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Basic question about dark matter interaction



 
 
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  #1  
Old July 11th 15, 07:57 PM posted to sci.astro.research
keybounce
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Posts: 2
Default Basic question about dark matter interaction

So I have a basic question on dark matter. As I understand it, it
does not interact with normal matter except by gravity, and according
to observation of the bullet cluster, does not interact with itself
except by gravity.

Yet it manages to lump together, to form the scaffolding that
generates the gravitational attraction for the rest of
matter/stars/galaxies.

I know enough to know that if the only interaction is gravity, then
there is no lumping -- gravity alone is conservative, so all of the
kinetic energy becomes potential energy becomes kinetic energy etc.,
and the particles would just bound around. In order to lump, some
of that energy has to go elsewhere, so there needs to be some
interaction other than gravity.

So somewhere in here, I must have an error. Either gravity alone
is able to cause loss of energy (perhaps from relativistic effects?),
or there must be some other form of interaction in there somewhere.
What am I not understanding about dark matter?

[[Mod. note -- Gravity can cause a loss of energy, by the mechanism
known as "dynamical friction". Basically, the kinetic energy of an
infalling dark (or other) matter object can be transferred to the
galactic stars. This happens even in Newtonian gravity -- it's not
a relativistic effect. See
https://en.wikipedia.org/wiki/Dynamical_friction

However, I do not personally know whether this is currently thought
to be the main mechanism allowing galaxies or galaxy clusters to
capture dark matter.
-- jt]]
  #2  
Old July 14th 15, 09:41 AM posted to sci.astro.research
Martin Brown
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Posts: 1,707
Default Basic question about dark matter interaction

On 11/07/2015 19:57, keybounce wrote:

So I have a basic question on dark matter. As I understand it, it
does not interact with normal matter except by gravity, and according
to observation of the bullet cluster, does not interact with itself
except by gravity.


Does it not also interact very infrequently by the weak interaction (or
something of similar strength) if it gets close enough to another WIMP
or an atomic nucleus?

Yet it manages to lump together, to form the scaffolding that
generates the gravitational attraction for the rest of
matter/stars/galaxies.

I know enough to know that if the only interaction is gravity, then
there is no lumping -- gravity alone is conservative, so all of the
kinetic energy becomes potential energy becomes kinetic energy etc.,
and the particles would just bound around. In order to lump, some
of that energy has to go elsewhere, so there needs to be some
interaction other than gravity.


As soon as you have a three body interaction there is scope for two of
them ending up bound and the third one being expelled to infinity.

So somewhere in here, I must have an error. Either gravity alone
is able to cause loss of energy (perhaps from relativistic effects?),
or there must be some other form of interaction in there somewhere.
What am I not understanding about dark matter?


I have a few other dark matter related questions of my own.
I'll be visiting the Boulby potash mine experiment shortly...

I presume that cold dark matter detectors on Earth are looking for
inbound particles with kinetic energy in the range characteristic of the
Earth's orbit and/or the suns motion around the galactic centre. These
interactions being incredibly rare and the experiments typically being
deep underground for screening. Hoping to see an annual variation.

Are there any decent theoretical bounds on the characteristics of
candidate CDM WIMPs/axions that allow the experimenters to know that
interactions with xenon or other crystal based scintillators will occur?

Are any of the detectors located in the tropics where there might be a
slight daily modulation from the gravitational focussing effect of the
WIMP particles as they pass through the Earth?

Since gravity is the only thing they really see does any of the Earth,
Jupiter or Sun offer enough gravitational pull x nuclear collision cross
section to ever capture some of them in bound orbits?

What is the fate of a gravitationally bound WIMP in the suns core?

[[Mod. note -- Gravity can cause a loss of energy, by the mechanism
known as "dynamical friction". Basically, the kinetic energy of an
infalling dark (or other) matter object can be transferred to the
galactic stars. This happens even in Newtonian gravity -- it's not
a relativistic effect. See
https://en.wikipedia.org/wiki/Dynamical_friction

However, I do not personally know whether this is currently thought
to be the main mechanism allowing galaxies or galaxy clusters to
capture dark matter.
-- jt]]


Do the simulated universe models not show which clumps first ordinary
matter or CDM - or are there sufficient free parameters available to get
the observed results with either being the driving force?

I am not up with current theory but this seems like a decent review:

http://www.kiss.caltech.edu/workshop...ack-galaxy.pdf

It is something of an embarrassment that CDM is so difficult to observe
when its influence on galaxy stellar dynamics is all too obvious.

--
Regards,
Martin Brown
  #3  
Old July 19th 15, 06:38 AM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Basic question about dark matter interaction

On Tuesday, July 14, 2015 at 4:42:01 AM UTC-4, Martin Brown wrote:
It is something of an embarrassment that CDM is so difficult to observe=20
when its influence on galaxy stellar dynamics is all too obvious.


The Fermi-LAT Collaboration posted a preprint to arxiv.org yesterday
reporting that sensitive searches for gamma-ray emission from a
high quality set of dwarf spheroidal galaxies found no indications
of particle dark matter "annihilations".

http://arxiv.org/abs/1507.03530=20

There is an exceedingly simple answer to the question of why there
has been no observation of any form of particle dark matter over
the last 3 decades despite a huge number of empirical searches. The
answer: DM is not in the form of subatomic particles. However, that
appears to be an answer that most physicists do not want to entertain.

RLO
Fractal Cosmology
  #4  
Old July 19th 15, 04:46 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Basic question about dark matter interaction

In article ,
"Robert L. Oldershaw" writes:

The Fermi-LAT Collaboration posted a preprint to arxiv.org yesterday
reporting that sensitive searches for gamma-ray emission from a
high quality set of dwarf spheroidal galaxies found no indications
of particle dark matter "annihilations".


Why the scare quotes?

http://arxiv.org/abs/1507.03530=20

There is an exceedingly simple answer to the question of why there
has been no observation of any form of particle dark matter over
the last 3 decades despite a huge number of empirical searches. The
answer: DM is not in the form of subatomic particles. However, that
appears to be an answer that most physicists do not want to entertain.


Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
OBSERVATIONS. That, and that alone, is why most people believe that DM
is some soft of elementary particle.

Yes, there could be some types of DM which self-annihilate. However,
this is not a necessary characteristic of DM. So, at best, this paper
rules out a certain class of DM particles. That is progress. Over the
years, various candidates---dark baryons, primordial black holes,
etc---have been ruled out.
  #5  
Old July 20th 15, 08:55 AM posted to sci.astro.research
Jos Bergervoet
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Posts: 126
Default Basic question about dark matter interaction

On 7/19/2015 5:46 PM, Phillip Helbig (undress to reply) wrote:
"Robert L. Oldershaw" writes:

..
There is an exceedingly simple answer to the question of why there
has been no observation of any form of particle dark matter


It's a pity you eluded it, Phillip, but Robert's response
was to Martin Brown's remark that it's "an embarrassment that
CDM is so difficult to observe when its influence on galaxy
stellar dynamics is all too obvious."

There was no mention of *particle* dark matter in particular,
only of an obvious influence of some form of dark matter.

answer: DM is not in the form of subatomic particles.


No, that does not remove the embarrassment at all! If it's
not particles, then we still haven't observed it.

Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
OBSERVATIONS.


No, suggesting candidates does not remove the embarrassment
either! Actually, a large number of candidates, without a
clue which one it is, would increase the embarrassment IMHO.

.. So, at best, this paper
rules out a certain class of DM particles. That is progress. Over the
years, various candidates---dark baryons, primordial black holes,
etc---have been ruled out.


Ruling out all candidates except one would completely solve
the problem with mathematical certainty. (Unfortunately,
many physicists claim that mathematics is no science, so
this will not suffice..)

--
Jos
  #6  
Old July 20th 15, 08:36 PM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Basic question about dark matter interaction

On Sunday, July 19, 2015 at 11:46:15 AM UTC-4, Phillip Helbig (undress to reply) wrote:

Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
OBSERVATIONS. That, and that alone, is why most people believe that DM
is some soft of elementary particle.

Yes, there could be some types of DM which self-annihilate. However,
this is not a necessary characteristic of DM. So, at best, this paper
rules out a certain class of DM particles. That is progress. Over the
years, various candidates---dark baryons, primordial black holes,
etc---have been ruled out.


Firstly, using all caps is considered to be shouting in a slightly
unhinged manner in discussion groups. Thus, it is considered bad form
and indicates that the author may have lost his/her grip on rational
thought. Just thought you might like to consider this.

Primordial black holes have most definitely not been ruled out, at
least not empirically or scientifically. There are mass ranges that
are still completely untested.

Also, not long ago I started a thread on MRS Hawkins' latest published
paper in which he argues that some galactic models could be consistent
with MACHOs constituting up to 100% of the Galactic dark matter.

To be sure, getting an idea published in a decent journal does not
mean the idea is true/correct. However, it does mean that the analysis
is plausible and defensible, and, more importantly for this
discussion, the idea(s) cannot be denied to exist or falsely claimed
to be wrong before being adequately tested.

The dark matter does not absolutely have to be in the form of
subatomic particles. Yet most physicists act virtually as if it must
be in this form. There is no evidence for this bias and it is more
like a religious liturgy than a scientific assessment.

RLO
Fractal Cosmology

[Mod. note: reformatted -- mjh]
  #7  
Old July 22nd 15, 01:17 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Basic question about dark matter interaction

In article ,
keybounce writes:
So I have a basic question on dark matter. As I understand it, it
does not interact with normal matter except by gravity, and according
to observation of the bullet cluster, does not interact with itself
except by gravity.


Those are the simplest assumptions. At the moment, all we can say
for sure is that interaction cross sections are too small to have
obvious astrophysical consequences. They are probably not strictly
zero.

I know enough to know that if the only interaction is gravity, then
there is no lumping


Sorry... that's not correct.

-- gravity alone is conservative, so all of the
kinetic energy becomes potential energy becomes kinetic energy etc.,


Yes, energy is conserved. That doesn't mean the density remains
uniform.

Think about the initial state: particles are (nearly) uniformly
distributed with finite, random motions. Potential energy is zero,
kinetic energy is positive, but the virial theorem tells us that
_cannot_ be an equilibrium state.

What actually happens is that density contrast grows over time, the
negative potential energy being offset by increased (on average)
kinetic energy. Numerous dark matter simulations show this very
clearly. There are a few movies of this process on youtube, but I've
seen much better ones. (Some movies illustrate the matter
distribution at the present epoch "z=0", but what you want is one
illustrating the evolution from high redshift to now.)

One (now older) simulation is described at
http://www.mpa-garching.mpg.de/galfo...um/index.shtml

At the bottom of the page, there are pictures illustrating the
density distributions at various epochs. I'm disappointed that there
doesn't seem to be a movie.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #8  
Old July 22nd 15, 01:17 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Basic question about dark matter interaction

In article ,
Martin Brown writes:
Are there any decent theoretical bounds on the characteristics of
candidate CDM WIMPs/axions that allow the experimenters to know that
interactions with xenon or other crystal based scintillators will occur?


I gather there are predictions for particular particle types and
vaguely remember an article, perhaps in _Physics Today_, from some
years ago. A very quick web search found
https://www.astro.umd.edu/~ssm/darkm...periments.html
I bet a careful search would turn up much more.

Are any of the detectors located in the tropics where there might be a
slight daily modulation from the gravitational focussing effect of the
WIMP particles as they pass through the Earth?


Wouldn't the major component of motion be the Sun's orbit around the
Milky Way center?

Since gravity is the only thing they really see does any of the Earth,
Jupiter or Sun offer enough gravitational pull x nuclear collision cross
section to ever capture some of them in bound orbits?


The cross sections are presumably similar to (or less than) those of
neutrinos, so I wouldn't think so. Maybe it could happen if the
elastic collision cross section is much higher than the inelastic
cross section. If it's that high, though, I'd expect (without having
done the calculation) major astrophysical consequences, which we
don't see.

What is the fate of a gravitationally bound WIMP in the suns core?


How did it get captured? Assuming it did, it probably just orbits
around the Sun's center, coming into thermal equilibrium with the
local gas if the elastic interaction cross section is high.
Eventually, assuming a non-zero inelastic cross section, the particle
would undergo some nuclear reaction with products depending on just
what the particle is.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #9  
Old July 22nd 15, 01:18 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Basic question about dark matter interaction

In article , "Robert L.
Oldershaw" writes:

Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
OBSERVATIONS. That, and that alone, is why most people believe that DM
is some soft of elementary particle.


Firstly, using all caps is considered to be shouting in a slightly
unhinged manner in discussion groups. Thus, it is considered bad form
and indicates that the author may have lost his/her grip on rational
thought. Just thought you might like to consider this.


An entire post in caps? Yes. As a substitute for italics, to
underscore an important point? No.

Primordial black holes have most definitely not been ruled out, at
least not empirically or scientifically. There are mass ranges that
are still completely untested.


And they are?

Also, not long ago I started a thread on MRS Hawkins' latest published
paper in which he argues that some galactic models could be consistent
with MACHOs constituting up to 100% of the Galactic dark matter.


But he claims to have detected these, so this is a different claim.

To be sure, getting an idea published in a decent journal does not
mean the idea is true/correct. However, it does mean that the analysis
is plausible and defensible,


It means that the referee thought that the paper should be published,
and the editor didn't disagree. No more, no less. (Of course, the
importance of this varies from journal to journal.)

and, more importantly for this
discussion, the idea(s) cannot be denied to exist or falsely claimed
to be wrong before being adequately tested.


I don't think anyone has denied that any IDEA (italics, not shout) does
not exist. (Denying the existence of an idea is logically close to
impossible, since formulating the negation essentially defines the
idea.)

You mentioned courtesy above. In the scientific literature, it is
considered common courtesy to at least acknowledge criticism of one's
work, and at least briefly address it, even if one does not agree with
it, assuming that it was published somewhere at least as reputable as
the publication it criticizes. Hawkins has stopped doing this and has
thus withdrawn himself from accepted modes of discussion. Which is a
shame.

The dark matter does not absolutely have to be in the form of
subatomic particles. Yet most physicists act virtually as if it must
be in this form. There is no evidence for this bias and it is more
like a religious liturgy than a scientific assessment.


There is a huge amount of evidence: other candidates have been ruled
out.
  #10  
Old July 23rd 15, 06:35 AM posted to sci.astro.research
Nicolaas Vroom
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Posts: 216
Default Basic question about dark matter interaction

Op woensdag 22 juli 2015 02:17:32 UTC+2 schreef Steve Willner:
In article ,
keybounce writes:


-- gravity alone is conservative, so all of the
kinetic energy becomes potential energy becomes kinetic energy etc.,


Yes, energy is conserved. That doesn't mean the density remains
uniform.


This requires more explanation. See below.

Think about the initial state: particles are (nearly) uniformly
distributed with finite, random motions. Potential energy is zero,
kinetic energy is positive, but the virial theorem tells us that
_cannot_ be an equilibrium state.


The virial theorem allows us how to calculate the average mass of
the objects of a cluster based on radius and average speed.
For more information about my simulations see:
http://users.telenet.be/nicvroom/virial.htm
The important thing is that each simulation depents very much on
initial condition (The accuracy of the simulation and the algorithm
used to calculate the next positions)
When your intial condition is a sphere and v=0 than all your objects
will move towards the center and collide.
When your initial condition is a sphere and (vR)n(vR)n10 and small
for each object than you will get a contracting / expanding simulation.
With vR(n) I mean the speed in radial direction for object n
In such a simulation the density at the center heavily fluctuates.
When the initial values for (vR)n are larger the fluctuations are less.

What I want to say that it requires a lot of thought to start with
a simulation in equilibrium which stays in equilibrium.
With equilibrium I mean that the density curve starting from the center
maintains the same shape for as long as possible.
IMO the virial theorem has nothing to do with this particular issue.

What actually happens is that density contrast grows over time, the
negative potential energy being offset by increased (on average)
kinetic energy. Numerous dark matter simulations show this very
clearly.

Why do you mention here dark matter?
What is the difference when you use only baryonic matter versus
only dark matter? (IMO there is none)
Or are you using a mixture of both?

There are a few movies of this process on youtube, but I've
seen much better ones. (Some movies illustrate the matter
distribution at the present epoch "z=0", but what you want is one
illustrating the evolution from high redshift to now.)


Why do you mention high redshifts?

In the simulations I have done are strictly based on baryonic matter
objects.
My understanding is that if the objects are dark matter there is
no difference.
Of course if is the space between the objects is not empty, than that
will influence the simulations and the final calculations of
the masses of the individual objects.

Nicolaas Vroom.
 




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