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## Dark Matter Puzzle

First, I've spent quite a bit of time over the past couple of days trying to find if this has been addressed anywhere; didn't find it so I'm going to ask.

Dark Matter (DM) is, if I understand it correctly, supposed to be interacting gravitationally with normal matter but not in normal ElectroMagnetic (EM) ways. This is used as a theory to explain such diverse things as why the Universe formed as it has & why there are apparent anomalies in the rotational speeds of galactic disks.

Now this may just be a math issue (as in I don't understand the math) but, there seems to be a requirement that most of the matter in the Universe is both DM AND closely allied to galaxies. Also, it is supposed to be mostly outside the galactic plane, comprising a sphere around the galaxy.

If this is so, it seems most of the galactic mass is not in the central sphere or in the disk, but it IS required to interact gravitationally. It also isn't supposed to interact in any EM way or we'd bee 'seeing' it other than via the lensing effect.

1. How is it that the disk forms at all?

2. Why doesn't the DM distribute the normal matter throughout the sphere as the galaxy forms?

3. Also, why doesn't the gravitational effect seem to work both ways?
If DM is evenly distributed in a shell or sphere, & we have normal matter in a concentrated disk, & the DM is not being kept in place by some sort of motion effect (orbit or repulsion etc.) then over time, the DM should be attracted down to the disk or into the central bulge. (if the bulge has significantly more mass than the disk)

4. For that matter, is it the case that the DM exists outside the disk & central sphere but is not distributed within the normal matter realm? If so, how can that be?

5. Wouldn't that require either EM interaction or physical interaction between normal matter & energy & DM?

I'm a bit puzzled by the explanations I've seen. It seems the DM shell is not required to rotate (or all velocities sum to zero) & it can't interact in any way that would provide a normal effect to the paths of normal matter. Yet it apparently can act upon normal matter in such fashion to produce the observations we have of the visible universe.

2. As Dark energy is not dark or even energy. As Dark matter is not dark but could be described as matter. The fact is the word dark is used because we do not know a better word as yet. Unknown matter and Unknown energy might be considered as a better explanation of these conditions...but still does not paint an image of any reality we can poke a stick at.
WE, (who ever they are) have drawn the conclusion that over 80% of the mater that makes up this universe is 'Dark' Replace with UNKNOWN for a better idea of clarity. We have not seen this stuff, but know it must be there. To study a Galaxy and calculate its rotational velocity requires a understanding of mass and orbital mechanics. From these calculations can we point to the gravity expected being exceeded. Thus forces unknown, dark whatever...

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Yes, I understand that the term Dark Matter is a placeholder. I'm wondering about the purported effects that are supposed to be there for the equations or theories to work out.

4. Originally Posted by Acolyte
Dark Matter (DM) is, if I understand it correctly, supposed to be interacting gravitationally with normal matter but not in normal ElectroMagnetic (EM) ways.
Yup, that's the general assumption.

most of the matter in the Universe is both DM AND closely allied to galaxies. Also, it is supposed to be mostly outside the galactic plane, comprising a sphere around the galaxy.
Yes.

If this is so, it seems most of the galactic mass is not in the central sphere or in the disk, but it IS required to interact gravitationally. It also isn't supposed to interact in any EM way or we'd bee 'seeing' it other than via the lensing effect.
Yes.

1. How is it that the disk forms at all?
The ordinary matter has some net angular momentum. It collides
with itself as it orbits the galaxy. The "vertical" motions cancel
each other out over the course of many collisions, and the
ordinary matter loses energy but retains the angular momentum.
The result is a disk.

2. Why doesn't the DM distribute the normal matter throughout the sphere as the galaxy forms?
Because the normal matter interacts electromagnetically
with itself; those interactions overwhelm the gravitational
forces on the ordinary matter from the dark matter.

3. Also, why doesn't the gravitational effect seem to work both ways?
If DM is evenly distributed in a shell or sphere, & we have normal matter in a concentrated disk, & the DM is not being kept in place by some sort of motion effect (orbit or repulsion etc.) then over time, the DM should be attracted down to the disk or into the central bulge. (if the bulge has significantly more mass than the disk)
Because there's a LOT more DM than ordinary matter, for one
thing. For another, the DM can't dissipate its energy via
electromagnetic radiation, so it can't get rid of the "vertical"
motions and settle into a disk.

4. For that matter, is it the case that the DM exists outside the disk & central sphere but is not distributed within the normal matter realm? If so, how can that be?
No, the DM is distributed throughout the bulge and disk and halo.
It's most concentrated near the bulge.

5. Wouldn't that require either EM interaction or physical interaction between normal matter & energy & DM?
No.

5. Also, bear in mind there are elliptical galaxies too. The normal matter in elliptical galaxies is behaving much as the dark matter.

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Originally Posted by StupendousMan
The ordinary matter has some net angular momentum. It collides with itself as it orbits the galaxy. The "vertical" motions cancel each other out over the course of many collisions, and the
ordinary matter loses energy but retains the angular momentum. The result is a disk.
This sounds a little 'special case' in that I'm curious as to why the horizontal motions don't also cancel out.
eg. When the 'cloud' of Normal Matter (NM) is collapsing to form the proto-galaxy, why would only the vertical motions be affected? I seem to have a failure of visualisation here.
To me, if the DM is affecting the formation of the galaxy gravitationally it should be attracting the cloud all through the sphere of DM.

Originally Posted by StupendousMan
Because the normal matter interacts electromagnetically with itself; those interactions overwhelm the gravitational forces on the ordinary matter from the dark matter.
Does that imply NM has EM effects even if it isn't charged? Or are you saying that the events occuring to atoms that strip an electron & charge it are common enough to have this effect on NM? (not quite sure if this question conveys what I'm trying to get at. Have to think more about it I guess)
Originally Posted by StupendousMan
Because there's a LOT more DM than ordinary matter, for one thing. For another, the DM can't dissipate its energy via electromagnetic radiation, so it can't get rid of the "vertical"
motions and settle into a disk.
If the DM can be affected by gravity, but is not otherwise constrained, & the NM is constrained into the disk & bulge, that would mean a concentration of attraction in the disk & bulge - surely that should affect the DM over the eons?
Is there some EM effect even with NM if it's attracted by a gravity source that changes it's motion? ie. the Earth radiates EM in some manner because it is being constrained by the Sun's gravity?
Originally Posted by StupendousMan
No, the DM is distributed throughout the bulge and disk and halo. It's most concentrated near the bulge.
Ah, I was wondering, if it's also present within the area of NM how that would affect our observations of the galaxy itself & also how it would affect our view of the surrounding universe. After all, light is affected by DM so wouldn't there be a variety of effects depending on where we look?

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Originally Posted by Ken G
Also, bear in mind there are elliptical galaxies too. The normal matter in elliptical galaxies is behaving much as the dark matter.
I was trying not to think about them for the nonce. I figured they just muddy the waters I am trying to look into. But I did wonder if they are examples where the DM has affected the formation by actually preventing the formation of the disk.

But that leads into more murky waters when I begin to wonder what would be different so as to cause the varied results. Particularly when collisions of disk galaxies apparently sometimes produce ellipticals & sometimes don't.

8. Originally Posted by Acolyte
But I did wonder if they are examples where the DM has affected the formation by actually preventing the formation of the disk.
Yes, that seems to be true.
But that leads into more murky waters when I begin to wonder what would be different so as to cause the varied results. Particularly when collisions of disk galaxies apparently sometimes produce ellipticals & sometimes don't.
It does get complicated, I think simulations of dark matter have succeeded, though there are some free parameters to adjust.

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## It Was the Best of Times, It Was the Worst of Times

Originally Posted by Ken G
Yes, that seems to be true.
It does get complicated, I think simulations of dark matter have succeeded, though there are some free parameters to adjust.
We're shaking out the simulations to see how the galaxies shake out their shapes.

We're living in interesting times, anyhow.

10. Originally Posted by Acolyte
Originally Posted by StupendousMan
No, the DM is distributed throughout the bulge and disk and halo. It's most concentrated near the bulge.
Ah, I was wondering, if it's also present within the area of NM how that would affect our observations of the galaxy itself & also how it would affect our view of the surrounding universe.
If there was a spherical shell of dark matter around the galactic bulge and disk, then the dark matter would have no effect (or a balancing effect such that the result is zero) on the gravity and hence the rotation of the disk (See Newton's Principia.) But the "too-fast" galactic rotation was what led to the dark matter hypothesis in the first place. (Well, it might have been the second place -- Zwicky noticed that galaxies in galaxy clusters were moving around much too fast to stay in the cluster.)

So anyway, as Stupendous Man says, the DM must be distributed throughout the disk and bulge. How does this affect our observations? Well, the observed rotation speeds at different radii make it appear that there must be a lot more mass distributed throughout the disk and bulge, keeping the galaxy from flying apart. Is that what you meant?

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Originally Posted by Ken G
Yes, that seems to be true.
It does get complicated, I think simulations of dark matter have succeeded, though there are some free parameters to adjust.
*grins* I think those free parameters make a regular attempt to adjust my mind when I try to work out this stuff.

It will be interesting times indeed to see just what simulation eventually reproduces the results... & then we can see if the galaxy mentioned in my post fits the model. (somehow the universe always manages to produce an exception or two)

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Originally Posted by Cougar
If there was a spherical shell of dark matter around the galactic bulge and disk, then the dark matter would have no effect...

...So anyway, as Stupendous Man says, the DM must be distributed throughout the disk and bulge. How does this affect our observations? Well, the observed rotation speeds at different radii make it appear that there must be a lot more mass distributed throughout the disk and bulge, keeping the galaxy from flying apart. Is that what you meant?
Partly that's what I meant, but also I'm wondering how the presence of DM through the NM areas would affect what we are currently seeing. DM apparently does affect light or we wouldn't have lensing effects.

If it is everywhere, would all the effects of lensing cancel out or could we be seeing a distortion of what is really out there when we look? Or does the answer to that depend on just what the distribution of DM is & whether or not there are concentrations?

Are there other explanations for what we see with lensing around galaxies and/or the Bullet Cluster collision?

13. Originally Posted by Acolyte
Partly that's what I meant, but also I'm wondering how the presence of DM through the NM areas would affect what we are currently seeing. DM apparently does affect light or we wouldn't have lensing effects.
It affects light in the same way it effects normal matter: gravitational interaction. If it didn't affect light, then we'd know something really weird was going on! Actually, if we didn't see a gravitational lensing signal from dark matter, that would be a very strong piece of evidence that it didn't exist.

Hmmm... I apologize for the convolution of that last sentence.

Originally Posted by Acolyte
If it is everywhere, would all the effects of lensing cancel out or could we be seeing a distortion of what is really out there when we look? Or does the answer to that depend on just what the distribution of DM is & whether or not there are concentrations?
Keep in mind that when we see gravitational lensing of galaxies and quasars through other galaxies and galaxy clusters, we are seeing the integrated effect of all the dark matter along the line of sight. And there's a lot of dark matter in a galaxy cluster. Looking toward a star or cluster of stars in our own galaxy would not produce much of a signal, because of how diffuse the dark matter is.

Whether dark matter is clumpy, smooth, whole 2% or skim is a current topic of research. If you haven't already, look up the MACHO studies. They were looking for large "particles" of dark matter, and didn't find a signal from clumps of dark matter above a certain size.

There is also current work looking for the weak lensing signal from the dark matter filaments that we expect to exist between clusters of galaxies (the fainter bits connecting the brighter bits in the, e.g. The Millennium Simulation). I seem to recall that there was a recent press release or astro-ph submission claiming a positive detection, though I could be wrong (and no, I don't mean the recent detection of hot filament gas, which is also an important result).

Originally Posted by Acolyte
Are there other explanations for what we see with lensing around galaxies and/or the Bullet Cluster collision?
There are some versions of MOND that claim to be able to reproduce the Bullet Cluster and other lensing results. But the best ones that I'm aware of either require a mass for the neutrino that is far too large (above the current threshold from particle physics experiments), or can't consistently account for all observations with the same set of parameters.

Beyond that? Nope, there are no other currently viable hypotheses that account for lensing + BAO + galaxy rotation curves + cluster motion + CMB anisotropies + (what am I missing?).

But there have been quite a few simulations of galaxies formation and evolution using dark matter. I don't have links to the best ones (with shiny movies!), but someone around here probably does.

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Don't go away y'all... I'll be back.

Loose translation is I'm heading off to do some more research given the information here. Thanks for that. Back soon...

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## Dark Matter Theory In Crisis?

Because there's a LOT more DM than ordinary matter, for one thing. For another, the DM can't dissipate its energy via electromagnetic radiation, so it can't get rid of the "vertical" motions and settle into a disk.
Normal matter interacts gravitational with “dark matter”, so dark matter can loss or gain energy from the galaxy. Unfortunately for the “dark matter theory”, hydro-dynamic simulations, fundamentally disagree with real galaxies. The simulations create a model disc that is an order of magnitude smaller than what is observed. This discovery, which is called the “angular momentum catastrophe”, was made 8 years ago. I believe there is no solution to the angular momentum catastrophe, which is not surprising; however, as more detail data and observations concerning spiral galaxies shows structures that could not possibly have been created by the interaction of “dark matter” and normal matter.

It should be noted that the "angular momentum catastrophe” problem and the “missing satellites problem” is leading some researchers to state that dark matter does not exist which is interesting as LCDM theory will need to change. The “angular momentum catastrophe” and the missing satellites problem” are not the only fundamental disagreements with the “dark matter” theory and reality.

New Problems for the Formation of Disk Galaxies, by Frank C. van den Bosch

http://arxiv.org/PS_cache/astro-ph/p.../0208524v1.pdf

The current paradigm for disk formation contains three important ingredients: (i) the angular momentum originates from cosmological torques; (ii) the gas and dark matter within virialized systems have initial angular momentum distributions (AMDs) that are identical; and (iii) the gas conserves its specific angular momentum when cooling. Under these assumptions the predicted scale lengths of disk galaxies are in excellent agreement with observations (Fall & Efstathiou 1980; de Jong & Lacey 2000), which has motivated the construction of more detailed models, but always under the three assumptions listed above (e.g., Mo, Mao & White 1998; van den Bosch 1998, 2000, 2001, 2002; Firmani & Avila-Reese 2000). Because of the overall success of these models in explaining a wide range of observed properties of disk galaxies, it has generally been assumed that the aforementioned assumptions are correct.
However, several recent results have started to cast some doubt as to the validity of this standard framework. First of all, detailed hydro-dynamical simulations of disk formation in a cold dark matter (CDM) Universe yield disks that are an order of magnitude too small (e.g., Steinmetz & Navarro 1999). This problem, known as the angular momentum catastrophe, is a consequence of the hierarchical formation of galaxies which causes the baryons to lose a large fraction of their angular momentum to the dark matter.

Numerical Simulation of Cosmic Structure Where are we now? By Hugh Couchman: McMaster University

•Large-Scale Structure
•Galaxy Formation -the angular momentum problem

http://www.physics.uoguelph.ca/poiss...a/couchman.pdf

16. Originally Posted by William
Unfortunately for the “dark matter theory”, hydro-dynamic simulations, fundamentally disagree with real galaxies. The simulations create a model disc that is an order of magnitude smaller than what is observed. This discovery, which is called the “angular momentum catastrophe”, was made 8 years ago.
That's interesting, because 10 years ago, Dominguez-Tenreiro, et al. published A Way Out of the Disc Angular Momentum Catastrophe in Hierarchical Hydrodynamical Simulations. They show that the angular momentum catastrophe "can be easily avoided by including star-forming processes."

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## How Do Spiral Galaxies Stay Spiral After Merges?

These authors raised the same question that was raised in the spiral vs elliptical thread. Ignoring the basic, fundamental, unanswered problem of where does the net angular momentum come to initially create a spiral galaxy, this paper asks the question "Why are 70% of galaxies spiral not elliptical?" A major merger of two spiral galaxies creates a elliptical galaxy that has roughly zero net angular momentum. i.e. Its constitute stars orbit the elliptical galaxy's gravitation centre both clockwise and anticlockwise such that the integrated angular momentum for the elliptical is typically small. (Which explains why the elliptical is sphere like rather than disk like.) A merger of an elliptical galaxy produces a elliptical galaxy. Most galaxies have been involved in a number of major mergers based on simulations, LCDM theory, and observations.

(Comment: And yes, parejkoj, the authors of this paper postulate that a spiral galaxy can be produced from a major merger of two spirals. Perhaps it can, however, there needs to be a new explanation as to what creates a spiral galaxy. Dark matter has nothing to do with the solution to this problem. Also as one digs deeper into the new observations, there are multiple problems with the connecting theories, so the solution is not just let's replace dark matter with another prop.)

Other authors are raising questions concerning the age differences of stars in a spiral galaxy and details concerning disk and halo of the spiral galaxy. Very difficult to explain. Others are asking where does the spiral galaxy gas and dust come from. Elliptical galaxies have very little gas and dust as compared to spirals. Is dust being created in spirals?

“The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies”, by F. Hammer, M. Puech, L. Chemin, H. Flores, M. Lehnert

http://arxiv.org/abs/astro-ph/0702585v1

Disk galaxies constitute the majority of the galaxy population observed in the local universe. They represent 70% of intermediate mass galaxies (stellar masses ranging from 3× 10^10 to 3 × 10^11 M⊙), which themselves include at least two-third of the present-day stellar mass (e.g., Hammer et al. 2005). Early studies of the Milky Way have led to a general description of the formation of a disk galaxy embedded in a halo (Eggen, Lynden-Bell, & Sandage 1962). Fall & Efstathiou (1980) set out a model of galaxy formation assuming that disks form from gas cooling and condensing in dark halos. Protogalactic disks are assumed to be made of gas containing substantial amount of angular momentum, which condenses into stars to form thin disks (Larson 1976). These disks then evolve only through secular processes.
However, there are several outstanding difficulties with this standard scenario. One such difficulty is the so-called angular momentum problem. That is, simulated galaxies cannot reproduce the large angular momentum observed in nearby spiral galaxies (e.g., Steinmetz & Navarro 1999). Another is the assumed absence of collisions during and after the gas condensation process. Indeed, the hierarchical nature of the LCDM cosmology predicts that galaxies have assembled a significant fraction of their masses through collisions with other galaxies. It is likely that such collisions would easily destroy galactic disks (e.g., Toth & Ostriker 1992).
Using arguments based on either dynamical friction (Binney & Tremaine 1987) or simple orbital time-scale (e.g.,Bell et al. 2006), this time scale has been estimated to be about 0.35Gyrs. Combining the pair fraction and characteristic time scale estimates suggests that for a present-day galaxy with a stellar mass larger than 3 × 10^10 M⊙, the chance it has experienced a major merger since z=1 is 50±17%, 75±25% and 70% according to Lotz et al. (2006), Hammer et al. (2005), and Bell et al. (2006), respectively1. Although less certain, integrating the merger rate to higher redshift implies that a typical bright galaxy may have experienced up to four to five major merging events since z=3 (Conselice et al. 2003).
The widely accepted assumption that a major merger would unavoidably lead to an elliptical is perhaps no longer tenable: accounting for the large number of major mergers that have apparently occurred since z=3 would imply that all present day galaxies should be ellipticals. This is obviously not the case. So it is likely that disks either can survive or are “rebuilt” after a major merger, through whatever mechanism as yet perhaps unknown in detail (see, for example, Robertson et al. 2006).

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## Spiral Galaxies' Missing Dwarf Satellite Problem

Based on hydro dynamic simulations with the dark matter models the Milky Way should have in excess of 250 satellite galaxies. The Milky Way has eleven significant dwarf galaxies. The problem of the missing satellite galaxies is found to occur with other spiral galaxies, so the solution is not that the Milky Way lost its satellites by some strange coincidence.

Somewhat interesting if you are interested in theories which might replace the "dark matter" based theory is that the satellite galaxies are found to orbit the Milky Way in a disk that is at right angles to the Milky Way's spiral disk.

Aside
One of the solutions to the dark matter spiral galaxy theory's angular momentum catastrophe, is that 80% of the spiral galaxy's disk material should be moved to the spiral halo which would create a massive set of spheroidal dwarf galaxies that would orbit the spiral. This is of course not observed. (i.e. The angular momentum catastrophe problem only adds to the satellite galaxy problem.

“The mass of dwarf spheroidal galaxies and the missing satellite problem” by Read, Wilkinson, Evans, Gilmore, & Kleyna

http://arxiv.org/abs/astro-ph/0505226v1
3. Conclusions
In conclusion, tidal stripping cannot be very strong for many, if not all, of the local group dwarf spheroid galaxies. Strong tidal stripping, which would produce distorted isodensity contours, also leads to velocity gradients and flat or rising projected velocity dispersions - neither of which are observed in the local group dSphs for which we have good kinematic data (but see also Munoz et al. 2005). This suggests that dSph galaxies must be sufficiently massive such that tidal stripping is of little importance for the stars. Either they are on orbits with large pericentres, in which case they can have masses as low as _ 10^8M⊙ (Kleyna et al. 2001); or they are on more extreme orbits in which case they must be _ 10^9 − 10^10M⊙ depending on the extremity of the orbit. Our current cosmological paradigm would favour the latter hypothesis, but this leaves us with a puzzle: if the dwarf spheroid galaxies are really as massive as _ 10^10M⊙ and have dark matter densities which are cosmologically consistent then they would have central velocity dispersions which are too large to be consistent with Draco or UMi - even after significant tidal stripping and shocking.

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## Angular Momentum Catastrophe Solved By Feedback?

That's interesting, because 10 years ago, Dominguez-Tenreiro, et al. published A Way Out of the Disc Angular Momentum Catastrophe in Hierarchical Hydrodynamical Simulations. They show that the angular momentum catastrophe "can be easily avoided by including star-forming processes."
My reply. No feedback appears to not have solved the "angular momentum catastrophe" problem. However, I am a confessed "dark matter" sceptic and only mention it in this thread, as I found the "angular momentum catastrophe" coming up at different recent conferences that I was reviewing looking for some actual useful information.

Better in terms of not getting stuck in a conversation of yes feedback can allow a large spiral disc to form rather than a massive halo to my no, would be to add the facts and observations concerning evolution, age, and structural details of the Milky Way's inner and outer disk and halo. I will start a thread in the astronomy section when I have more material and something to hopefully contribute.

I has surprise to find a number of recent presentations concerning basic fundamental issues which appear to point in a new direction.

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## 2006 Angular Momentum Catastrophe Still Not Resolved?

Cougar, this seminar is recent and indicates that the "angular momentum catastrophe" is not resolved. I believe the solution presented at this seminar in 2006 is different than the feed back paper you presented in 1998.

Angular Momentum Problems in Disk Formation

http://www.mpia.de/homes/vdbosch/disks2.pdf

21. You know, William, I can't understand how you can learn so many incorrect things by (apparently) reading so many papers. You seem to always come away with the opposite of what a given paper says!

1. Yes, it is certainly possible for a major merger between two spiral galaxies to produce another spiral, if the impact parameters and angular momentum are correct. And certainly a minor merger between a spiral and something else will likely remain a spiral. Tell me, William, what galaxies constitute the main component of clusters? And what galaxies are most common in underdensities and voids? And why would there be such a difference?

2. The "angular momentum catastrophe" is really just a limitation due to the resolution of our models. We can't model small enough particles for a long enough time while including all the relevant physics. And you know what, that's exactly what all those papers and talks say!

Originally Posted by Couchman, 29 May 2003, 9th slide
Suppose that we wanted to achieve resolution independence by simulating across the “cooling gap”

what is N?
N ~ 106 ×100 × 10 = 109 !
But if we include the relevant feedback by hand, the results look about right. If you don't include all the relevant physics, you don't get the right answer. But until recently, we didn't have the computational power to include the correct physics, even if we knew what it was. And, as Cougar pointed out, we did. And the talk you just posted from van den Bosch mentions just that on the 7th slide! And, as always, the things you post directly contradict the point you are trying to make:

Originally Posted by Couchman, 29 May 2003, final slide
Plausible models for disc galaxy formation in models are beginning to emerge (cannot say that CDM is inconsistent with the existence of disc galaxies).
In addition, the papers resulting from the van den Bosch conference proceeding that you cited both contradict your claim that there is "...no solution to the angular momentum catastrophe..." I'm sensing a trend here...

3. As to your misquoting of Hammer et al. (2005), they contradict your claims that we cannot properly simulate the properties of spiral galaxies right in the abstract!! Did you somehow miss this line?

Originally Posted by Hammer et al. (2005), abstract
However, the so-called spiral-rebuilding scenario proposed two years ago by Hammer et al. is consistent with the properties of both distant galaxies and of their descendants, the local spirals.
4. And finally, William? Can you please describe to us your proposed solution to all these so-called "impossible problems" that keep bring up? I've asked for it several times now. You could at least provide a hint...

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Originally Posted by parejkoj
It affects light in the same way it effects normal matter: gravitational interaction. If it didn't affect light, then we'd know something really weird was going on! Actually, if we didn't see a gravitational lensing signal from dark matter, that would be a very strong piece of evidence that it didn't exist.
Hmmm... I apologize for the convolution of that last sentence.
S'OK... I think I didn't misunderstand what you might not have meant... *grins*
Originally Posted by parejkoj
Keep in mind that when we see gravitational lensing of galaxies and quasars through other galaxies and galaxy clusters, we are seeing the integrated effect of all the dark matter along the line of sight. And there's a lot of dark matter in a galaxy cluster. Looking toward a star or cluster of stars in our own galaxy would not produce much of a signal, because of how diffuse the dark matter is.
OK, but unless DM is pread evenly through ALL space, in galaxy or out, then wouldn't there be effects that vary depending on where we look as well as the means by which we look? If the effects balance out in all directions & by all methods of looking, doesn't that imply DM is either evenly spread everywhere or there is some other explanation? If it's evenly spread, how can it influence galaxy parameters or formation - it becomes the equivalent of a universal field doesn't it?
Originally Posted by parejkoj
Whether dark matter is clumpy, smooth, whole 2% or skim is a current topic of research. If you haven't already, look up the MACHO studies. They were looking for large "particles" of dark matter, and didn't find a signal from clumps of dark matter above a certain size.
I have yet to find anything about the size of clumpiness, but so far what I have seen suggests they are looking for 'normal' matter that for various reasons doesn't emit light. My impression is the standard for DM is that it isn't normal ie. baryons & so wouldn't fit the MACHO model.

Originally Posted by parejkoj
There are some versions of MOND that claim to be able to reproduce the Bullet Cluster and other lensing results. But the best ones that I'm aware of either require a mass for the neutrino that is far too large (above the current threshold from particle physics experiments), or can't consistently account for all observations with the same set of parameters.
Is this part of the reason for the LHC? If they find Higgs, could that provide at least a partial solution? Or is that a different problem?

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Originally Posted by William’s quote
Using arguments based on either dynamical friction (Binney & Tremaine 1987) or simple orbital time-scale (e.g.,Bell et al. 2006), this time scale has been estimated to be about 0.35Gyrs.
So, 0.35Gyrs = 350,000,000 years? And the estimated time for one galactic orbit by our solar system is about 250,000,000 years IIRC? So if I am reading this right, a galactic disk forms in the gas condensation process in slightly over 1 revolution of the galaxy? Am I missing something or is that one heck of a powerful influence forming the disk?
Originally Posted by William’s quote
Fall & Efstathiou (1980) set out a model of galaxy formation assuming that disks form from gas cooling and condensing in dark halos. Protogalactic disks are assumed to be made of gas containing substantial amount of angular momentum, which condenses into stars to form thin disks (Larson 1976)
But if the gas is cooling & condensing within the halo of DM, wouldn’t gravitational interaction act to hamper the formation of a spiral galaxy? The ‘local’ attraction of DM would influence the NM to form clumps close by rather than passing it down to a central area. Unless the angular momentum was there first, like maybe a whirlpool of gas with a centre that funnels down through a clump of DM to form the galaxy?

24. Originally Posted by Acolyte
This sounds a little 'special case' in that I'm curious as to why the horizontal motions don't also cancel out.
eg. When the 'cloud' of Normal Matter (NM) is collapsing to form the proto-galaxy, why would only the vertical motions be affected?
If the net angular momentum of NM in a galaxy was zero, then, if it was able to collide with itself efficiently -- that is, if it was gas, rather than stars -- then it would indeed collapse into a little ball. The "vertical" motions would all cancel out, as would all the "horizontal" motions.

However, if the net angular momentum of NM is not zero, then by definition, there will be some axis around which there is more momentum orbiting in one direction than in the other. I using the term "horizontal" to mean "in the plane perpendicular to this axis", and "vertical" to mean "parallel to this axis." When gas particles collide in this situation, by definition, there will be an equal number going "up" vertically and "down" vertically, so the result of collisions will be to cancel all "vertical" motions. By definition, there are more particles going one way -- say, counterclockwise -- than the other way in the "horizontal" direction. So, after many collisions, all the "clockwise" motions are cancelled, and we are left with some residual "counterclockwise" motion.

Hence, a disk.

Does that imply NM has EM effects even if it isn't charged?
Yes. When a physicist writes "electromagnetic forces", he often means "forces between the electron shells surrounding atoms which come close to each other." As in, for example, the interaction between your feet and the floor. Your feet don't fall through the floor because of the EM interactions between the electrons in your feet and the electrons in the floor -- even though both floor and feet are neutral.

If the DM can be affected by gravity, but is not otherwise constrained, & the NM is constrained into the disk & bulge, that would mean a concentration of attraction in the disk & bulge - surely that should affect the DM over the eons?

25. Originally Posted by Acolyte
This sounds a little 'special case' in that I'm curious as to why the horizontal motions don't also cancel out.
eg. When the 'cloud' of Normal Matter (NM) is collapsing to form the proto-galaxy, why would only the vertical motions be affected? I seem to have a failure of visualisation here.
To me, if the DM is affecting the formation of the galaxy gravitationally it should be attracting the cloud all through the sphere of DM.

Does that imply NM has EM effects even if it isn't charged? Or are you saying that the events occuring to atoms that strip an electron & charge it are common enough to have this effect on NM? (not quite sure if this question conveys what I'm trying to get at. Have to think more about it I guess)
If the DM can be affected by gravity, but is not otherwise constrained, & the NM is constrained into the disk & bulge, that would mean a concentration of attraction in the disk & bulge - surely that should affect the DM over the eons?
Is there some EM effect even with NM if it's attracted by a gravity source that changes it's motion? ie. the Earth radiates EM in some manner because it is being constrained by the Sun's gravity?
Ah, I was wondering, if it's also present within the area of NM how that would affect our observations of the galaxy itself & also how it would affect our view of the surrounding universe. After all, light is affected by DM so wouldn't there be a variety of effects depending on where we look?
Man, EM effects are EVERYTHING that you know as "touchable matter".In fact, atoms are mostly empty space and never collide at ordinary conditions, when you touch something hard ,the EM forces of the matter repulse the EM forces of your finger/whatever you touch with.
When you cut something you overcome the EM forces holding the matter together, not cut the actual atoms.

26. Originally Posted by Acolyte
OK, but unless DM is pread evenly through ALL space, in galaxy or out, then wouldn't there be effects that vary depending on where we look as well as the means by which we look? If the effects balance out in all directions & by all methods of looking, doesn't that imply DM is either evenly spread everywhere or there is some other explanation? If it's evenly spread, how can it influence galaxy parameters or formation - it becomes the equivalent of a universal field doesn't it?
Hmmm... I think I understand what you're getting at, but I think I already answered it. The effect of the small amount of dark matter on our line of sight to a star in the milkyway is so small (because there isn't much in that small space) that it wouldn't produce an appreciable lensing signal.

Again, the gravitational lensing that we observe from galaxy clusters is due to all the dark matter in and between the galaxies in the cluster. That's a lot more than exists between stars in our own galaxy.

And it isn't completely evenly spread: there is more in the center of the galaxy, and in the disk, because of the attraction of the baryons. Look up Baryon Acoustic Oscillations for one way that the dark matter and the baryons imprint signals on each other in the early universe.

Originally Posted by Acolyte
I have yet to find anything about the size of clumpiness, but so far what I have seen suggests they are looking for 'normal' matter that for various reasons doesn't emit light. My impression is the standard for DM is that it isn't normal ie. baryons & so wouldn't fit the MACHO model.
The MACHO project was looking for any "clumps" of material that were large enough to cause a lensing signal as they passed infront of stars. Could have been baryonic, dark matter, small black holes or something else. The MACHO searches started before we knew as much about cosmology, so there was still a hope that we'd find the missing mass in such "dark stars." But based on all the recent observations (I listed many above), the bulk of the matter density of the universe must be collisionless -- does not interact via E&M.

Most of the MACHO signals were from baryonic matter that doesn't emit much light: brown dwarfs, red dwarfs (Kryten!), isolated neutron stars, small black holes, etc. But there could have been a signal from dark matter, if it is significantly clumpy.

Originally Posted by Acolyte
Is this part of the reason for the LHC? If they find Higgs, could that provide at least a partial solution? Or is that a different problem?
Partial solution to what? The Higgs is a prediction of the standard model of particle physics. But we measure the mass of the neutrino though other experiments -- the wikipedia page lists some of them.

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## Dark Matter is it Real?

The "angular momentum catastrophe" is really just a limitation due to the resolution of our models. We can't model small enough particles for a long enough time while including all the relevant physics. And you know what, that's exactly what all those papers and talks say!
Please provide a quote from the papers to back up your statement. You did say all papers.

Is the feedback mechanism required or not required? No issue with number of dwarf satellite galaxies?

I will start a thread in the astronomy section concerning the Milky Way observations.

Why is there a difference in the age of stars in the Milky Way inner disk and outer disk? Why do the spiral Halo's have net zero angular momentum?

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Originally Posted by Acolyte
3. Also, why doesn't the gravitational effect seem to work both ways?
If DM is evenly distributed in a shell or sphere, & we have normal matter in a concentrated disk, & the DM is not being kept in place by some sort of motion effect (orbit or repulsion etc.) then over time, the DM should be attracted down to the disk or into the central bulge. (if the bulge has significantly more mass than the disk)

4. For that matter, is it the case that the DM exists outside the disk & central sphere but is not distributed within the normal matter realm? If so, how can that be?
Perhaps the DM isn't bound by gravity, but rather, by EM itself? In other words, the DM is affected by light, but not by gravity, and normal matter is affected by gravity, but not by light.

I don't know if that's even possible, but it's the only rational explanation I can think of that fits what we do know.

And, it might just be the link between gravity and EM, or GR and QM.

5. Wouldn't that require either EM interaction or physical interaction between normal matter & energy & DM?
I'm a bit puzzled by the explanations I've seen. It seems the DM shell is not required to rotate (or all velocities sum to zero) & it can't interact in any way that would provide a normal effect to the paths of normal matter. Yet it apparently can act upon normal matter in such fashion to produce the observations we have of the visible universe.[/QUOTE]

That's what I'm thinking.

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PS:

Acolyte, your/our theories sounds like a candidate for the ATM thread. Since you posted it first, I'll let you do the honors.

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Originally Posted by mugaliens
PS:

Acolyte, your/our theories sounds like a candidate for the ATM thread. Since you posted it first, I'll let you do the honors.
*grins* As yet I don't have a theory, just lotsa questions. In trying to understand what they are talking about with DM I find little things that don't seem to settle out so I ask.

But I agree there seems to be a discontinuity regarding the effects of DM, which was why I skirted around one-way interactions. It's as if the model seems to require interaction to produce an effect but the 'normal' back reaction doesn't happen. eg. DM is supposedly not affected normally by EM forces so can't reaxt that way to the gravity of NM, but I don't see why it couldn't release the results of interaction by changing location.

It may be I have a failing of understanding of normal physics here & that with NM, when one particle grvitically attracts another the change in energy (momentum) is balanced by an EM effect & that is what can't happen with DM & so it doesn't change momentum.

Or it may be I'm not making sense at all *grins*

But given I'd have to defend & not just ask questions, I'd rather have the ability to do so before posting a half-baked theory in ATM.

I can't quite grasp it yet but from the responses (which are very much appreciated BTW) I seem to be skirting around something that will hopefully resolve in my head soon.

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