# Thread: Galactic Rotation... no need for dark matter.

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Originally Posted by Tensor
The mean free path for the particles is ~1000 parsecs, compared to the size of the entire system which is on the order of ~1,000,000.
By baryonic particle, I assume you mean atoms or hydrogen molecules which are diffusely distributed. We do see the IGM gas component colliding. What if there were additional clumps of matter particles each consisting of 1,000,000 molecules or more? In that case, the chances of collision would be much smaller (for particles containing 1M-molecules , about 100 times smaller). If I'm not mistaken the probability of even galaxies colliding is not large in a cluster collision.

2. Originally Posted by TooMany
By baryonic particle, I assume you mean atoms or hydrogen molecules which are diffusely distributed.
Baryons comprise most normal matter (you, me, stars, atoms). Dark matter is not made of baryons. A quick web search could give you the definition.

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Originally Posted by TooMany
By baryonic particle, I assume you mean atoms or hydrogen molecules which are diffusely distributed. We do see the IGM gas component colliding.
That was the point. The baryonic matter is colliding, but lensing shows most of the mass in a different part of system. The collision is producing x-rays, in the area where the gas is observed. But, what is producing the lensing found in the B

Originally Posted by TooMany
What if there were additional clumps of matter particles each consisting of 1,000,000 molecules or more?
I should have given the dimension of the system, ~1,000,000 parsecs. The free path for particles is 1000 parsecs, compared to the size of the entire system, which is on the order of ~1,000,000 parsecs. A particle travels ~1,000 parsecs, on average, before colliding with another particle. If you average the chance of collision, over the entire system, it would almost certainly collide with something, separating the baryonic matter from dark matter. The difference here is that with the length of the free path and the size of the system, you can treat the ICM as a collisional fluid.

Originally Posted by TooMany
In that case, the chances of collision would be much smaller (for particles containing 1M-molecules , about 100 times smaller).
First off, it doesn't quite work that way. Colliisons are dependent on thermal energy and (or proton or other particles) temperature. There's also the matter of the interaction radius changing with the temperature change. However, with additional clumps containing molecules the probability would increase, not decrease, the chances of collision, because one of the parameters that the mean free path depends on is the density of the particles. Additional clumps would INCREASE the density and this would lead to a higher collisional probabilities, which leads to a greater separation of dark and baryonic matter.

Originally Posted by TooMany
If I'm not mistaken the probability of even galaxies colliding is not large in a cluster collision.
Yeah, and in galactic collisions, the probability of stars colliding isn't that large either, but as in cluster collisions, the gas still collides.

My objection was to your statement in Post #59:

Originally Posted by TooMany
The point is that it's hard to explain this tight correlation if baryonic and non-baryonic matter are easily separated (as claimed in the Bullet Cluster).
You implied that it was easy to separate the two kinds of matter. I was just pointing out that it is not. There has to be specific conditions to produce the separation that is observed in the Bullet Cluster. I provided the conditions required to produce the observed separation and what we do see matches those conditions. I specifically pointed out that a smaller system size or different collisional speeds(or several other things that would prevent the ICM from being treated as collisional) would prevent the two from separating from each other. Which is why I asked about other models that would match observations.

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Originally Posted by Tensor
However, with additional clumps containing molecules the probability would increase, not decrease, the chances of collision, because one of the parameters that the mean free path depends on is the density of the particles. Additional clumps would INCREASE the density and this would lead to a higher collisional probabilities, which leads to a greater separation of dark and baryonic matter.
I think you are mistaken about the effect of clumping on collisions. The formula for mean free path is:

p = (dx)^-1

where:

p is the mean free path
d is the particle density in number of particles per unit volume
x is the effective cross section of the particles

Consider a fixed amount of matter distributed as particles with size (diameter) l in a unit volume.

The mass of a (~spherical) particle proportional to it's volume, (4/3)pi*l^3. Therefore the particle density d is proportional to l^-3. The effective cross section is proportional to l^2, so:

p is proportional to (l^-3*l*2)^-1 = l

Thus the mean free path of a given mass of particles in a given volume increases directly with particle size.

The size of a hydrogen molecule is about 10^-10m. The size of a very tiny dust particle is 1 micron or 10^-6m. So the mean free path of the 1 micron particles is 10,000 times longer than the hydrogen molecule.

Moreover, at a given equilibrium temperature, the thermal velocity of a particle is proportional to 1/sqrt(pm) where pm is the particle mass. The ratio of the mass of the 1 micron particle to the single molecule is (10^-6)^3/(10^-10)^3 which is 10^12, so the ratio of thermal velocity is 10^-6. Hence collisions due to thermal motion are 10^-6/10,000 or 10 billion times fewer for the dust particles than the gas molecules.

Relative to gas, 1 micron particles are non-colliding.

Originally Posted by Tensor
Yeah, and in galactic collisions, the probability of stars colliding isn't that large either, but as in cluster collisions, the gas still collides.
And above is the reason that gas collides but stars don't when galaxies collide even though the masses of the components are comparable.

Originally Posted by Tensor
You implied that it was easy to separate the two kinds of matter. I was just pointing out that it is not. There has to be specific conditions to produce the separation that is observed in the Bullet Cluster.
Wouldn't we also expect separation in galaxy collisions? I wonder what conclusions have been drawn from studying those.

Also there are claims that some dwarf galaxy are 99% non-baryonic dark matter. How does that happen?

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Originally Posted by TooMany
I think you are mistaken about the effect of clumping on collisions. The formula for mean free path is:

p = (dx)^-1

where:

p is the mean free path
d is the particle density in number of particles per unit volume
x is the effective cross section of the particles
d is equal to number of particles in a given volume, it has nothing to do with the mass of each of the particles. As you add more particles to the given volume, the denominator on the right side get bigger, meaning the value of the right side gets smaller. Since the right side gives the distance, if the right side gets smaller, the mean free path gets smaller.

Originally Posted by TooMany
Consider a fixed amount of matter

snip...

micron particles are non-colliding.
I believe you have an error somewhere in there. I think it has to do with considering the mass of the particles within the volume of the particle. The density in the mean free path equations have to do with the number of particles in a given volume of space.

take the following mean free path equation, which accounts for the volume and number of particles:

Where V is the volume, d is the diameter of the particles and n is the number of particles. Since Pi, 2 and d are all constants, and in the case of the cluster collision, V is also (for a static amount of time to check the effect of increasing or decreasing the number of particles) a constant. That leaves the number of particles, or n as the only variable. If the number of particles increases, the value of the denominator increases, which means the value of the right side of the equation DECREASES. Which means the length of the mean free path will also decrease or the probability of collision increases, they mean the same thing. As I said, if you increase the number of particles, there would be more collisions and the baryonic matter and non-collisional dark matter would separate even further than they have.

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Originally Posted by Tensor
d is equal to number of particles in a given volume, it has nothing to do with the mass of each of the particles.
Where did I say it did?

Originally Posted by Tensor
As you add more particles to the given volume, the denominator on the right side get bigger, meaning the value of the right side gets smaller. Since the right side gives the distance, if the right side gets smaller, the mean free path gets smaller.
True, but I think you are missing the point. The point is that for a given total mass, the mean free path gets longer in direct proportion to the size of the particles. If the particles are sufficiently large, their collisions with gas molecules are relatively harmless. So I'm not talking about those collisions. The collisions of importance with large particles (say > 10^12 molecules) are collisions with other large particles and those are much rarer (>10,000 times in my example). When clusters collide stars do collide with the gas, but not with each other.

To explain a bit further my point is that there can be lots of additional non-colliding baryonic matter in clusters as long as it is in a sufficiently condensed form. Moreover the particles don't need to be very large in order to be non-colliding and to add substantial mass.

Originally Posted by Tensor
I believe you have an error somewhere in there.
Please explain the error in what I stated.

Originally Posted by Tensor
take the following mean free path equation, which accounts for the volume and number of particles:

Where V is the volume, d is the diameter of the particles and n is the number of particles. Since Pi, 2 and d are all constants, and in the case of the cluster collision, V is also (for a static amount of time to check the effect of increasing or decreasing the number of particles) a constant. That leaves the number of particles, or n as the only variable. If the number of particles increases, the value of the denominator increases, which means the value of the right side of the equation DECREASES. Which means the length of the mean free path will also decrease or the probability of collision increases, they mean the same thing. As I said, if you increase the number of particles, there would be more collisions and the baryonic matter and non-collisional dark matter would separate even further than they have.
I don't disagree that more particles mean more collisions. The point is that larger particles mean fewer collisions for a given total mass. Thus a large amount of additional mass (say the expected mass of the dark matter) can be in form of large particles and thereby also be essentially collision-less, just like stars and the proposed non-baryonic matter. Large particles may have some inconsequential collisions with gas molecules but they will also have very few collisions with one another.

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Originally Posted by TooMany
Where did I say it did?
Well, you're trying to convert larger dust grains into baryonic particles. I would agree if you were talking about baryonic matter vs dark matter. But this discussion was about particles with a mean free path, in a collisional fluid. I gave the mean free path of ~1000 parsecs, which is consistent with particles, such as protons, electrons, even atoms, but not something large enough to be considered a dust grain.

Originally Posted by TooMany
If the particles are sufficiently large, their collisions with gas molecules are relatively harmless.
While that may be true for the large dust grains, the small particles would heat up and radiate and it's possible for the small particle to break up the dust grain or gas. If charged, you would have bremsstrahlung from the small particle. It's not seen, outside of the x-ray emission.

Originally Posted by TooMany
So I'm not talking about those collisions. The collisions of importance with large particles (say > 10^12 molecules) are collisions with other large particles and those are much rarer (>10,000 times in my example).
No, small particles, electrons, protons, and gas particles (think H1 or H3, due to their charge), are important, due to the effects on the smaller particles. Ions or plasmas would have bremsstrahlung. That would be noticed, it isn't. With the possibility of breaking up the large grains and gas.

Originally Posted by TooMany
When clusters collide stars do collide with the gas, but not with each other.
Can you show that electrons, protons, and smaller gas particles would produce no effects on your proposed dust or gas? Moving at the speed they are moving at, your dust or gas would either break up, or the smaller particles would produce bremsstrahlung, from the stopping (or slowing down) of the charged particles.

Originally Posted by TooMany
To explain a bit further my point is that there can be lots of additional non-colliding baryonic matter in clusters as long as it is in a sufficiently condensed form. Moreover the particles don't need to be very large in order to be non-colliding and to add substantial mass.
You're still missing it. The ICM in the Bullet Cluster is collisional. Small particles will collide within the system of clusters. Electron, and protons have a mean free path that is ~1000 parsecs. Compare that to the system size of ~1,000,000 parsecs and it's probable that the small particles will collide. If the small particles will collide, the small particles will collide with the larger groups of molecules you have proposed.
How large do the clumps have to be to show no effects from the electrons, protons, and dust moving at collision speeds of ~ 1% the speed of light? You haven't specified.
The dust collisions themselves can produce x-ray emissions from radiative friction. But you haven't mentioned this. Can you show the dust grains are small enough, and rare enough that the ICM in the Bullet Cluster can be considered collisionless?
Are the size of the clumps graded or are there sharp cutoffs for the clumps? What is the composition of the clumps?

Originally Posted by TooMany
Please explain the error in what I stated.
Particles do not contain a million molecules. Particles or baryons are protons, electrons(even though they are technically Leptons), I could even be convinced that a particle could be an atom. But not a million molecules or something 1 micron across. You are talking about dust grains or gas, and there would be extinction problems considering the amount of dark matter needed.

Originally Posted by TooMany
I don't disagree that more particles mean more collisions. The point is that larger particles mean fewer collisions for a given total mass. Thus a large amount of additional mass (say the expected mass of the dark matter) can be in form of large particles and thereby also be essentially collision-less, just like stars and the proposed non-baryonic matter.
It could, if you can show that there wouldn't be any kind of radiation (either thermal or bremsstrahlung) or extinction from those large particles.

Originally Posted by TooMany
Large particles may have some inconsequential collisions with gas molecules but they will also have very few collisions with one another.
Those inconsequential collisions would be visible due to radiation. But, you have a different problem. Why are there so many baryonic collisions that the collisions are producing x-ray emission, but, there aren't enough baryonic collisions to prevent two different mass centers from forming? The two conditions contradict each other, unless you can show the dust or bas you propose is entirely large enough and can't be affected at all by collisions with other particles. Unless you have specific calculations, showing specific sizes of the clumps, outlining why observations don't see those clumps(either though radiation or extinction), and showing that the ICM is collisionless at the size of those clumps, and yet can produce x-ray emissions, you have a problem.

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Tensor and others the MOND people certainly do not think the Bullet Cluster is any serious threat to their theory:

QUOTE:
The accepted picture has it that about 4 percent of the mass in the universe is in the form of baryons (standard matter). Of this we have so far seen (detected) only a tenth. So we know anyway that there is still much standard matter in the universe to be detected (beside the putative DM). What MOND still requires in clusters is a small fraction of that, so really it's no big deal.

From

http://www.astro.umd.edu/~ssm/mond/moti_bullet.html

Also have a look at Stacy McGaugh's web site and click on his MOND pages link. Scroll down at the end of that to find material addressing the Bullet Cluster ("Stacey" is a girl's name in the UK, so I was a bit taken aback wiht the photo on this site when it loaded up at first

Stacy McGaugh at University of Maryland:

http://www.astro.umd.edu/~ssm/

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Originally Posted by Tensor
You're still missing it. The ICM in the Bullet Cluster is collisional. Small particles will collide within the system of clusters. Electron, and protons have a mean free path that is ~1000 parsecs. Compare that to the system size of ~1,000,000 parsecs and it's probable that the small particles will collide. If the small particles will collide, the small particles will collide with the larger groups of molecules you have proposed.
No I'm not missing that at all. What I'm trying to show you is that ordinary baryonic could exist in large quantities as chunks and be largely (but not perfectly) non-collisional.

Originally Posted by Tensor
If the small particles will collide, the small particles will collide with the larger groups of molecules you have proposed.
Yes they would collide with gas molecules but even a 1 micron bit of condensed matter has about a trillion molecules in it.

Originally Posted by Tensor
How large do the clumps have to be to show no effects from the electrons, protons, and dust moving at collision speeds of ~ 1% the speed of light? You haven't specified.
A clump 1cm in diameter has 100 million-trillion molecules in it. How much damage do you think a collision with a proton is going to do?

Originally Posted by Tensor
The dust collisions themselves can produce x-ray emissions from radiative friction.
The collisions are too rare to produce detectable emissions.

Originally Posted by Tensor
But you haven't mentioned this. Can you show the dust grains are small enough, and rare enough that the ICM in the Bullet Cluster can be considered collisionless?
I thought I already showed you that. It's not how small, it's how big they are that allows them to be collisionless and yet account for a large baryonic mass.

Originally Posted by Tensor
Are the size of the clumps graded or are there sharp cutoffs for the clumps?
I have no idea.

Originally Posted by Tensor
What is the composition of the clumps?
Well, it would have to be mostly hydrogen perhaps with some small quantities of metal.

Originally Posted by Tensor
You are talking about dust grains or gas, and there would be extinction problems considering the amount of dark matter needed.
I just used dust grains to demonstrate even if matter is condensed into small chunks it becomes collisionless. No extinction if the chunks are large enough.

Originally Posted by Tensor
It could, if you can show that there wouldn't be any kind of radiation (either thermal or bremsstrahlung) or extinction from those large particles.

Those inconsequential collisions would be visible due to radiation. But, you have a different problem. Why are there so many baryonic collisions that the collisions are producing x-ray emission, but, there aren't enough baryonic collisions to prevent two different mass centers from forming? The two conditions contradict each other, unless you can show the dust or bas you propose is entirely large enough and can't be affected at all by collisions with other particles. Unless you have specific calculations, showing specific sizes of the clumps, outlining why observations don't see those clumps(either though radiation or extinction), and showing that the ICM is collisionless at the size of those clumps, and yet can produce x-ray emissions, you have a problem.
Do you accept the existence of the Oort cloud? It is thought to contain about 1 trillion bodies larger than 1 km. Most likely it contains a range of sizes with smaller bodies being more common. And as yet we cannot directly detect any matter in that cloud except when a chunk is deflected and enters the inner system as a comet. Those chunks of matter are at least 4 billion years old and are doing just fine. They are surviving within the ISM so I'm sure they could survive in the IGM.

How do clumps form? Gravitational condensation of gas and a bit of dust. However, once formed collisions are rare.

In a cluster collision, enough chunks of condensed matter could exist to account for all the mass and yet that matter would be essentially collisionless. I cannot prove that this matter exits, I'm just saying that it's a possible explanation that does not require non-baryonic matter.

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Originally Posted by kzb
QUOTE:
The accepted picture has it that about 4 percent of the mass in the universe is in the form of baryons (standard matter). Of this we have so far seen (detected) only a tenth. So we know anyway that there is still much standard matter in the universe to be detected (beside the putative DM). What MOND still requires in clusters is a small fraction of that, so really it's no big deal.
Finding that "missing baryonic matter" is just as important to LCDM as not finding too much. The LCDM theory has to propose that a substantial portion of the expected baryonic matter is simply undetected.

What form does this matter take? Where is it? How much is there?

These are open questions that may be very difficult to answer through observations.

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What form does this matter take? Where is it? How much is there?

There are hints in the literature that things are being found. The following paper found lots of structured gas clouds around the MW.

Absorption and emission line studies of gas in the Milky Way halo

N. Ben Bekhti et al.

QUOTE:
The VLA
resolves the HVC into several compact, cold clumps. ...[EDIT]...In all four observed
directions we found small-scale structure embedded in the more diffuse component of
the absorbing cloud ....[EDIT]... Furthermore, it is remarkable that
the smallest spatial structures observed along the four sightlines are of similar size as
the synthesised WSRT and VLA beams. Therefore, it is possible that the clumps are
still not resolved and contain structures on even smaller scales.

http://arxiv.org/abs/1007.3366

This is even more interesting (will re-find paper later, but it's the same people):

EBHIS/GASS data resolve the
previously known ”large” clouds into tiny objects
[EDIT]
even unresolved within the new survey data while no extended
diffuse emission is detected [EDIT] Will the observed fragmentation
continue when going to even higher resolution and if so,
what are the properties of this scaling?

12. Originally Posted by kzb
...the MOND people certainly do not think the Bullet Cluster is any serious threat to their theory:

QUOTE:
The accepted picture has it that about 4 percent of the mass in the universe is in the form of baryons (standard matter). Of this we have so far seen (detected) only a tenth. So we know anyway that there is still much standard matter in the universe to be detected (beside the putative DM). What MOND still requires in clusters is a small fraction of that, so really it's no big deal.

http://www.astro.umd.edu/~ssm/mond/moti_bullet.html
You mean one MOND person.

"No big deal"?... That doesn't sound very quantitative. And that link goes to a page with a whole lot of words, but finally McGaugh explains his Bullet Cluster argument in a single sentence:

"When two clusters collide head on the gas components of the two just stick together and stay in the middle, while the rest (galaxies plus this extra component I spoke of) just go through and stay together."

Wave to McGaugh. I think he's handwaving to us.

BTW, the "extra component" he mentions is.... more baryons. He simply claims there are 10 times more baryons than we currently detect. As if dozens of teams of astrophysicists have not gone out and looked for all those missing baryons over the last several decades. But they have found only a few percent of what is needed.

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Originally Posted by kzb
This is even more interesting (will re-find paper later, but it's the same people):

EBHIS/GASS data resolve the
previously known ”large” clouds into tiny objects
[EDIT]
even unresolved within the new survey data while no extended
diffuse emission is detected [EDIT] Will the observed fragmentation
continue when going to even higher resolution and if so,
what are the properties of this scaling?
I'm interested. Thanks.

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Originally Posted by Cougar
BTW, the "extra component" he mentions is.... more baryons. He simply claims there are 10 times more baryons than we currently detect. As if dozens of teams of astrophysicists have not gone out and looked for all those missing baryons over the last several decades. But they have found only a few percent of what is needed.
No, what he is saying is the current LCDM theory requires that there are more baryons than detected in order to reach the 4% requirement. Are you suggesting that LCDM is wrong because astrophysicists have "looked" but cannot find the required missing baryons?

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The amount of extra baryonic matter for MOND to make a success of the Bullet Cluster is only about a factor of 2, not 10. It says that on his other pages.

Baryonic matter could just about exist in forms not detected by current methods. As Nereid has said, 10^3km planetoids. It can't be snowflake sized though, because that would be opaque. Also the planetoids could only exist in the requisite number in the outer regions of the galaxy, because the mass budget around here does not allow for them.

when you have eliminated the impossible, whatever remains, however improbable, must be the truth

16. Originally Posted by kzb
... As Nereid has said, 10^3km planetoids. ...
Of course, to account for the third peak in the CMB, these planetoids would have to have existed before the final CMB signal. It is difficult to imagine how balls of molecular Hydrogen ice could have formed when the universe' ambient temperature was thousands of degrees.

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Originally Posted by TooMany
I'm interested. Thanks.
The quote came from this paper:

On the origin of gaseous galaxy halos - Low-column density gas in the Milky Way halo

http://arxiv.org/abs/1102.5205

What's missing though is an estimate of the total mass involved. It'd probably be possible to integrate their column densities for the gas over the whole sky. But then again, if the resolution is still not good enough to resolve the densest objects that could be an underestimate.

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Originally Posted by antoniseb
Of course, to account for the third peak in the CMB, these planetoids would have to have existed before the final CMB signal. It is difficult to imagine how balls of molecular Hydrogen ice could have formed when the universe' ambient temperature was thousands of degrees.
Maybe they didn't have to be frozen at that point, just in the process of fragmentation and collapse. In any case, the missing baryon problem is a problem for LCDM, so any baryons I can find is a favour to anyone at this point in the argument.

19. Originally Posted by TooMany
No, what he is saying is the current LCDM theory requires that there are more baryons than detected in order to reach the 4% requirement.
Where does he say that? I don't believe he does. What he does say is he takes the failure of MOND to match observations in galaxy clusters and claims it would match if the cluster had twice as much mass, then he disingenuously compares that to dark matter, which has to account for 10 times the mass of the visible matter. Well, gee, the dark matter scenario does not inject an arbitrary, unjustified, and illogical adjustment to gravity as we know it, such that when the effect of gravity falls below a certain arbitrary threshold, it suddenly gains more force and prevents stellar orbits from escaping their galaxy. Hoh-kay.

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Originally Posted by Cougar
Where does he say that? I don't believe he does. What he does say is he takes the failure of MOND to match observations in galaxy clusters and claims it would match if the cluster had twice as much mass, then he disingenuously compares that to dark matter, which has to account for 10 times the mass of the visible matter. Well, gee, the dark matter scenario does not inject an arbitrary, unjustified, and illogical adjustment to gravity as we know it, such that when the effect of gravity falls below a certain arbitrary threshold, it suddenly gains more force and prevents stellar orbits from escaping their galaxy. Hoh-kay.
Hard to say whether that's more or less concocted than CDM. MOND has been made into a relativistic theory in recent years. I don't like it much either but I feel the same about CDM.

21. kzb
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Cougar wrote:
Well, gee, the dark matter scenario does not inject an arbitrary, unjustified, and illogical adjustment to gravity as we know it

Neither does the exponent in the Tully-Fisher (now Baryonic Tully Fisher) relationship fall out of LCDM naturally, like it does from MOND.

The Baryonic Tully-Fisher Relation of Gas Rich Galaxies as a Test of LCDM and MOND

http://arxiv.org/abs/1107.2934

My biggest problem with LCDM is this BTFR. It just does not seem right that it could be such a tight relationship if 80% of the mass is non-baryonic, and that it can be separated from baryonic matter over cosmic history.

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Originally Posted by kzb
My biggest problem with LCDM is this BTFR. It just does not seem right that it could be such a tight relationship if 80% of the mass is non-baryonic, and that it can be separated from baryonic matter over cosmic history.
Moreover there are galaxies thought to contain far more DM than 80% and they still obey the relationship. Not proof in itself but it implies that DM tends to behave like baryonic matter. Hence WDM or somewhat-interacting DM are proposed. If DM is actually BDM, some problems go away including the need for a new stable particle.

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Originally Posted by kzb
My biggest problem with LCDM is this BTFR. It just does not seem right that it could be such a tight relationship if 80% of the mass is non-baryonic, and that it can be separated from baryonic matter over cosmic history.
A small point: DM is not "separated from baryonic matter over cosmic history". DM and baryonic matter interact via gravity. Thus they tend to be together. DM though tends to spread out because it interacts weakly via EM.

Personally I am not that surprised by the BTFR result. MOND works really well for most types of galaxies - even better than DM. The cited paper shows a good fit to a certain class of gas-rich galaxies.
It is at higher scales that MOND tends to fail, see the blog post Dark Matter: Just Fine, Thanks by Sean Carroll who lists that MOND currently fails for
* galactic clusters.
* dwarf spheroidals.
* explaining the direction of gravity in galaxy cluster collisions ("The evidence from gravitational lensing is absolutely unambiguous: to fit the data, you need to do better than just modifying the strength of Newtonian gravity.")
* the cosmic microwave background.

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Originally Posted by TooMany
Not proof in itself but it implies that DM tends to behave like baryonic matter. Hence WDM or somewhat-interacting DM are proposed. If DM is actually BDM, some problems go away including the need for a new stable particle.
As I noted above, DM tends to behave like baryonic matter gravitationally. The observationally different behaviour comes when there are EM interactions that separate the two.
The problem with proposing that DM is actually BDM is that you still need DM to explain collisions such as the Bullet Cluster which MOND cannot explain.
You also have to explain why you still need a lot of non-baryonic matter to explain the CMB data using standard models. No existing MOND theory can explain that.
The Train Wreck cluster (Abell 520) presents problems for both DM and MOND theories. There is a possible DM explanation: Cluster Abell 520: a perspective based on member galaxies. A cluster forming at the crossing of three filaments?

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Originally Posted by Reality Check
The problem with proposing that DM is actually BDM is that you still need DM to explain collisions such as the Bullet Cluster which MOND cannot explain.
The Bullet cluster implies that the unseen mass is not in form of gas, correct? It seems to be associated with the galaxies. Is there something beyond that we need to conclude from the Bullet Cluster?

Originally Posted by Reality Check
The Train Wreck cluster (Abell 520) presents problems for both DM and MOND theories. There is a possible DM explanation: Cluster Abell 520: a perspective based on member galaxies. A cluster forming at the crossing of three filaments?
There's a lot of hard work evident in that Abell 520 paper but the conclusion is rather speculative. No doubt it's very complex to analyze this one.

26. Originally Posted by TooMany
The Bullet cluster implies that the unseen mass is not in form of gas, correct? It seems to be associated with the galaxies. Is there something beyond that we need to conclude from the Bullet Cluster?
Yes. The unseen mass fits the signature of dark matter - weakly interacting massive particles that interact so little, they don't even interact with themselves. The unseen mass as dark matter also fits into all the other independent observations of galaxy and cluster dynamics where the detectable mass is not nearly enough to maintain the structure, which is nevertheless relatively stable.

The scientific community is forced to concude the existence of some exotic dark matter since regular baryonic matter has essentially been ruled out by numerous independent observational methods.

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Originally Posted by Cougar
The scientific community is forced to concude the existence of some exotic dark matter since regular baryonic matter has essentially been ruled out by numerous independent observational methods.
I cannot subscribe to that conclusion because we simply do not know much about the distribution of baryonic matter beyond the gas, the stars and the dust. Between brown dwarfs and gas molecules there is a ranges of sizes of baryonic matter clumps that would simply go undetected. The range of sizes is over many orders of magnitude. They are not ruled out.

28. Originally Posted by TooMany
I cannot subscribe to that conclusion because we simply do not know much about the distribution of baryonic matter beyond the gas, the stars and the dust. Between brown dwarfs and gas molecules there is a ranges of sizes of baryonic matter clumps that would simply go undetected. The range of sizes is over many orders of magnitude. They are not ruled out.
Can you walk us through the methods used by researchers, in appropriate mathematical and technical detail?

Can you show us, again in appropriate detail, where you think they may be missing something?

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Originally Posted by Hornblower
Can you walk us through the methods used by researchers, in appropriate mathematical and technical detail?

Can you show us, again in appropriate detail, where you think they may be missing something?
They cannot detect anything smaller than Jupiter size with current microlensing. Particles much larger than dust are also not directly detectable. It's that simple. Coincidentally, all the things that we cannot yet detect do not exist is your claim, so the burden of proof is yours not mine. I'm just saying we do not really know.

30. Originally Posted by TooMany
They cannot detect anything smaller than Jupiter size with current microlensing. Particles much larger than dust are also not directly detectable. It's that simple. Coincidentally, all the things that we cannot yet detect do not exist is your claim, so the burden of proof is yours not mine. I'm just saying we do not really know.
You really need to go back and do the calculations about the required metallicity of your various not-dust scale things, and compare that with the measured metallicity of the Galaxy. I think you'll find that your proposal creates a whole host of problems.

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