on the edge. I agree that the term "halo" is a bad one, because it seems to imply a high density in some kind of ring or hollow sphere, but that's not what astronomers really mean when they talk about dark matter halos.


In your referenced Dark Matter and the Rotation of the Galaxy (http://www.hectorparr.freeuk.com/hcp/galaxy.htm), it states:

"Its area is proportional to r, but its attractive force on m is also proportional to 1/r^2 by Newton's law, so its net effect is proportional to 1/r. This tends to infinity as r tends to zero, so that the attractive force of the whole sector must be infinite. (Notice that this argument does not apply in the three dimensional case, where we deal with an elementary surface, whose mass is proportional to r^2, rather than an arc whose mass is proportional to r)."

This 1/r^2 would be modified to 1/r in the outer sections of galaxy if the gravitational constant G grows by the same proportion as r from its center. This may not be the place to discuss this, since it is an ATM idea (http://www.bautforum.com/showthread.php/38753-Jerry-Jensen-s-ATM-idea?p=887293#post887293), but the results would be the same, if this is true, which means the G in GR would have to be 'tweaked'.

forrest noble
2010-Jul-18, 05:57 PM
Grey,


You've claimed that you have an alternate theory that can explain the flat rotation curves just as well, but I confess that I'm now much more skeptical about that, since it seems like you don't understand standard orbital mechanics, or how to determine what the orbital velocity of an object would be, given some specific mass distribution.

My gravitational theory, that you inquired about, can be found at pantheory.com. pages 57A to 57E. If you have further questions about it contact me by e-mail on the website or PM me if you wish.

Nereid
2010-Jul-18, 08:47 PM
Grey,



Let's take a static sphere of 10 times the mass of the interior galaxy (your link) and place it as a hollow globe with the visible galaxy stars orbiting inside it with an inverse square mass distribution. Don't you think in time that those outer stars of the visible galaxy that are closer to this giant static mass would in time start to lose their angular momentum eventually moving outward (or inward) causing a domino effect on the visible galaxy?
There are many papers on this topic, but I have been unable to find an easy-to-read, yet accurate, website.

This paper is as good a place to start as any: On the Correlation between Spin Parameter and Halo Mass (http://iopscience.iop.org/0004-637X/678/2/621/fulltext). For more information, check out the references therein (http://adsabs.harvard.edu/cgi-bin/nph-ref_query?bibcode=2008ApJ...678..621K&refs=REFEREN CES&db_key=AST).

Very briefly, dark matter halos will have a spin (i.e. a non-zero angular momentum, rotation), and the spin of the galaxies which form within them will be related to that spin ... but galaxy collisions, mergers, and interactions will leave a complicated imprint ...

Grey
2010-Jul-18, 08:54 PM
Let's take a static sphere of 10 times the mass of the interior galaxy (your link) and place it as a hollow globe with the visible galaxy stars orbiting inside it with an inverse square mass distribution. Don't you think in time that those outer stars of the visible galaxy that are closer to this giant static mass would in time start to lose their angular momentum eventually moving outward (or inward) causing a domino effect on the visible galaxy?If we take a uniform hollow sphere, with the galaxy at the center? No, actually I don't think that will have any effect at all on the rotation of the stars. And I know that because I've worked out the gravitational effects of a hollow sphere on the matter inside, and the result is zero. It's a bit counterintuitive, but it's also a very straightforward calculation; it's a common homework problem in undergraduate classes on mechanics.

If instead we assume that there's a dark matter sphere composed of individual particles, and that they move around interacting only gravitationally with each other and with the visible matter, then we don't get a hollow sphere, we got a sphere with a higher density in the center than the outside (which is why nobody thinks that dark matter forms hollow spheres, except people who have misinterpreted the unfortunate term "halo"). And if we're modeling the dark matter as individual particles, then yes, the overall angular momentum of the dark matter and the angular momentum of the visible matter will affect each other, but they won't just magically disappear. Angular momentum is a conserved quantity. So if the cloud that formed the original galaxy had some initial angular momentum, it will maintain that same angular momentum indefinitely. If the visible matter loses angular momentum to the dark matter, the dark matter gains an equal amount of angular momentum, and that keeps happening until you reach a stable equilibrium. The reason that we know that the cold dark matter model is a good one is precisely because we've done lots of simulations like that. The results turn out to be consistent with the galaxies that we observe.

Grey
2010-Jul-18, 09:02 PM
In your referenced Dark Matter and the Rotation of the Galaxy (http://www.hectorparr.freeuk.com/hcp/galaxy.htm), it states:

"Its area is proportional to r, but its attractive force on m is also proportional to 1/r^2 by Newton's law, so its net effect is proportional to 1/r. This tends to infinity as r tends to zero, so that the attractive force of the whole sector must be infinite. (Notice that this argument does not apply in the three dimensional case, where we deal with an elementary surface, whose mass is proportional to r^2, rather than an arc whose mass is proportional to r)."

This 1/r^2 would be modified to 1/r in the outer sections of galaxy if the gravitational constant G grows by the same proportion as r from its center. This may not be the place to discuss this, since it is an ATM idea (http://www.bautforum.com/showthread.php/38753-Jerry-Jensen-s-ATM-idea?p=887293#post887293), but the results would be the same, if this is true, which means the G in GR would have to be 'tweaked'.He's just pointing out why you can't easily model the gravitational effects of a two dimensional disk on objects within the disk. As the author explains, that's essentially because if you assume a truly flat disk, then the density of the disk is infinite. The problem goes away if you assume some real three dimension shape. I don't think that obvious limitation in modeling mass distributions has any bearing on the ideas proposed in the ATM thread you linked.

forrest noble
2010-Jul-19, 05:36 PM
Nereid,


Very briefly, dark matter halos will have a spin (i.e. a non-zero angular momentum, rotation), and the spin of the galaxies which form within them will be related to that spin ... but galaxy collisions, mergers, and interactions will leave a complicated imprint ...

Thanks for the links and relevant summation.

icarus2
2010-Jul-22, 11:35 AM
Im sorry. I cant English well. My native language is not English.
I have one idea for the dark matter and dark energy

Link redacted

It is explain that
- Almost uniformly distribution of the dark matter,
- Non-observation of the star and galaxy consists of dark matter,
- Dark matter clumping around galaxy,
- Rotation curve of the galaxy -Centripetal force effect,
- Low interaction between dark matter when collision occurs between dark matter,
- Collision of Bullet Cluster,
- Non-observation of dark matter at the Earth and Solar system (Xenon100, CDMS II)
- Non-baryonic dark matter condition,

- Observed value of the dark energy(10^(-47)GeV^4),
- Repulsive force needed for accelerating expansion,
- It is show that result of original field equation(Λ=0) of general relativity is right.

Please view to the link and critical comment to me! There are none who comment to my explaination.
Again I'm sorry!

forrest noble
2010-Jul-22, 09:28 PM
icarus2,

Nice graphics and interesting presentation concerning negative matter but this forum, astronomy, does not allow such discussions to take place. Discussions here are generally limited to ideas that are presently considered to be mainstream rather than generally unknown alternative theory. I'll drop by your ATM thread and give a quick comment. Good luck on your presentation and on learning more English which will be beneficial not only in your future presentation(s), but to better understand the questions and answers given.

PetersCreek
2010-Jul-22, 09:48 PM
icarus2,

You may discuss your theory in your thread only. Do not post about it in any other thread, like this one. Also, please read our rules. A link to our rules appears at the bottom of this post.

forrest noble,

Your comments are also against the rules. Kindly use the report button next time and leave the moderating to the moderators.