Originally posted by twhitehead
You have it all wrong. Dark matter theories put a dark matter halo around the galaxy - not the solar system.
The reason why it doesn't all gather in the centre is because it is weakly interacting. In general, any particle in the galaxy orbits the centre. The only reasons for orbit decay (and thus migration to the centre) are:
1. collisions with other pa ...[text shortened]...
However the characteristics of dark matter suggest it consists of much more massive particles.
2. gravitational interactions which create some drag.Well there you are then. In multiple body systems orbits are not stable. This even applies to the orbits of the planets in the solar system, which are only quasi-stable. The earth's orbit changes over time. Gravitational interactions should drive some dark matter into the sun and the rest out of the solar system. Other interactions are not needed for this.
There is no indication of any WIMPs in particle physics experiments. Neutrinos come under the heading of Hot Dark Matter, they have an extremely small mass of the order of electron volts, but energies of the order of keV, and so tend to travel close to the speed of light. They easily have solar and for that matter galactic escape velocity. The dark matter models need the stuff to hang around, so they need very heavy particles of the order of 100GeV (for comparison an electron has mass about 500keV and nucleons slightly under 1GeV). These things should shed energy through gravitational interactions and clump in the centre of the sun. I have not read a convincing explanation as to why they don't.
Dark matter candidates include WIMPs, but no one has ever seen one (except for DAMA/Nal which no one else really believes), MACHOs but there aren't enough black holes, and the lightest Supersymmetric particles, but target space supersymmetry is all but ruled out.
One option would be a brane-world explanation where there is a mirror universe which interacts with ours gravitationally. One would need some explanation of why the mirror universe contained 5 times the mass of ours.
Originally posted by DeepThoughtCan you give me a times scale for this?
Gravitational interactions should drive some dark matter into the sun and the rest out of the solar system.
Note that there are still planets, and plenty of smaller objects around the sun, and there would be a lot more if it wasn't for collisions. On the galactic scale, I believe an even greater proportion of mass remains in orbit rather than in a central black hole. If anything the difficulty has been to explain why the central black holes are so massive, not the other way around.
There is no indication of any WIMPs in particle physics experiments.
Nor is there any indication that there aren't WIMPs. In other words all experiments to date are neutral on the matter.
The dark matter models need the stuff to hang around,
Yes, analysis suggests they are cold rather than hot.
These things should shed energy through gravitational interactions and clump in the centre of the sun.
Show me your calculations to that effect.
I have not read a convincing explanation as to why they don't.
Do you know enough relevant physics to think that they should, or are you just guessing?
Dark matter candidates include WIMPs, but no one has ever seen one ....
And until very recently, nobody had seen a Higgs boson. But that doesn't mean it wasn't there.
There is nothing whatsoever ruling out WIMPs or even indicating that they don't exist. There is however plenty of evidence for dark matter and WIMPs is currently the most viable explanation. It is not however conclusive.
Originally posted by twhitehead
Can you give me a times scale for this?
Note that there are still planets, and plenty of smaller objects around the sun, and there would be a lot more if it wasn't for collisions. On the galactic scale, I believe an even greater proportion of mass remains in orbit rather than in a central black hole. If anything the difficulty has been to explain why the ...[text shortened]... or dark matter and WIMPs is currently the most viable explanation. It is not however conclusive.
Do you know enough relevant physics to think that they should, or are you just guessing?My first degree was in theoretical physics from the University of Exeter and my D.Phil. (Oxon) thesis title was Polymerization of 2D Quantum Gravity Models. I've been out of the field for 15 years and what I did wasn't directly relevant to this question, but I'll stand by my intuition - there should be a significant amount of dark matter in the Sun - how much is another matter, I'd need to run a simulation and that is not the most straightforward thing to do from scratch.
If WIMPs are in the 100 GeV scale you'd expect some kind of signature at LHC, it depends on the cross-section for WIMP production. Always be sceptical of theories that require exotic matter. Of the various ideas I listed I like the brane world one best as it doesn't require any exotic matter, just at least one extra unobserved dimension.
By the way, the Higgs boson is a pretty much essential part of the electro-weak model. The observation of the W+/- and Z particles provided significant evidence that it was there.
Originally posted by DeepThoughtSince we haven't seen WIMP's in the LHC, could that mean they could still exist but be say 500 Gigs mass? Great degree! What is Oxon? What does polymerization of 2d gravity models mean? I think of polymerization as a chemical relation where units are added up making larger and larger molecules.Do you know enough relevant physics to think that they should, or are you just guessing?My first degree was in theoretical physics from the University of Exeter and my D.Phil. (Oxon) thesis title was Polymerization of 2D Quantum Gravity Models. I've been out of the field for 15 years and what I did wasn't directly relevant to this questio ...[text shortened]... l. The observation of the W+/- and Z particles provided significant evidence that it was there.
You say you have been out of the field for 15 years, what did you do when you finished your thesis, did you go industrial or stay in Academia?
Originally posted by DeepThoughtBut if you are that confident, it would be worth doing. If you are right, you would make a name for yourself in astronomy.
but I'll stand by my intuition - there should be a significant amount of dark matter in the Sun - how much is another matter, I'd need to run a simulation and that is not the most straightforward thing to do from scratch.
My intuition however tells me you are wrong. Without collisions there is no easy way to radiate energy. In order to the WIMPS to loose their potential energy and settle into the sun, they would have to radiate that energy. The possible mechanisms include gravitational interaction with ordinary matter, (and letting ordinary matter do the radiating), ejection of some WIMPs at escape velocities. But this latter process takes time, and is counteracted by other WIMPs entering the system from outside.
Ordinary matter achieves the radiation of energy through physical interaction which then results in photon emission.
Always be sceptical of theories that require exotic matter.
There is nothing exotic about WIMPS.
Of the various ideas I listed I like the brane world one best as it doesn't require any exotic matter, just at least one extra unobserved dimension.
That is more exotic than WIMPS. A WIMP is similar to a neutrino but heavier. It would fit reasonably well into the standard model.
By the way, the Higgs boson is a pretty much essential part of the electro-weak model.
Yes, it was predicted. That doesn't mean there aren't others out there we didn't predict. There is nothing whatsoever telling us what is out there and what isn't. Its pretty much unknown territory.
Originally posted by sonhouseWIMPs may be difficult to produce because of other reasons than mass. With a collision energy of 7 TeV they should be able to produce something with a mass up to 3.5 TeV. The W and Z particles have masses around 80 GeV and were spotted at LEP (beam energy of 200 GeV). The Higgs weighs in at 125 GeV, but it took them a while to spot it as the signature is quite weak. So there are a few possibilities. One is they don't exist, one is that they have a very low cross-section for production at accessible energies, and one is that they are so massive they are outside the range even of LHC. The difficulty with the last possibility is that then has to explain why they are so numerous. If they are that hard to produce they should be rare, rather than ubiquitous. One other possibility is that the Level 1 Trigger is incorrectly dumping all the relevant events as it misidentifies them as uninteresting, but I doubt that.
Since we haven't seen WIMP's in the LHC, could that mean they could still exist but be say 500 Gigs mass? Great degree! What is Oxon? What does polymerization of 2d gravity models mean? I think of polymerization as a chemical relation where units are added up making larger and larger molecules.
You say you have been out of the field for 15 years, what did you do when you finished your thesis, did you go industrial or stay in Academia?
Oxon is an abbreviation for Oxford. I spent a while trying to work out what to do next and ended up as a computer programmer optimizing signal and image processing code.
A naive approach to quantizing gravity in four dimensions doesn't work, but standard techniques do work for pure gravity in two dimensions. If one then adds matter fields it changes the geometry of the space. 1 species of electron makes the space more crinkly, more than that and the theory ceases to be renormalizable - meaning the theory doesn't exist. We used dynamical triangulation to produce a lattice model of 2 dimensional Euclidean gravity (that is to say there were 2 space like dimensions rather than 1 dimension of time and 1 space-like one) in order to study this. One considers all ways of gluing together triangles and takes a weighted average to extract the dynamics. We could show that the reason 2D gravity ceases to exist when coupled to too many matter fields is that the partition function (weighted sum) becomes dominated by surfaces that consist of thin branching tubes - the width of each tube is of the order of the lattice size (size of each triangle). My work was to show that the breakdown happened in various mean field inspired models. The term "branched polymer" is by analogy with the term from chemistry.
Originally posted by twhiteheadOne minor point, my argument should work at a galactic level, where one would expect the WIMPS to fall into the central black hole or be ejected from the galaxy. If they were uniformly distributed throughout the solar system then they should affect the orbits of the planets.
But if you are that confident, it would be worth doing. If you are right, you would make a name for yourself in astronomy.
My intuition however tells me you are wrong. Without collisions there is no easy way to radiate energy. In order to the WIMPS to loose their potential energy and settle into the sun, they would have to radiate that energy. The poss ...[text shortened]... thing whatsoever telling us what is out there and what isn't. Its pretty much unknown territory.
There is nothing exotic about WIMPS.Yes there is, they aren't in the standard model and can't simply be fitted into it. If a particle interacts via the weak force (like a neutrino) then it must come with a partner which is charged - this follows from the symmetry group that the weak force is based on. So there isn't just 1 extra particle hypothesised.
A very heavy neutrino implies a fourth generation of particles, so an extra electron and 3 new quarks. A fourth generation is "ruled out" at 5.3 sigma, so it's not quite good enough to be considered formally ruled out, but it is very very unlikely.
So if these aren't neutrinos they are something else, and we are either talking about something really exotic like an axion or the lightest supersymmetric state and as I mentioned earlier target space supersymmetry is all but ruled out.
That [brane worlds] is more exotic than WIMPS.You mean it involves extra currently undetected dimensions. Granted, but I find it less of a problem observationally. Brane worlds could solve various hierarchy problems as well.
There is nothing whatsoever telling us what is out there and what isn't.Well there are several bounds on what can be "out there" based on cosmology and we can pretty much rule out new matter which weighs less than 100GeV, as we would have seen that by now.
I take it that both dark matter theory and MOND currently each have their own strengths and weaknesses over the other making it currently hard to rationally determine which is more probable.
I have some questions:
Just like it has been possible to determine that a quantum entity can be a particle and a wave (matter-wave duality ) despite that being anti-intuitive, is it possible that they could somehow be both right! Or is there simply no way to reconcile the existence of dark matter with any MOND theory without either logical contradiction or some ridiculous assumptions that violates Occam's razor?
Does anyone here think it is scientifically credible that BOTH dark matter theory and MOND be wrong!? if so, can you give an example of a scientific theory with some possible credence that can assume both no dark matter and no MOND?
Originally posted by humyYour first sentence sounds fine to me.
I take it that both dark matter theory and MOND currently each have their own strengths and weaknesses over the other making it currently hard to rationally determine which is more probable.
I have some questions:
Just like it has been possible to determine that a quantum entity can be a particle and a wave (matter-wave duality ) despite that being anti-int ...[text shortened]... a scientific theory with some possible credence that can assume both no dark matter and no MOND?
is it possible that they could somehow be both right!I don't think they contradict each other. For example one type of dark matter is simply normal matter which is non-incandescent. Coming under the generic name MaCHOs - massive condensed halo objects - they are simply objects which are normal baryonic matter or black holes which are non-incandescent. All dark matter theories contain them, but they cannot exist in large enough quantities to account for the entire effect. They are entirely compatible with MOND. I'd imagine that it is possible to tune parameters in the models to allow them to co-exist without creating too much gravitational attraction.
...some ridiculous assumptions that violates Occam's razor?Occam's Razor is not a law of nature, it is a guideline when choosing a theory. It should only be invoked when there is no better way of distinguishing between a group of theories.
Does anyone here think it is scientifically credible that BOTH dark matter theory and MOND be wrong!?Well either one has to change the rules of gravity or introduce extra matter, the only other possibility I can think of is some previously unknown 5th force.
Maybe Quantum Gravity effects create extra terms in the Einstein Field Equations that do what we need them to - depending on if you think that counts as different from MOND or not. Also my brane world idea is different from standard dark matter theories, but would still, I think, count as a dark matter theory.
That leaves a 5th force which only manifests itself on galactic distance scales. I find that unlikely, but then again why not?
Originally posted by DeepThoughtWow, that's pretty heavy stuff. You didn't want to do post doc stuff, I gather.
WIMPs may be difficult to produce because of other reasons than mass. With a collision energy of 7 TeV they should be able to produce something with a mass up to 3.5 TeV. The W and Z particles have masses around 80 GeV and were spotted at LEP (beam energy of 200 GeV). The Higgs weighs in at 125 GeV, but it took them a while to spot it as the signature ...[text shortened]... field inspired models. The term "branched polymer" is by analogy with the term from chemistry.
Did you feel you didn't have anything more to add to Quantum gravity at that point?
Have you followed work along your lines and see where it led in the last 15 years? Anything spectacular stand out?
Originally posted by DeepThoughtinteresting
Your first sentence sounds fine to me.is it possible that they could somehow be both right!I don't think they contradict each other. For example one type of dark matter is simply normal matter which is non-incandescent. Coming under the generic name MaCHOs - massive condensed halo objects - they are simply objects which are normal baryon ...[text shortened]... nly manifests itself on galactic distance scales. I find that unlikely, but then again why not?
Occam's Razor is not a law of nature, it is a guideline when choosing a theory.
yes, I knew that. I didn't think it was a law of nature -I should know because I have intensively been studying that principle for years and probably have a far better idea of what it is than most laypeople.
By far the most common misunderstanding by laypeople is not that it is a law of nature (that is a new one on me! ) but that the principle simply says the simplest theory is always the most probable one -which is false!
If either dark matter or MOND alone explains all observations so that having just one renders the other as being totally unnecessary to explain any observation then Occam's razor would recommend rejecting the unnecessary one.
Originally posted by DeepThoughtTo be honest this is a bit beyond me, but I would think that if there were large numbers of all types of particles created shortly after the big bang, then the most weakly interacting ones would be the most numerous in the end because they wouldn't interact and change into others (typically less massive because of the way interactions work as the universe cools).
The difficulty with the last possibility is that then has to explain why they are so numerous.
Originally posted by DeepThoughtYes, but at slower rates than the physical matter does, and most galaxies are a long way from being completely exhausted of non-black hole physical matter.
One minor point, my argument should work at a galactic level, where one would expect the WIMPS to fall into the central black hole or be ejected from the galaxy.
If they were uniformly distributed throughout the solar system then they should affect the orbits of the planets.
Yes, they presumably would, but not enough to measure. They do affect the orbits of stars in the galaxy - which is one of the key pieces of evidence for dark matter.
I think the biggest misconception you have is about just how much dark matter there is in the solar system. It is not 63% of matter in the solar system. It is 63% of matter in the galaxy.
http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html
Also see:
http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/
Originally posted by twhitehead
Yes, but at slower rates than the physical matter does, and most galaxies are a long way from being completely exhausted of non-black hole physical matter.
[b]If they were uniformly distributed throughout the solar system then they should affect the orbits of the planets.
Yes, they presumably would, but not enough to measure. They do affect the or ...[text shortened]... html
Also see:
http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/[/b]
I think the biggest misconception you have is about just how much dark matter there is in the solar system. It is not 63% of matter in the solar system. It is 63% of matter in the galaxy.
Yes, and assuming dark matter actually exists, with the vast bulk of that dark matter not being within any solar system but between them and around the galaxy. This is simply because the total volume taken up by solar systems is a tiny fraction of the total volume of the whole galaxy and dark matter, assuming it exists, wouldn't be concentrated within the solar systems in particular.
According to that link, 1.07 X 10^20 kg of dark matter have been captured by our solar system. The mass of our solar system (excluding dark matter ) is about 1.99 X 10^30 kg so that should mean that the percentage of dark matter in our solar system out of all the matter would be very roughly about 0.000000005% I doubt that such a tiny percentage would have a easily detected effect on the orbits of our planets.
Originally posted by twhiteheadThis is another thing. WIMPs have to be pretty stable. At very high energies everything behaves as if it's massless. In units where the speed of light is 1, E^2 = p^2 + m^2, and if p >> m, we have E = p. So all particles should be created in roughly equal numbers. If WIMPs have a mass of ~100GeV then they stop being produced as the temperature drops below around 10^15 K. Now we have a problem, normal Baryonic matter has a slight asymmetry between matter and anti-matter which prevents all of it from annihilating. If this does not apply to WIMPs they would not exist any more. If they aren't at least mostly annihilated then they would dominate the mass of the universe by something like 10 orders of magnitude. But then why is there more dark matter than there is of the ordinary stuff? If they exist in the same numbers they should make up 100 times the mass of normal matter. What is more there has to a quantum number (let's call it darkness number) to protect them from decaying into lighter particles. As a hack's guess, given a muon has a half-life of 2 microseconds and decays via the weak interaction you'd expect a WIMP to decay in of the order of a nanosecond. A conserved quantum number implies a symmetry, so what is the symmetry?
To be honest this is a bit beyond me, but I would think that if there were large numbers of all types of particles created shortly after the big bang, then the most weakly interacting ones would be the most numerous in the end because they wouldn't interact and change into others (typically less massive because of the way interactions work as the universe cools).
To reply to your other post, that solves my problem as to why the sun hasn't gone out. However my mechanism was correct, they do get trapped in the centre of the system.