How can dark matter explain the need for modifications to the CDM model?

In summary, dark matter plays a role in the formation of galaxies and clusters, and is essential to the universe.
  • #1
mesogen
25
0
Just a layman's question about dark matter.

Dark matter, like regular matter, is associated with the gravitational force. Dark matter attracts other dark matter as well as regular matter. Regular matter attracts dark matter also.

With that said, wouldn't dark matter be all around us? Wouldn't it be part of the Earth, or at least the solar system? Also, since dark matter doesn't interact with regular matter in any way except through the gravitational force, maybe the center of planets and stars contain large amounts of dark matter? The dark matter wouldn't bond chemically to regular matter and would presumably just float on by, straight to the center of large objects.

I would think this would be the case, especially since there is supposed to be at least 5 times the amount of dark matter in the universe as regular matter.

I'm sure I'm missing something fundamental about the nature of dark matter, so any explanations will be appreciated.

Thanks.
 
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  • #2
mesogen said:
Just a layman's question about dark matter.

Dark matter, like regular matter, is associated with the gravitational force. Dark matter attracts other dark matter as well as regular matter. Regular matter attracts dark matter also.

With that said, wouldn't dark matter be all around us? Wouldn't it be part of the Earth, or at least the solar system? Also, since dark matter doesn't interact with regular matter in any way except through the gravitational force, maybe the center of planets and stars contain large amounts of dark matter? The dark matter wouldn't bond chemically to regular matter and would presumably just float on by, straight to the center of large objects.

I would think this would be the case, especially since there is supposed to be at least 5 times the amount of dark matter in the universe as regular matter.

I'm sure I'm missing something fundamental about the nature of dark matter, so any explanations will be appreciated.

Thanks.

The problem is that just like there is nothing to prevent the dark matter from reaching the center of the Earth, neither is there any thing to cause it to stop there. Ordinary matter tends to clump together to form large bodies because it interacts electromagnetically. So when it collides due to its mutual gravitational attraction it "sticks" together and sheds its excess kinetic energy by radiating it away. Dark matter neither sticks together nor can it shed energy by radiating. As it falls toward the center of the Earth, it picks up kinetic energy, and with no interaction or radiation, it has all that kinetic energy when it reaches the center, and this kinetic energy carries it back out of the Earth. There is no way for it to collect there.
 
  • #3
Really? So because there is no friction of any sort, all dark matter can do is slingshot through a planet or star?

Surely dark matter would tend to clump together with itself gravitationally, but let's say a large clump of dark matter fell into a planet. Would we be able to see any perturbation in the planet's orbit as the dark matter clump interacts with the planet and then slignshots away? It would have to be a pretty large clump of dark matter.

Also, I guess the dark matter would scatter from this interaction since it can't clump together chemically, only gravitationally.
 
  • #4
mesogen said:
Really? So because there is no friction of any sort, all dark matter can do is slingshot through a planet or star?

Surely dark matter would tend to clump together with itself gravitationally, but let's say a large clump of dark matter fell into a planet. Would we be able to see any perturbation in the planet's orbit as the dark matter clump interacts with the planet and then slignshots away? It would have to be a pretty large clump of dark matter.

Also, I guess the dark matter would scatter from this interaction since it can't clump together chemically, only gravitationally.

Dark matter won't tend to clump together for the same reasons that it doesn't collect at the center of planets. There is no "friction" between the particles of dark matter that allows for shedding of the kinetic energy.
 
  • #5
Why wouldn't dark matter clump together gravitationally?
 
  • #6
mesogen said:
Why wouldn't dark matter clump together gravitationally?

It does to some extent. It's a matter:devil: of degree.

Nobelist George Smoot has a 20 minute talk you should watch where he traces the clumping of ordinary matter back to the clumping of dark matter.
Dark matter gathering together was essential to the formation of structure----including concentrations of ordinary matter that became galaxies and clusters thereof.

Janus is right comparatively speaking. Dark matter doesn't clump very easily because it has only two ways to get rid of excess kinetic energy, that we know of.

But there is a lot of dark matter, so even if it is awkward for it to clump it plays a very important role in structure formation. Computer dynamical simulations of dark matter show it forming cobwebby structures, strands and voids. It forms the kind of qualitative largescale structure that we observe.

(This is according to standard mainstream computer models and conventional notions. It's not written in stone, new discoveries could make the pros change their mind.)

In his talk, Smoot plays computer movies of dark matter clumping---numerical simulations by Kravtsov. Great. Links are probably in the Cosmology sticky. If not I'll dig them up.
==================

How does dark matter jettison excess kinetic energy?

Gravitational (e.g. slingshot) interaction. DM can give away KE to ordinary or to other DM. Then it has less and can clump.

The other DM that took away the extra KE will itself eventually be slowed down by the universe expansion process and may get a second chance to clump. Steven Weinberg has a proof in his Cosmo text that expansion drains kinetic energy. But Hongbao Zhang has a simpler proof posted on arxiv.

We know that expansion drains energy from light, from radiation---that is the usual cosmo redshift. So it should not be completely surprising that particles with mass are gradually slowed down by expansion (seen from standpoint of observer at rest relative to cosmic Background.)
====================

We have contour maps showing concentration of vast clouds of dark matter around clusters of galaxies. DM must be able to clump otherwise it would be spread uniformly. But large differences in DM density have evolved. Smoot's talk is great on this.

Here are some links: Kravtsov computer simulations
http://cosmicweb.uchicago.edu/filaments.html
http://cosmicweb.uchicago.edu/group.html
I watched the halfsize MP4 version of the movie because it is very easy to download, only about 2.4 MB.[/QUOTE]

The mp4 version of Smoot's talk is slow to download.
http://video.ted.com/talks/podcast/GeorgeSmoot_2008P_480.mp4
You click on it, press pause, and go away and do something else for 5 or 10 minutes. Then come back and press run, that way the streaming has a head start.
 
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  • #7
There is no observational evidence of DM.
We observe normal matter in the sky, thru matter radiated energy (photons).
We look to the patterns of distribution of matter and do gravitational simulations usually using the Newtonian approximation model (infinite speed of propagation).
Things got wrong now, because only after injecting more mass (less than 5% observed and 95% of DM) the observed patterns (galaxy or overall observed universe) got similar to the simulation.
It is only a parameter in the simulation code, and something yet to be observed.
As there is no other known acceptable model we stick to LCDM that now reaches more consensuses, but several problems persist.
 
  • #8
heldervelez said:
There is no observational evidence of DM.

..., because only after injecting more mass (less than 5% observed and 95% of DM) the observed patterns ...

You appear to be confused, heldervelez, and to have not been paying attention to recent observations mapping DM concentrations by weak lensing.

This is a trivial matter but your post seems to equate DM with DE (dark energy).

DM is not estimated at 95%. That would be both darks combined.

DM is estimated at about 24%.

Dark matter and dark energy are very different and present different observational challenges, so it is important to distinguish between them.

Dark matter is comparatively easy to map because it condenses into clouds of higher-than-average density surrounding galaxy clusters, and these clouds can be mapped by weak lensing of more distant galaxies in the background.

When clusters of galaxies collide, as in the "Bullet Cluster" case, their clouds of DM behave differently from their clouds of ordinary matter.

Try Wikipedia on "Dark Matter". I will try to help you by getting some links, but you may be able to find adequate ones on your own.

Yeah, here's something:
http://arxiv.org/abs/astro-ph/0608407
Look at Figure 1.
By coincidence I've snorkeled in the Caribbean with Maxim, one of the authors. :biggrin: That was a couple of years before he became famous.
They titled their article in the Astrophysical Journal A DIRECT EMPIRICAL PROOF OF THE EXISTENCE OF DARK MATTER and they weren't kidding. Their work has been supported by other studies with other systems of galaxies.

Hey! I just tried wikipedia. The article is quite good! It gives the various kinds of evidence that have been accumulating during the past few of years.
http://en.wikipedia.org/wiki/Dark_Matter
 
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  • #9
So, as dark matter is made of WIMPs, they don't interact - like an ideal gas.
What is a temperature of that gas?
 
  • #10
Dmitry67 said:
So, as dark matter is made of WIMPs, they don't interact - like an ideal gas.
What is a temperature of that gas?

To me that seems like a beautiful question.
Excuse me if I seem too enthusiastic. But I really like it.

What would you say? 3/2 times the average kinetic energy?

Maybe someone else will jump in on this one.

I don't want to answer---if I were forced to guess I would say the same temperature as the primordial neutrino background, or almost exactly the same temperature as the CMB but not quite.

Expansion gradually deprives massive particles of kinetic energy. So the WIMPs must have cooled quite a bit.
 
  • #11
Hm... But WIMPs are quite heavy.
I am afraid in that case they would be too slow, so they would not be able to escape even from the Earth gravity. We don't know the mass of the WIMPs, so let me ask my question differently, not 'what is the temperature', but 'how fast do they move'?

Well, in any case, no matter how fast WIMPs move, there is a very small percentage of extremely cold ones. During 4 billion years, Earth could capture some dark matter cirling inside the core now on different orbits...

But measuring G using the balls we completely ignore the fact that Earth can contain some percentage of the dark matter.
 
  • #12
P.S.

I know there are many simulations of the early galaxy formation. Different % of the Dark matter leads to different results. However, I believe they must somehow define a mass of a WIMP (what if there are >1 sorts of stable WIMPs?)
 
  • #14
Thank you for the link
So avg speed = 9km/s?
Temperature = 10000K?
Doesn't it give us an exact MASS of a WIMP?
Looks like it is very light (based on these 2 numbers)... I expected it to be much heavier. Heavy enough, for example, to decay into "oh my God" particles.
 
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  • #15
Well, so far this has been very educational for me. Thanks for all the links. It may take me a while to get through the video. I'll get to that later. Thanks guys.
 
  • #16
can a black hole capture DM?
esp the huge galaxy center type of BH
if so what % of a multi million solar mass BH would be DM ?
could DM capture explain the huge mass of these BH's?

if a BH cannot capture DM does that mean DM could be a tachyon?
 
  • #17
making the history of the % of DM+DE versus matter since the BB cosmology tell us something:
The existence of DM & DE and the amount (and now Dark Flow ?) had not been derived from any principle nor predicted.

The initial model of BB disagrees with measured data (now at 96% of darkness versus 4% of matter/light).
Two ways to evade from this conflict.
Get another model or supply a different data set and make a patch to the theory.
A new model is not around the corner. So we are left with the second alternative.

quoting http://en.wikipedia.org/wiki/Dark_Matter
quote
...
It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita."[2]
...
Recently too there is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. There are also a small number of galaxies, like NGC 3379 whose measured orbital velocity of its gas clouds, show that it contains almost no dark matter at all.[9]
...
end-quote

see more puzzles at https://www.physicsforums.com/showthread.php?t=276174
where
wolram and markus introduced to us this very recent article
at http://arxiv.org/abs/0811.4684 (Submitted on 28 Nov 2008 (v1), last revised 22 Dec 2008 (this version, v2)

I hope that sometime in future we can discuss a model that fits 100% of matter/light, no more, no less. I do preffer this kind of solution.
 
  • #18
Dark matter is absolutely required for the formation of the galaxies. Without dark matter background radiation would be the same from any directs and no galaxies will be formed.

If you don't like DM how do you explain it?
 
  • #19
ray b said:
can a black hole capture DM?
esp the huge galaxy center type of BH
if so what % of a multi million solar mass BH would be DM ?
could DM capture explain the huge mass of these BH's?

if a BH cannot capture DM does that mean DM could be a tachyon?

Yeah. I hadn't thought of that before. Black holes should be gobbling up huge amounts of dark matter. I don't think we'd see any evidence of this because the DM couldn't radiate anything to tell us that it was being sucked in. All that would happen is the mass of the black hole would increase. It would be very difficult if at all possible to observe that sort of thing.
 
  • #20
Dmitry67 said:
Dark matter is absolutely required for the formation of the galaxies. Without dark matter background radiation would be the same from any directs and no galaxies will be formed.

If you don't like DM how do you explain it?

I'm not taking any position on the existence of DM, but why would DM in particular be unevenly distributed right after the BB? Wouldn't it be equally distributed just like regular matter?
 
  • #21
mesogen said:
I'm not taking any position on the existence of DM, but why would DM in particular be unevenly distributed right after the BB? Wouldn't it be equally distributed just like regular matter?

DM is evenly distributed after the BB, like a normal matter.
All types of matter are subjects to the gravitational instability: http://en.wikipedia.org/wiki/Jeans_mass

So the radiation pressure blocks the formation of any dense clouds of the normal matter until the light is separated from matter.

But DM does not interact with light, so, contrary to the normal matter, it begins to form clouds almost immediately after the BB.

So even the density fluctations of all sorts of matter were tiny right after the BB, DM had an extra epoque to form a structure.
 
  • #22
quoting the above mentioned paper
http://arxiv.org/PS_cache/arxiv/pdf/0811/0811.4684v2.pdf
quote
...
From the theoretical viewpoint the main weak points of CDM include [10]:
• The Fine Tuning Problem: What is the physical mechanism that sets the value of  to its observed value which is 120 orders of magnitude smaller than the physically anticipated value?
• The Coincidence Problem: Why is the energy density corresponding to the cosmological constant just starting to dominate the universe at the present cosmological time?
...
end-quote

beeing the above statements without formal denial I think that each one is a theory killer.

the first statement I read: the theory, at least this one, can make whatever value has a prediction and make an error to the observed value of the magnitude of the universe.
p= prediction
o=observed and p~= o^120 or p~=o^(-120) (is irrelevant a + or - in the exp.)
the 2nd statement I read: forget Galileu and Einstein, after all we live at the center of the Universe (some kind of center).

to me, those statements are deadly killers, and the paper continues with some 6 minor puzzles (compared to that tantalizing statements), and finally the documents concludes, in proper words: we have to do a major patch in the theory.
 
  • #23
But the problem with 120 orders of magnitude is not related to DM. It is a general problem of QM. If we assume that virtual particles boiling in vacuum participate in gravity we get the density of vacuum extremely high.

Also, there are MANY fine tuning problems and most of them do not intersect with the DM at all.
 
  • #24
Dmitry67 said:
Dark matter is absolutely required for the formation of the galaxies. Without dark matter background radiation would be the same from any directs and no galaxies will be formed.

If you don't like DM how do you explain it?

I don't like or dislike DM, DE or any darkness.
It's a question of theory and the blindness that pervades our minds when the issue is the failing without alternatives.

By the rules of this forum I can not say 'look here, look there' outside of the 'mainstream'.

it may help to explore the answers to the conceptual experiment at:
https://www.physicsforums.com/showthread.php?t=279200
 
  • #25
Dmitry67 said:
But the problem with 120 orders of magnitude is not related to DM. It is a general problem of QM. If we assume that virtual particles boiling in vacuum participate in gravity we get the density of vacuum extremely high.

Also, there are MANY fine tuning problems and most of them do not intersect with the DM at all.

do you remember to read something of Einstein related to virtual particles? I dont.
and the actual mainstream cosmology at least, try to adhere to GR.
I think that we can have a talk about cosmology without virtual particles.

If QM introduces an error of the order of magnitude of 120 then clearly QM must be excluded from the solution, don't you think? we will have fewer problems.
 
  • #26
heldervelez said:
making the history of the % of DM+DE versus matter since the BB cosmology tell us something:
The existence of DM & DE and the amount (and now Dark Flow ?) had not been derived from any principle nor predicted.
Which makes this part of science pretty normal, doesn't it?

I mean, a substantial proportion of the new ideas in science, over at least the last few centuries, have come from observations that implied something not predicted by the then current theory.
The initial model of BB disagrees with measured data (now at 96% of darkness versus 4% of matter/light).
Presumably you mean GR-based cosmological models whose only mass-energy content is baryonic (and leptonic) matter + photons; do you?

Also, I'm not quite sure what you mean by "measured data (now at 96% of darkness versus 4% of matter/light)": these proportions of darkness vs light are anything but "data"!
Two ways to evade from this conflict.
Get another model or supply a different data set and make a patch to the theory.
A new model is not around the corner. So we are left with the second alternative.

quoting http://en.wikipedia.org/wiki/Dark_Matter
quote
...
It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita."[2]
...
Recently too there is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. There are also a small number of galaxies, like NGC 3379 whose measured orbital velocity of its gas clouds, show that it contains almost no dark matter at all.[9]
...
end-quote

see more puzzles at https://www.physicsforums.com/showthread.php?t=276174
where
wolram and markus introduced to us this very recent article
at http://arxiv.org/abs/0811.4684 (Submitted on 28 Nov 2008 (v1), last revised 22 Dec 2008 (this version, v2)
I think you have glossed over some rather important aspects of the current situation!

The extent to which the galactic-scale puzzles are related to the cosmological ones is an active area of research, and it's far too premature (IMHO) to be making guesses as to how they will be answered (Perivolaropoulos is quite clear about this in his paper). For example, if one of the properties of CDM is that it forms constant-density cores of size ~1 kpc, does this necessarily have a profound implication for cosmology?

In any case, how remarkable is it that such a diverse set of observations can be explained, to within ~2σ (or better) with such a simple model (CDM)! No doubt scientists in many fields would die for such consistency.

I hope that sometime in future we can discuss a model that fits 100% of matter/light, no more, no less. I do preffer this kind of solution.
If wishes were horses, beggars would ride.

And the universe seems to pay no attention whatsoever to the hopes and preferences of us Homo saps.
 
  • #27
mesogen said:
Yeah. I hadn't thought of that before. Black holes should be gobbling up huge amounts of dark matter.

[...]
Why?
 
  • #28
heldervelez said:
quoting the above mentioned paper
http://arxiv.org/PS_cache/arxiv/pdf/0811/0811.4684v2.pdf
quote
...
From the theoretical viewpoint the main weak points of CDM include [10]:
• The Fine Tuning Problem: What is the physical mechanism that sets the value of  to its observed value which is 120 orders of magnitude smaller than the physically anticipated value?
• The Coincidence Problem: Why is the energy density corresponding to the cosmological constant just starting to dominate the universe at the present cosmological time?
...
end-quote

beeing the above statements without formal denial I think that each one is a theory killer.

the first statement I read: the theory, at least this one, can make whatever value has a prediction and make an error to the observed value of the magnitude of the universe.
p= prediction
o=observed and p~= o^120 or p~=o^(-120) (is irrelevant a + or - in the exp.)
As Dmitry67 has already noted, the relevant theory is QM, or at least the standard model of particle physics.

But it's been known for some time now that there is a mutual incompatibility between QM and GR, and also that the universe is perfectly capable of showing us new physics in energy domains beyond our current reach; for example, how much new physics is there between 1eV and 1 MeV (say), some 6 orders of magnitude (OOM)? between 1 MeV and 1 TeV (another 6 OOM)? What about between 1 TeV and 1 EeV? And for context, let's keep in mind that UHECRs (ultra-high energy cosmic rays) have been observed with energies just shy of 1 ZeV.

Maybe you have a somewhat unrealistic view of what scientific theories can - and cannot - do?
the 2nd statement I read: forget Galileu and Einstein, after all we live at the center of the Universe (some kind of center).

to me, those statements are deadly killers, and the paper continues with some 6 minor puzzles (compared to that tantalizing statements), and finally the documents concludes, in proper words: we have to do a major patch in the theory.
It does?

Here's what my copy of the paper says, in its last para:
In conclusion, the six puzzles for CDM discussed in the present study provide a fertile ground for the development of both new theoretical model predictions on the corresponding observables and new observational data that would either establish or disprove these challenges for CDM.
May I ask how you read "major patch" into this?
 
  • #29
Nereid said:
Why?

Well, if there is a lot of dark matter hanging around in galaxies, wouldn't it tend to also hang around black holes, especially the supermassive ones at the centers? Any dark matter that veered near a black hole would most likely be attracted to it, no?
 
  • #30
mesogen said:
Well, if there is a lot of dark matter hanging around in galaxies, wouldn't it tend to also hang around black holes, especially the supermassive ones at the centers? Any dark matter that veered near a black hole would most likely be attracted to it, no?

Because of friction, a black hole is more of en effective target for ordinary matter than it is for dark matter. See

https://www.physicsforums.com/showthread.php?p=2000505#post2000505.
 
  • #31
George Jones said:
Because of friction, a black hole is more of en effective target for ordinary matter than it is for dark matter. See

https://www.physicsforums.com/showthread.php?p=2000505#post2000505.

But if there is 5x as much dark matter, it should maybe compete to some degree with regular matter. I wonder if anyone has estimated the ratio of matter to dark matter that would tend to find its way into a black hole.
 
  • #32
From the other thread that you linked to (where I also see that, yes, someone has estimated the ratio):

oldman said:
This is why, I think, in the link you gave, Peirani and Pacheko say that "dark matter particles constitute a collisionless fluid".

So, if the particles of dark matter cannot collide at all, then can they superimpose will nilly? As in, dark matter particles can be in the same place at once, or a trillion of them can also occupy the same space all at once? There is absolutely no "collision" whatsoever? Wow.
 
  • #33
mesogen said:
As in, dark matter particles can be in the same place at once, or a trillion of them can also occupy the same space all at once? There is absolutely no "collision" whatsoever? Wow.

1 Pauli exclusion principle can be applicable to these particles if they are fermions, so 'trillions' can not gather in the same place

2 Collision is possible but the probability is extremely low
As I understand, the collision occurs at the Sypersymmetry breaking energy
Such virtual particles are very rare.
 
  • #34
Nereid said:
...

Here's what my copy of the paper says, in its last para:
"..."
May I ask how you read "major patch" into this?


quoting from the above paper
"Even though some of the puzzles discussed here may be resolved by more complete observations or astrophysical
effects, the possible requirement of more fundamental modifications of the CDM model remains valid.
...
This improved effectiveness could possibly be provided by a mild
evolution of Newton’s constant G (higher G at z > 0.5)"

in the first bold I read "major patch" and in the second bold I read "lets rewrite the more fundamental laws of physics".
An hipotetical evolution of G, of T, of alfa, of L and may others and combinations of those constants or magnitudes have been tried. I've found pappers on G and on alfa.

Triyng to mess with G without providing a mechanism for such change is data fit. But until now the pursue has been unfruitfull.
 

1. What is dark matter and how does it relate to the CDM model?

Dark matter is a hypothetical type of matter that does not interact with light or other forms of electromagnetic radiation. It is believed to make up about 85% of the total matter in the universe. The CDM (Cold Dark Matter) model is a widely accepted theory that explains the large-scale structure of the universe, but it requires modifications to account for the presence of dark matter.

2. How does dark matter explain the need for modifications to the CDM model?

The CDM model predicts that the universe is dominated by dark matter, which forms large, diffuse halos around galaxies. However, observations have shown that the distribution of dark matter is not as smooth as predicted. This has led to the need for modifications to the CDM model, such as including interactions between dark matter particles or considering alternative theories of gravity.

3. What evidence supports the existence of dark matter and the need for modifications to the CDM model?

There is a wealth of evidence supporting the existence of dark matter, including observations of the rotation curves of galaxies, gravitational lensing effects, and the cosmic microwave background radiation. Additionally, simulations based on the CDM model have failed to accurately reproduce the observed structures in the universe, indicating the need for modifications.

4. Are there any alternative explanations for the observed discrepancies in the CDM model?

While the presence of dark matter is the most widely accepted explanation for the need for modifications to the CDM model, there are some alternative theories that have been proposed. These include modified theories of gravity, such as MOND (Modified Newtonian Dynamics), and theories that propose a modification of the laws of gravity on large scales.

5. How do scientists plan to further study and understand dark matter and its implications for the CDM model?

There are ongoing efforts to study dark matter through various methods, such as particle colliders, astronomical observations, and simulations. These studies aim to better understand the properties of dark matter and its interactions with other forms of matter. Additionally, scientists are also exploring alternative theories and modifications to the CDM model in order to better explain the observed discrepancies and make more accurate predictions about the structure of the universe.

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