|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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. |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Complete dark matter theory opens door to weight/energy potential(Dark matter is considered to be the top mystery in science today, solved,really.) And more finding on dark matter ebergy science from the 1930's. | [email protected] | Astronomy Misc | 0 | September 14th 08 03:03 AM |
Just solved the whole dark matter puzzle with my last question (belowDark Matter/Mark Datter thread) | gb[_3_] | Astronomy Misc | 23 | April 21st 08 09:45 PM |
Basic Dark Matter question. | Charles Cagle | Astronomy Misc | 0 | July 23rd 05 12:42 AM |
Basic Dark Matter Question | Rantrod | Research | 41 | June 26th 05 11:26 AM |