Question about dark matter


by mesogen
Tags: dark matter, gravity
mesogen
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#19
Dec22-08, 10:44 AM
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Quote Quote by ray b View Post
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.
mesogen
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#20
Dec22-08, 10:48 AM
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Quote Quote by Dmitry67 View Post
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 dont 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?
Dmitry67
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#21
Dec22-08, 10:57 AM
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Quote Quote by mesogen View Post
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.
heldervelez
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#22
Dec22-08, 04:20 PM
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quoting the above mentioned paper
http://arxiv.org/PS_cache/arxiv/pdf/...811.4684v2.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 statments without formal denial I think that each one is a theory killer.

the first statment 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 statment I read: forget Galileu and Einstein, after all we live at the center of the Universe (some kind of center).

to me, those statments are deadly killers, and the paper continues with some 6 minor puzzles (compared to that tantalizing statments), and finally the documents concludes, in proper words: we have to do a major patch in the theory.
Dmitry67
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#23
Dec22-08, 04:26 PM
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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.
heldervelez
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#24
Dec22-08, 04:36 PM
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Quote Quote by Dmitry67 View Post
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 dont like DM how do you explain it?
I dont 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:
http://www.physicsforums.com/showthread.php?t=279200
heldervelez
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#25
Dec22-08, 04:45 PM
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Quote Quote by Dmitry67 View Post
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, dont you think? we will have fewer problems.
Nereid
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#26
Dec25-08, 01:32 PM
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Quote Quote by heldervelez View Post
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 http://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.
Nereid
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#27
Dec25-08, 01:33 PM
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Quote Quote by mesogen View Post
Yeah. I hadn't thought of that before. Black holes should be gobbling up huge amounts of dark matter.

[...]
Why?
Nereid
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#28
Dec25-08, 01:46 PM
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Quote Quote by heldervelez View Post
quoting the above mentioned paper
http://arxiv.org/PS_cache/arxiv/pdf/...811.4684v2.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 statments without formal denial I think that each one is a theory killer.

the first statment 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 statment I read: forget Galileu and Einstein, after all we live at the center of the Universe (some kind of center).

to me, those statments are deadly killers, and the paper continues with some 6 minor puzzles (compared to that tantalizing statments), 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?
mesogen
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#29
Dec29-08, 12:23 PM
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Quote Quote by Nereid View Post
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?
George Jones
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#30
Dec29-08, 01:13 PM
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Quote Quote by mesogen View Post
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

http://www.physicsforums.com/showthr...05#post2000505.
mesogen
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#31
Dec29-08, 02:56 PM
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Quote Quote by George Jones View Post
Because of friction, a black hole is more of en effective target for ordinary matter than it is for dark matter. See

http://www.physicsforums.com/showthr...05#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.
mesogen
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#32
Dec29-08, 02:59 PM
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From the other thread that you linked to (where I also see that, yes, someone has estimated the ratio):

Quote Quote by oldman
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.
Dmitry67
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#33
Dec29-08, 03:30 PM
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Quote Quote by mesogen View Post
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.
heldervelez
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#34
Jan20-09, 11:26 AM
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Quote Quote by Nereid View Post
...

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.


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