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Basic question about dark matter interaction
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keybounce
Jul 11, 2015, 2:57:42 PM7/11/15
to
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]]
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]]
Martin Brown
Jul 14, 2015, 4:42:01 AM7/14/15
to
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
> 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.
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.
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'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
> [[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]]
>
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/workshops/darkmatter2009/papers/primack-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
Robert L. Oldershaw
Jul 19, 2015, 1:38:58 AM7/19/15
to
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
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
> 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
Phillip Helbig (undress to reply)
Jul 19, 2015, 11:46:15 AM7/19/15
to
In article <7e3b8c64-fa95-4de9...@googlegroups.com>,
> 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.
"Robert L. Oldershaw" <rlold...@amherst.edu> 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?
> 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.
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.
Jos Bergervoet
Jul 20, 2015, 3:55:29 AM7/20/15
to
On 7/19/2015 5:46 PM, Phillip Helbig (undress to reply) wrote:
> "Robert L. Oldershaw" <rlold...@amherst.edu> writes:
..
> "Robert L. Oldershaw" <rlold...@amherst.edu> 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
>> has been no observation of any form of particle dark matter
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.
not particles, then we still haven't observed it.
> Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
> OBSERVATIONS.
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
> years, various candidates---dark baryons, primordial black holes,
> etc---have been ruled out.
the problem with mathematical certainty. (Unfortunately,
many physicists claim that mathematics is no science, so
this will not suffice..)
--
Jos
Robert L. Oldershaw
Jul 20, 2015, 3:36:50 PM7/20/15
to
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
>
> 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.
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]
Steve Willner
Jul 21, 2015, 8:17:32 PM7/21/15
to
In article <5907d804-c9e2-4f56...@googlegroups.com>,
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/galform/virgo/millennium/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 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
keybounce <keyb...@gmail.com> 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
> 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.
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
> -- gravity alone is conservative, so all of the
> kinetic energy becomes potential energy becomes kinetic energy etc.,
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/galform/virgo/millennium/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 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
Steve Willner
Jul 21, 2015, 8:18:00 PM7/21/15
to
In article <mo03l8$jrs$1...@speranza.aioe.org>,
vaguely remember an article, perhaps in _Physics Today_, from some
years ago. A very quick web search found
https://www.astro.umd.edu/~ssm/darkmatter/WIMPexperiments.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.
Martin Brown <|||newspam|||@nezumi.demon.co.uk> 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
> 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?
vaguely remember an article, perhaps in _Physics Today_, from some
years ago. A very quick web search found
https://www.astro.umd.edu/~ssm/darkmatter/WIMPexperiments.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?
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?
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?
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.
Phillip Helbig (undress to reply)
Jul 21, 2015, 8:18:16 PM7/21/15
to
In article <mt2.0-30608...@hydra.herts.ac.uk>, "Robert L.
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.
Oldershaw" <rlold...@amherst.edu> 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.
> > 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
> 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.
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.
> 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 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.
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.
out.
Nicolaas Vroom
Jul 23, 2015, 1:35:57 AM7/23/15
to
Op woensdag 22 juli 2015 02:17:32 UTC+2 schreef Steve Willner:
> 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)n1<>0 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.
> In article <5907d804-c9e2-4f56...@googlegroups.com>,
> keybounce <keyb...@gmail.com> 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.
> keybounce <keyb...@gmail.com> 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.
> 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 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)n1<>0 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.
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.)
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.
Richard D. Saam
Jul 23, 2015, 1:36:42 AM7/23/15
to
On 7/21/15 7:17 PM, Steve Willner wrote:
>
> 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/galform/virgo/millennium/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.
>
There is an entirely different field of study
>
> 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/galform/virgo/millennium/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.
>
related to wastewater treatment
with its own terminology
that deals with density distributions
in a fluid medium
in hydraulic shear state:
Google:activated sludge filamentous bacteria images
https://en.wikipedia.org/wiki/Activated_sludge_model
These filamentous forms seem to have the same visual characteristics
as DM filaments.
Richard D Saam
Martin Brown
Jul 23, 2015, 1:37:26 AM7/23/15
to
On 22/07/2015 01:17, Steve Willner wrote:
> In article <mo03l8$jrs$1...@speranza.aioe.org>,
> Martin Brown <|||newspam|||@nezumi.demon.co.uk> writes:
>> Are there any decent theoretical bounds on the characteristics of
>> candidate CDM WIMPs/axions that allow the experimenters to know that
>> interactions with xenon or other crystal based scintillators will occur?
>
> I gather there are predictions for particular particle types and
> vaguely remember an article, perhaps in _Physics Today_, from some
> years ago. A very quick web search found
> https://www.astro.umd.edu/~ssm/darkmatter/WIMPexperiments.html
> I bet a careful search would turn up much more.
Thanks. That link is very interesting. I had thought that MOND was now
> In article <mo03l8$jrs$1...@speranza.aioe.org>,
> Martin Brown <|||newspam|||@nezumi.demon.co.uk> 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/darkmatter/WIMPexperiments.html
> I bet a careful search would turn up much more.
also ruled out of the game now but that page seems to say not.
>
>> 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?
Yes. And any passing CDM will still be fast enough that the Earths
>> 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?
gravity isn't going to alter their trajectory by much. The Boulby DRIFT
detector exploits this to distinguish real signal by direction.
>
>> 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.
I found some claims that they might and that as a result WIMP anti-WIMP
>> 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.
interactions might lead to annihilation reactions with observable high
energy neutrino and gamma emission as a consequence.
I presume with such a low interaction cross section the residual mix of
WIMP and anti-WIMP is much closer to being 50:50 than for real matter.
>
>> 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.
Thanks. It is hard to see the wood for the trees sometimes. I can find
>> 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.
all sort of claims and counter claims about CDM in a web search.
It was very interesting to see the lab where Zepplin lived. A cleanroom
built inside a working potash mine (in the more stable salt layer) is an
interesting thing to visit. Outside everything is coated in dirty
crystalline salt which glistens in the helmet lamp light. Keeping that
muck from getting inside and wrecking experiments is non trivial.
They are presently qualifying materials for the next generation of CDM
detectors in a screened environment where background radioactivity is
about a million times lower than at the surface. All we saw go through
it in realtime was alpha particles not even a neutron :(
The detectors are inside "castles" of nested lead and copper block
shielding from locally generated radiation eg. from visiting humans!
Also interesting geology and tests for exobiology in the same vicinity
since they have brine seeps with extremophile archeobacteria in too. An
exobiology lab is colocated with the dark matter facility and in
addition they have some beautiful mineral samples from the mine.
--
Regards,
Martin Brown
Phillip Helbig (undress to reply)
Jul 23, 2015, 1:37:57 AM7/23/15
to
In article <mom48b$q10$1...@dont-email.me>, Steve Willner
modulation.
<wil...@cfa.harvard.edu> writes:
> > Are any of the detectors located in the tropics where there might be a
> > slight daily modulation from the gravitational focussing effect of the
> > WIMP particles as they pass through the Earth?
>
> Wouldn't the major component of motion be the Sun's orbit around the
> Milky Way center?
Of the motion, yes, but the timescale is too long to notice any
> > Are any of the detectors located in the tropics where there might be a
> > slight daily modulation from the gravitational focussing effect of the
> > WIMP particles as they pass through the Earth?
>
> Wouldn't the major component of motion be the Sun's orbit around the
> Milky Way center?
modulation.
Phillip Helbig (undress to reply)
Jul 24, 2015, 2:59:25 AM7/24/15
to
In article <monsq7$j5d$1...@speranza.aioe.org>, Martin Brown
the need for dark matter. Of course it still works for this. There is
only one adjustable parameter (which seems to have a "natural" value),
and over time many other phenomena could be explained with the same
value of the adjustable parameter. This has not changed either. This
original MOND was not really a theory, just an empirical fit.
Certain theories based on MOND have been ruled out.
There is now much evidence for dark matter apart from flat rotation
curves of spiral galaxies. This cannot be explained by MOND, at least
not by the empirical MOND or a small extrapolation of it. However, it
could of course be the case that the MOND effect is real but that there
is also dark matter. There is no reason this could not be the case.
Psychologically, it might not be desirable to supporters of MOND who
don't like dark matter, but the universe is not obliged to conform to
our tastes. It might not be desirable to supporters of dark matter, but
the simplest explanation is not always the best. For example, it turned
out that neutrinos have mass, and are thus non-baryonic dark matter, but
the masses are too small to allow neutrinos to make up all non-baryonic
dark matter, so we already have a case where the simplest explanation
didn't turn out to be right.
Of course, one should prefer the simplest explanation unless a more
complicated one works substantially better (that a more complicated one
could work somewhat better is not sufficient). The question is whether,
in the case of spiral galaxies, MOND or DM is the better explanation.
Note that MOND doesn't give just approximately flat rotation curves,
which DM simulations can also give, but predict many details. It is not
obvious that these simply "fall out" of DM, and at least some claims
that they do is not convincing.
Personally, I am a bit sceptical of MOND, at least until some underlying
theory predicts something like it, but on the other hand my impression
is that the supporters of MOND in general know more about astronomy and
astrophysics than many of the detractors, and many "refutations" of MOND
are quite superficial and show that the detractors haven't investigated
MOND in detail.
Note that Oxford University professor James Binney, who literally wrote
the book on galactic dynamics, has been a MOND supporter for a while
now. MOND is not some fringe or crackpot theory, but an alternative
hypothesis which has not yet been ruled out.
<|||newspam|||@nezumi.demon.co.uk> writes:
> Thanks. That link is very interesting. I had thought that MOND was now
> also ruled out of the game now but that page seems to say not.
MOND was developed to explain flat rotation curves of galaxies without
> Thanks. That link is very interesting. I had thought that MOND was now
> also ruled out of the game now but that page seems to say not.
the need for dark matter. Of course it still works for this. There is
only one adjustable parameter (which seems to have a "natural" value),
and over time many other phenomena could be explained with the same
value of the adjustable parameter. This has not changed either. This
original MOND was not really a theory, just an empirical fit.
Certain theories based on MOND have been ruled out.
There is now much evidence for dark matter apart from flat rotation
curves of spiral galaxies. This cannot be explained by MOND, at least
not by the empirical MOND or a small extrapolation of it. However, it
could of course be the case that the MOND effect is real but that there
is also dark matter. There is no reason this could not be the case.
Psychologically, it might not be desirable to supporters of MOND who
don't like dark matter, but the universe is not obliged to conform to
our tastes. It might not be desirable to supporters of dark matter, but
the simplest explanation is not always the best. For example, it turned
out that neutrinos have mass, and are thus non-baryonic dark matter, but
the masses are too small to allow neutrinos to make up all non-baryonic
dark matter, so we already have a case where the simplest explanation
didn't turn out to be right.
Of course, one should prefer the simplest explanation unless a more
complicated one works substantially better (that a more complicated one
could work somewhat better is not sufficient). The question is whether,
in the case of spiral galaxies, MOND or DM is the better explanation.
Note that MOND doesn't give just approximately flat rotation curves,
which DM simulations can also give, but predict many details. It is not
obvious that these simply "fall out" of DM, and at least some claims
that they do is not convincing.
Personally, I am a bit sceptical of MOND, at least until some underlying
theory predicts something like it, but on the other hand my impression
is that the supporters of MOND in general know more about astronomy and
astrophysics than many of the detractors, and many "refutations" of MOND
are quite superficial and show that the detractors haven't investigated
MOND in detail.
Note that Oxford University professor James Binney, who literally wrote
the book on galactic dynamics, has been a MOND supporter for a while
now. MOND is not some fringe or crackpot theory, but an alternative
hypothesis which has not yet been ruled out.
brad
Jul 25, 2015, 4:15:20 AM7/25/15
to
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.
mimetic dark matter http://arxiv.org/abs/1308.5410
> OBSERVATIONS.
sincerely, Brad Johnson (no! not that one, or that one, or that one
Jos Bergervoet
Jul 28, 2015, 12:34:49 PM7/28/15
to
On 7/24/2015 8:59 AM, Phillip Helbig (undress to reply) wrote:
..
> ... For example, it turned
particle! Luminous matter is a mix of many particle types and
there is 4 times more dark than luminous matter, so shouldn't
it (DM) be entitled to consist of a complete new particle
zoo?
--
Jos
..
> ... For example, it turned
> out that neutrinos have mass, and are thus non-baryonic dark matter, but
> the masses are too small to allow neutrinos to make up all non-baryonic
> dark matter, so we already have a case where the simplest explanation
> didn't turn out to be right.
>
> Of course, one should prefer the simplest explanation unless a more
> complicated one works substantially better
But it would be strange if DM consisted of only one type of
> the masses are too small to allow neutrinos to make up all non-baryonic
> dark matter, so we already have a case where the simplest explanation
> didn't turn out to be right.
>
> Of course, one should prefer the simplest explanation unless a more
> complicated one works substantially better
particle! Luminous matter is a mix of many particle types and
there is 4 times more dark than luminous matter, so shouldn't
it (DM) be entitled to consist of a complete new particle
zoo?
--
Jos
Phillip Helbig (undress to reply)
Jul 28, 2015, 12:35:01 PM7/28/15
to
In article <704afb3d-bb19-4da8...@googlegroups.com>, brad
candidate. Like MOND, the idea is that a modification of the laws of
gravity can give effects similar to dark matter.
<lbjohn...@yahoo.com> writes:
> > Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
> > OBSERVATIONS.
>
> mimetic dark matter http://arxiv.org/abs/1308.5410
Actually, this is more a theory of modified gravity than a dark-matter
> > Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY
> > OBSERVATIONS.
>
> mimetic dark matter http://arxiv.org/abs/1308.5410
candidate. Like MOND, the idea is that a modification of the laws of
gravity can give effects similar to dark matter.
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