Does dark matter come in two types?
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From: Yousuf Khan on 3 Jul 2010 07:49 On 7/3/2010 12:18 AM, Raymond Yohros wrote: > On Jul 2, 6:47 am, Yousuf Khan<bbb...(a)spammenot.yahoo.com> wrote: >>> do you really think dark matter stay with the galaxy? >> >> Yes, that is their only purpose in life: to beef up the mass of >> galaxies. Intergalactic gas is actually more massive than the galaxies, >> but not very dense. Dark Matter is supposed to stay with the dense >> regions that are galaxies. >> > > but that beefing up comes from baryonic matter > and gravitation > observation shows that stars do not move faster near > the centers of galaxies all the times > and thats when people start speculating about > intergalactic dark matter. There are alternative theories to Dark Matter within the galactic realm that involve theories of modified gravity instead. One such early theory is called MOND, a couple of newer derivatives are called TeVeS and MOG. A third class of theories is the unified Dark Fluid theory, which incorporates all of the Dark Matter, Dark Energy, modified gravity theories into a single theory. The problem with Dark Matter is that it really doesn't do a good job of explaining galactic rotational curves, precisely the problem which it was originally designed to solve. MOND is far better in that realm. Dark Matter instead seems to be better at explaining lensing in galaxy clusters, which is one step above galaxies in the cosmic hierarchy. It fits observations of the Bullet Cluster much better than other theories; however, there is one galaxy cluster which defies explanation by Dark Matter (and most other theories) which is the Train Wreck Cluster/Abell 520, this cluster has gravity lenses in places there shouldn't be gravity lenses. Modified gravity theories on the other hand don't envision there being new of matter, but just an alternative behaviour for gravity itself. It envisions that as matter density in certain regions become smaller, e.g. in the disks of galaxies, that gravity starts to exert a stronger hold on that matter. The original modified gravity theory, MOND, was found to be an excellent at predicting velocity curves of most standard spiral galaxies, however it was found wanting when predicting the gravitational lensing effect of the galaxy clusters. TeVeS was the next best known modified gravity theory. It was as good as MOND in predicting spiral galaxy curves, naturally since it was built on MOND. It was better than MOND in explaining cluster lensing. And it was claimed to be able to explain the Bullet Cluster as well as Dark Matter can, but no word on whether it could explain the Train Wreck cluster either. TeVeS is a bit more ambitious than MOND or Dark Matter too, as it also tries to model Dark Energy! TeVeS however was found to suffer from two fatal flaws during simulations. (1) Simulations showed that it was too good at forming black holes in space, that is black holes would appear at random all throughout the universe inside galaxies, including within our own solar system (a black hole could appear inside our solar system every 10 hours!). (2) When they simulated, the evolution of a section of the universe out to about about 3 billion light years, they found that it wasn't as good as simulating it as General Relativity with a Cosmological Constant, or another less-ambitious modified gravity theory called "f(R) theory". For the moment, TeVeS looks like it will need to be sent back to the shop to be fixed up. Dark Energy theories are a bit more democratically spread around, with no theory taking a commanding lead yet. The theories for Dark Energy are (1) General Relativity's Cosmological Constant, (2) Quintessence, (3) Phantom Energy, and (4) f(R) theory. The Cosmological Constant is currently the favoured theory in Dark Energy, but it's problem is that it's not got an explanation of where this energy is coming from. The other theories try to explain the origins of this expansion. Dark Fluid on the other hand, is trying to combine all of these theories together into a single continuum of a theory. So far, because Dark Fluid has both a repulsive and an attractive force component, it has avoided the "black hole trap" of TeVeS. The authors feel confident that it can explain the Train Wreck cluster unlike any other theory yet proposed. But it's too new to have been thoroughly simulated yet. > >> >>> wimps may not exist at all! >> >> We're pretending here, for argument's sake. That's all. >> > > yes but for how long will this go on? > not even the LHC has shown any sign that this > particles may exist It should be noted that the current theories of Dark Matter are not the theories of Dark Matter that existed more than 15 years ago. These are a new breed of Dark Matter theories with little or nothing in common with the original ideas of Dark Matter. When the Dark Matter problem was first noticed in 1930's and proposed, the original thinking behind Dark Matter was that we don't see additional matter because our telescopes aren't powerful enough yet. Basically an instrumentation problem. That this Dark Matter would eventually turn out to be relatively normal stuff like neutron stars, old white dwarfs, and black holes, too dim to see at that time. These were the MACHO Dark Matter theories. As telescopes got better and better, it was found that we still weren't finding enough of these types of objects to account for the rotation curves. So eventually, the paradigm within Dark Matter shifted from MACHOs to WIMPs. There is a lot of hopes and dreams tied up in the LHC. Particle physicists want it to discover Supersymmetry (SUSY), so that there will be a whole families of brand new particles to discover and then they can justify asking for even newer more powerful accelerators. Astrophysicists are hoping that one of those SUSY particles will explain their WIMPs. If they've bet wrong, then they're going to have to go back to the drawing board, or at least start looking at the alternative theories a bit more seriously. >>> neutrinos may not be the kind of dark matter >>> everyone expects but they move and spread out >>> like light! >> >>> almost nothing stops them >> >> That's why they were known as the "hot" dark matter candidates, since >> they are relativistic. >> > > maybe there is no such thing as cold dark matter > just normal hot and cold matter and a huge load > of neutrinos! Neutrinos, being relativistic as they are, wouldn't affect the shapes of galaxies or galaxy clusters, but they would affect the shape of the entire universe. Yousuf Khan
From: Yousuf Khan on 4 Jul 2010 03:52 On 7/3/2010 4:06 AM, BURT wrote: > Dark matter is wrong because it would be everywhere with an origin > with normal matter gravitationally coallescing. It would make up most > of everything including the Sun and solar sytem. But it does not. > > There is a better explanation than dark matter for what we see. > > Mitch Raemsch Well, the point of WIMP-based DM is that it is everywhere, but in very low densities. And because it doesn't interact electromagnetically, it can't slow down to coalesce, so it stays close to its average density no matter where it is; whether it's on the outskirts of a galaxy, or near a blackhole. Yousuf Khan
From: eric gisse on 6 Jul 2010 01:47 Yousuf Khan wrote: [...] > The problem with Dark Matter is that it really doesn't do a good job of > explaining galactic rotational curves, precisely the problem which it > was originally designed to solve. MOND is far better in that realm. Bad. BAD. You know better. Dark matter explains the rotation curves quite well. > Dark > Matter instead seems to be better at explaining lensing in galaxy > clusters, which is one step above galaxies in the cosmic hierarchy. It False equivalency. MOND *COMPLETELY FAILS* at explaining lensing, while Dark matter explains both rotation curves and lensing observations. Not just cluster mergers, but ALL lensing observations. BTW - TeVeS still requires dark matter, just a significantly smaller amount. > fits observations of the Bullet Cluster much better than other theories; > however, there is one galaxy cluster which defies explanation by Dark > Matter (and most other theories) which is the Train Wreck Cluster/Abell > 520, this cluster has gravity lenses in places there shouldn't be > gravity lenses. We've been over this. Abell 520 does not 'defy explanation by dark matter' in any way. > > Modified gravity theories on the other hand don't envision there being > new of matter, but just an alternative behaviour for gravity itself. It No. You are wishing and hoping. The fields the modified theories of gravitation throw out there are not sourced from the matter stress tensor. > envisions that as matter density in certain regions become smaller, e.g. > in the disks of galaxies, that gravity starts to exert a stronger hold > on that matter. Which sounds nice until you think about the implications. I wonder when someone is going to throw the theory through something similar to the Millennium Simulation or its' followup. > The original modified gravity theory, MOND, was found to > be an excellent at predicting velocity curves of most standard spiral > galaxies, however it was found wanting when predicting the gravitational > lensing effect of the galaxy clusters. Or describing the dynamics of globular clusters. > > TeVeS was the next best known modified gravity theory. It was as good as > MOND in predicting spiral galaxy curves, naturally since it was built on > MOND. It was better than MOND in explaining cluster lensing. And it was > claimed to be able to explain the Bullet Cluster as well as Dark Matter > can Generating the lensing peaks is a good start. But all that is done by TeVeS is to shifting the argument around a little bit. Now you have to ask where the arbitrary fields TeVeS uses happen to come from, and why it makes 'more sense' than dark matter. > , but no word on whether it could explain the Train Wreck cluster > either. TeVeS is a bit more ambitious than MOND or Dark Matter too, as > it also tries to model Dark Energy! BFD? > TeVeS however was found to suffer > from two fatal flaws during simulations. (1) Simulations showed that it > was too good at forming black holes in space, that is black holes would > appear at random all throughout the universe inside galaxies, including > within our own solar system (a black hole could appear inside our solar > system every 10 hours!). (2) When they simulated, the evolution of a > section of the universe out to about about 3 billion light years, they > found that it wasn't as good as simulating it as General Relativity with > a Cosmological Constant, or another less-ambitious modified gravity > theory called "f(R) theory". Imagine that. > For the moment, TeVeS looks like it will > need to be sent back to the shop to be fixed up. Any theory that seeks to replace GR will have to be something along the lines of an f(R) theory that preserve GR's minimal curvature coupling while not deviating too strongly. Essentially a perturbation upon GR. > > Dark Energy theories are a bit more democratically spread around, with > no theory taking a commanding lead yet. The theories for Dark Energy are > (1) General Relativity's Cosmological Constant, (2) Quintessence, (3) > Phantom Energy, and (4) f(R) theory. The Cosmological Constant is > currently the favoured theory in Dark Energy, but it's problem is that > it's not got an explanation of where this energy is coming from. The > other theories try to explain the origins of this expansion. > > Dark Fluid on the other hand, is trying to combine all of these theories > together into a single continuum of a theory. So far, because Dark Fluid > has both a repulsive and an attractive force component, it has avoided > the "black hole trap" of TeVeS. The authors feel confident that it can > explain the Train Wreck cluster unlike any other theory yet proposed. > But it's too new to have been thoroughly simulated yet. Except the 'dark fluid' model is mislabeled as there is no fluid at all present, and is merely a recast of TeVeS as something else with even more tunable parameters. Dark fluid has a Lagrangian with _AT THE VERY LEAST_ the vector field with 4 dynamical degrees of freedom, the scalar field with its' degree of freedom, and rather arbitrary couplings to the Ricci tensor and higher order factors of the divergence of the vector field. It is a numerologist's wet goddamn dream. It is impossible to pin such theories down because there are so many goddamn tunable parameters. [...] > Neutrinos, being relativistic as they are, wouldn't affect the shapes of > galaxies or galaxy clusters, but they would affect the shape of the > entire universe. Neutrinos are relativistic. Dark matter is not, otherwise it wouldn't remain bound. > > Yousuf Khan
From: Raymond Yohros on 20 Jul 2010 13:13
On Jul 3, 6:49 am, Yousuf Khan <bbb...(a)spammenot.yahoo.com> wrote: > There is a lot of hopes and dreams tied up in the LHC. Particle > physicists want it to discover Supersymmetry (SUSY), so that there will > be a whole families of brand new particles to discover and then they can > justify asking for even newer more powerful accelerators. > Astrophysicists are hoping that one of those SUSY particles will explain > their WIMPs. If they've bet wrong, then they're going to have to go back > to the drawing board, or at least start looking at the alternative > theories a bit more seriously. > not discovering anything its also a great discovery because it helps to put the cards on the table. if something does not exist, will be harder to discover than the most undetectable particle ever! i think the LHC will be a great help in understanding how BH dynamics can explain with elegance the cosmos complexities we see today. new neutrino findings will also come out of this great machine. regards r.y |