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Dark matter particles

  1. Feb 24, 2010 #1
    if dark matter they beleive is not made of atoms it must be made of some substence. is it made of basicly a big bubble of some kind of bubble of particles? and how can this be are the particles made of string also like atoms , according to string theory that is
     
  2. jcsd
  3. Feb 24, 2010 #2
    Who is 'they
    Definine "some substance"
    How do you arrive at a "bubble of particles"?
    Why do you feel this is related to string theory?
     
  4. Feb 24, 2010 #3

    jambaugh

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    A note on definitions....

    Dark matter is simply non-luminous matter i.e. non-stellar matter. In this sense you and I are made up of "dark matter". One may consider however exotic dark matter in the form new, as yet undiscovered, elementary particles. This is much more speculative and has been given various descriptive names such as WIMPS=(weakly interacting massive particles).

    You should read up on the history of the "missing matter problem" which is a problem of computer simulations (based on simplifications of Einstein's full theory of gravitation) not matching observed data about the current age and size of the universe and also the rotation rates of galaxies.

    Note that recent observations indicate much more traditional (baryonic) matter (protons and neutrons) in the space between stars and galaxies than first thought.
    See: http://dsc.discovery.com/news/2008/05/21/universe-missing-matter.html" [Broken]

    It is also possible that the simplifications of the gravitational theory used to make the calculations are introducing sufficient error, or that the theory itself is sufficiently off, that the "missing matter" may simply be the need for a correction in the calculation of the dynamics of galaxies and the universe as a whole.

    In short there may not be any need to invoke exotic forms of dark matter once theories and observations are further refined. Time will tell. One should also be aware that as scientific speculation filters down to the laymen through various levels of media, the more fantastic and romantic notions (such as exotic matter and "dark energy") get much more "air-time" relative to e.g. questions of systemic error or widths of uncertainty in calculations.
     
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  5. Feb 24, 2010 #4
    ok what would dark matter made if it is real. would be be made of particles given off by the vibrations of anergy strings like photons and such
     
  6. Feb 24, 2010 #5
    They wouldn't emit photons, and string theory isn't the commonly accepted theory of everything you seem to think it is. Jambaugh answered your question perfecftly, giving you the best candidate particle time (WIMPs), and a likely other scenario which arises from the measurements and the math (no exotic dark matter).

    Remember, dark matter would participate in normal gravitational interactions, but nothing else. There is no complete or even well accepted physical theory to explain that, outside of pure conjecture.

    EDIT: Just for the record, asking what something would be made of, 'if it were real' isn't a meaningful question. The answer coming from pure imagination could be anything! This is partly the source of the joke about Black Holes containing "green slime and lost socks".
     
  7. Feb 24, 2010 #6
    What about dark energy? The opposing force that is theorized to be pushing the universe apart. Little is known about these forces and what comprise them, but it seems there is definitely some sort of energy there.
     
  8. Feb 24, 2010 #7
    That article is a very poor summary of what has been found. Right now the best guess for the fraction of the universe which is made up of baryons (i.e. ordinary matter) based on things like the amount of deuterium in the universe and the CMB is 0.04. The curious thing is that of this 4 percent of the universe, we have only been able to account for a small fraction of that in "bright matter", and it appears that the majority of ordinary matter in the universe is also dark.

    Now if you find more "ordinary dark matter" that still doesn't change the fraction of "weird dark matter" that you need to explain cosmological observations.

    At this point it's unlikely. The different between what we see and what we expect to see if the universe is not made of weird dark matter is so different that it's improbable that it's the result of refinement. It's possible that there is something very basic that we are not understanding, and that we are fundamentally misunderstanding what we are seeing, but I think that simply refining the observations isn't going to help. The difference is much too large.


    That happens sometimes, but it's not happening in this particular situation. It's good to read the original paper about dark energy because they go through all of the possible explanations for why this could be observation error, and conclude that the effect is much too large to be an observational issue.

    http://adsabs.harvard.edu/abs/1998AJ....116.1009R

    Since 1998, the observations are gotten more clear that something weird is going on. It's either dark matter/dark energy or some gravity effect, or something else that really weird.
     
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  9. Feb 25, 2010 #8
    Call me crazy but seeing as how it is gravity that shapes space-time and it is caused by mass, then why are we looking for a new particle? If every particle is skidding along curved space-time, and that curve "is" gravity then gravity is an inherent property of all particles isn't it?
     
  10. Feb 25, 2010 #9

    jambaugh

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    There are two distinct sources for inference about dark matter, exotic or not. One is the dynamics of galaxies, in particular the distribution of velocities. The second is the cosmological models which I view are much more speculative given one is extrapolating theory to the entire cosmos. In the first case the missing matter problem seems to be solved by more accurate application of gravitational theory to computational models and updated empirical evidence of interstellar hadronic dark matter. See e.g. http://meetings.aps.org/Meeting/APR06/Event/47584" [Broken].

    Now these references have been discussed already in this forum and are by no means the final word. I simply point out that as far as galaxy rotation goes, the predictions and scope of current gravitational theory must be fully mapped before we infer the existence of exotic dark matter (EDM) and until then there may be no need, and even possibly evidence for its absence within the structure of galaxies.

    It is from this end (galactic dynamics) far from certain that EDM is necessary or inferred to exist.

    As far as BB cosmology goes, I am still waiting for a consistent predictive theory rather than the ad hoc fitting used to infer dark matter and values for the cosmological constant (a.k.a. dark energy). Especially worrisome is the "artificial" pasting together of QFT and GR cosmology short of the ever elusive unified theory.

    This is not just a matter of observational error or insufficiency. It is the number of layers of extrapolation of theory one must invoke to project back to the BB and then forward again to current observations.

    Certainly EDM is a possibility, EDM as the majority of matter in the universe is a possibility. However until direct observation such as is being now attempted (http://blogs.physicstoday.org/update/2009/12/a-hint-of-wimps.html" [Broken]) gives significant evidence I think that we should remain skeptical and view EDM as speculative (though certainly valid speculation).

    My main concern is that, especially to the semi-layman the terms "dark matter" and "exotic dark matter" have become synonymous when they are clearly not.

    That is refinement of observation. But the inference is filtered through plural levels of theoretical interpretation and refinement of theory may (and I suspect will) eliminate the need for EDM.

    Agreed. I think this possibility is near certainty...as it always will be as we continue to advance our understanding. My suggestion was not that refinement of observation alone would solve the MM problem but rather that the interacting refinement of both theoretical models with empirical data may very well do so (with the elimination of the inferred EDM).

    As I see it now I'd wager its about 50-50 whether we will discover EDM or eliminate it from consideration in the future. Of course I'm not the best "bookie" for this bet having only sniffed around the periphery of the research. But I do have some perspective and am willing to take (small) cash wagers on it.

    I won't be sold until I see repeatable collider experiments with signatures for an EDM particle on par with the original evidence used to infer the existence of neutrino's.
     
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  11. Feb 25, 2010 #10
    I see the cosmological models as being a lot *less* speculative since they are based on pretty well know laws of gas dynamics and nuclear fusion. There are fewer moving parts.

    But then you still have a mystery of explaining the CMB background and deuterium abundances

    Correct.

    You get that with nucleosynthesis and CMB predictions. None of these require any exotic physics in the model.

    All that is totally irrelevant for dark matter predictions. The issue with dark matter is in order to get a prediction that something is weird, you can basically ignore all physics that we can't replicate on the earth. What happens at extremely high energies doesn't matter. The weird results come out of either radiation physics or nuclear physics. Quantum cosmology is completely irrelevant.

    There really aren't that many layers. To get CMB results, all you need to assume are standard gas dynamics and radiation physics, none of which is exotic.

    Yes they are. The consensus within the cosmological community is that "dark matter" *cannot* be baryonic without some extremely odd physics.

    That's not true. There *aren't* that many theoretical assumptions that you have to make in order to get to the result that dark matter is exotic. And the theoretical assumptions are firm enough that it one of them *were* wrong, it would be major news. For example, it's quite possible that we mis-understand fusion in some basic way, which will explain the presence of deuterium. But not understanding fusion is a big, big deal.

    What I'm saying is that we are well passed the point where a small refinement of theory can get you non-exotic dark matter. It's just not possible to get baryonic dark matter unless we fundamentally misunderstand something extremely basic. There is just no gap for the elephant to hide.

    And this is not the consensus of cosmologists. There is simply no way that you can get non-exotic dark matter by tweaking current cosmological models. It's just not possible. I'm not saying that the universe *isn't* made of baryonic dark matter, I'm saying

    1) that I happen to think that you can't get baryonic dark matter without a *big* change in how our models works and at this point the possibility of exotic dark matter is less odd than than those changes, and
    2) that my view on this is the consensus view among astrophysicists.
     
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  12. Feb 25, 2010 #11
    Never has a theory been more ad hoc than literally seeing holes, and inventing invsible matte to fill them.

    It may be true, but then again, it may all be "green slime and lost socks."

    Dark Matter seems to be a useful theoretical placeholder to explain some very odd behaviour and maybe a long-lived placeholder at that! Never should we forget however, that we're talking about a term that refers to nothing. Try to pin down what it is, and it all comes back to someone filling holes with words, and not real physical concepts.

    What's so wrong about saying, "We don't understand X right now, and instead of a bum rush to figure out some star trek theory to explain X, lets try to re-examine our approach, while we also give my theory some examination."
    In other words... is it a sin to say, "We don't have a clue what, or if anything is out there. Some kind of gravitational interaction is occuring, but no source of mass is evident. We postulate that it is our inability to detect the source of this behaviour (of galaxies, clusters, etc) that is the problem." END OF SENTENCE. No "Oh yeah, but for now lets just call it dark matter and throw it to the media and public like raw beef."

    Irresponsible is what that is... then we wonder why the public has such misconceptions about the subject. We don't actually have to BUY INTO every provisional theory just because its du jour.

    I accept that the total apparant mass of the universe is far greater than can be accounted for by baryonic matter. That is a QUESTION without an answer, and to pretend otherwise is to lie like a politician.
     
  13. Feb 25, 2010 #12
    Because we think we do understand what is going on. Now obviously there is something we don't understand, but it's not obvious what it is. No one has come up with an explanation of dark matter with non-strange physics

    In order to get a cosmological matter, you have to have

    a theory of gravity
    a theory of how gas behaves
    a theory of how radiation behaves
    a theory of how nuclear reactions occur

    Right now people are focusing on gravity because that's the part that we think we understand the least.

    The problem is that we have so much data coming in, that "we can't see anything" is not an excuse. The fact that we *can't* see something is significant, because our theories say we *should* see something, and the fact we don't is mind-blowing. It means that we basically don't understand what is going on.

    What I do believe is the *either*

    1) most of the universe is not baryonic, or
    2) there is something very seriously wrong with our models of the universe.

    I don't know which, but my point is that just saying "the problem will resolve itself if we have more data or more accurate models" just won't work. We have enough data and our models are accurate enough to know that we have a pretty serious problem.
     
  14. Feb 25, 2010 #13
    Hmmm... I see your point, but 1 and 2 are choices that one way or another, will require a massive change in our view of the universe. The very notion that the universe we detect (Whole EM spectrum) doesn't represent even a meaningful MINORITY of its mass is as you say, astonishing.

    That said, I agree with your final statement completely, either the something is out there that isn't baryonic, or we're missing something huge (or small, but pivotal).

    What you said about the confusion as to the source of... well... the confusion clarifies some matters for me. Until Grav, Gas, Rad, Nuc are ruled out as candidates for these imbalances, other theories are premature. I am 100% on board with that, unless someone produces something shocking and verifiable from the fringe (doubtful).

    Thanks!

    So... Given the problem... I'd guess a combo of 1 & 2 that you stated are probably true, implicit in which is that #2 includes this meanignful explanation Dark Matter, but also that it is at least as predictive as GR/QM.

    Wow... glad I'm not researching THAT! Or maybe I'm jealous? I can't tell... hmmm
     
  15. Feb 25, 2010 #14
    Goes with the territory. The reason that dark matter is preferred is that it's the least weird explanation.

    This is one problem that I have the popular press coverage of dark matter. Usually when people talk about the early universe they talk about weird physics like quantum gravity or string theory or something that we *know* we know nothing about. The problem with dark matter is that it involves pretty firm everyday physics. The temperature of the cosmic background radiation is about 3000 K. I'm staring at a light bulb that's more than 3000 K. Gas dynamics. I'm running through a gas. Self-gravitating gas. Look at the sun.

    Also if you look at deuterium abundances. All that involves are standard nuclear reactions. We think we understand those. If it turns out that we don't, then we really have bigger problems than the early universe because it means that those nuclear missiles that major countries have hidden away will either not work at all or a single bomb could destroy the planet. Suffice to say that there are *lots* of people that have spent a lot of time and effort trying to make sure that we really do understand what happens when you mash two hydrogen atoms to form helium, and there aren't likely to be too many surprises there.

    And all of the computer simulations that people use to calculate the early universe are basically the same sorts of computer programs that people use to simulate hydrogen bombs to make sure that they go boom if someone presses the button (and won't go boom of they don't), People are confident enough about the computer simulations that there is an international test ban treaty against exploding nukes. So if it turns out that we are wrong about the early universe, then they will instantly change the world balance of power.

    Every wonder *why* you see so many astrophysicists work at Los Alamos?

    And the physics is pretty straightforward. Deuterium burns easily. If you have a lot of ordinary matter then you end up with more nuclear reactions and less deuterium. If you assume that most of the universe is something that doesn't participate in nuclear reactions, then you have less burning before the universe cools and you end up more deuterium. Couldn't you produce deuterium after the big bang? People have tried. The basic problem is that if you hit two atoms hard enough to make deuterium it's hard enough to cause it to fragment before you make large amounts of deuterium.

    The physics that gets you galaxy distributions is also pretty straightforward. The thing about normal matter is that you can send sound waves through ordinary gas. The denser the gas, the stronger and the faster the sound waves can be. If you assume that the universe is all baryons then you end up with sound waves that are stronger than what we see from the distribution of the galaxies. So whatever makes up most of the universe, it's something that doesn't conduct sound very well.

    I don't see why. You don't know what's going on. You come up with a new idea. You get a paper out of it.

    Everyone is on the fringe. The point that I'm trying to make is that people that assume that most of the universe is made up of baryonic matter, have to make some nutty assumptions to get things to work. The big problem is that things don't work even if you *do* make nutty assumptions.

    It's not enough to just say "maybe your basic assumptions are wrong*. We know that. Now if you can guess as to *which* assumption is wrong, then you might get somewhere. Maybe if you have uneven temperatures, you can manage not to burn deuterium. Maybe..... Scribble, scribble. scribble. It's might make enough difference so that you can write a paper.

    In order to work out what is going on you have to do the math. You come up with a model, work through the model. About 90% of the time, it just won't work. If you can get it to the point where it isn't *obviously* wrong, you publish.

    If you understood what was going on, there wouldn't be anything to research.
     
  16. Feb 25, 2010 #15
    I see... I didn't realize the everyday of physics so resembled other branches of scientific research. You're right, I am influenced by popular (not always media, lets be honest sometimes professionals too) views.

    If I'm correct, then what you're saying is that this is an issue of nuclear physics, BB nucleosynthesis and all of the strange imbalances (matter over antimatter, non-baryonic over baryonic, the emergence of deuterium) and not open to the "weirdness" of the theoretical extremes.

    So, let me ask you, as you clearly have a grip on this... if I'm a numerical relativist crunching HPDEs for the 2 body problem, I get that. Am I to understand that research in this area is really research AROUND this area, which is expected to produce incremental results?

    Oh, and as for confidence in computer models, I have my doubts. Confidence in models led to issues such as Ivy Mike's overyield, although with the models of the day... fair enough. However, to pretend that the test-ban treaty is a reflection of our confidence, I would point to the NIF, which presumably must be part of the research into the nuclear fusion and related topics.

    We need the real thing, or something closer. The history of these experimental apparatus is that new information is gleaned only once the experiment begins. Bose-Einstein Condensates (He3 superfluid for instance) exhibited behaviour that was not predicted, if only because no one ever thought of the physical implications. Obviously a mechanism DOES exist to either preserve Deuterium, or create it outside of BB nucleosynthesis. That said, I don't feel the need to fill the space with two words which have no physical or scientific meaning: Dark Matter.

    This brings me back to my original point, which is that at the end of the day you either look at this as swiss cheese, needing holes to be filled, or as you do; in which the issue is that the HOLES are what have been defined, but we are yet to grasp the nature of the surrounding cheese. I appreciate the conundrums, and I have no unique insight of course, but in the absence of anything real I would rather not preconcieve what would be the majority of matter/mass in the universe.

    Poor sound conduction, participation in the gravitational, but not the EM, weak or strong nuclear forces? That's not Dark Matter, that's a question without an answer, and framing it as anything else is dishonest and leads to the kind of media scrum we see around these issues.

    Those computer simulations are great, but they still don't answer some basic questions about saaaaay... the exact means by which the secondary of a Teller-Ulam design is compressed. Is it tamper ablation? Radiation pressure? Some mystical plasma created by polystyrene foam as some on Wikipedia seem to believe? (tamper ablation is my view) The issue is that the computer models aren't conclusive.

    So... I'm going to be very patient when it comes to understanding how non-baryonic matter would dominate the universe. If that's true, then I think I sprained my anthropic principle.

    ...And yes, I used a cheese analogy. Gruyere baby.
     
  17. Feb 25, 2010 #16
    If you want to make up something about quantum gravity, you can. We don't know what's going on there. if you want to make up something about how hydrogen behaves at one atmosphere at 400 Kelvin, you can't.

    The dark matter problem is (as far as we know) unrelated to the matter/antimatter problem. By the time the nuclear reactions start, whatever caused there to be more matter than anti-matter has already happened. Part of the problem with dark matter is that there aren't that many places to hide the elephant.

    It's not directly related, but you never know. One of the ideas that Wiltshire has come up with is that lumpiness in the universe causes effects that create the illusion of an accelerating universe. It's possible that if you are a numerical relativist, you'll find something related. Or not.

    If confidence in nuclear weapons was based on things like NIF, then the Russians and the Chinese would have never agreed to the comprehensive test ban treaty. Russia and China have decent computers and they can feed the results of their experiments into their computer models. Computer models generally have a lot of fudge factors which you can set to make them match observations.

    Computers models work start by assuming X, Y, and Z. Crunch numbers. Do we get reasonable results. No. Keep tweaking X, Y, and Z until you something that matches reality.

    You find out what something is by eliminating what it isn't. You assume the universe is made up I don't know, heavy tau neutrinos. See what happens. If it doesn't work try something else (and it doesn't because you can calculate the rate at which tau neutrinos are formed by nuclear reactions, and if you multiply this by the number of tau neutrinos, the universe collapses).

    Over time, the gaps become smaller and smaller.

    Dark matter could interact with the weak nuclear force. It's a question with an answer. As of 2010, we don't know what the answer is, but I think we'll know by 2025.

    The people that write the computer models, certainly know how an H-bomb works. The problem is that knowing "in general" how an H-bomb, the early universe, or a car works isn't good enough. You need to know how the the burning within the piston produces nitrogen-based exhaust gasses so that you can tune the engine.

    The thing that most people don't realize is that we are way, way, way beyond general vague ideas about how the early universe works. We have enough data from the WMAP so that we are at the nuts and bolts level. And something basically doesn't make sense.
     
  18. Feb 25, 2010 #17
    Agreed, and thanks for that because if they could, they would.

    I've seen the WMAP data and computer models and it DOES seems as though something very like clumped matter dominates the space between galaxies and shapes clusters. I rememer when the tau neutrino bit was introduced, but that never seemed likely. WIMPs have always seemed the most appealing, if only as a catch-all.


    I didn't mean to imply that something as extreme as the NIF was required for an understanding of nuclear weapons. To be blunt, the first poor bastard who experienced a criticality accident learned a fair deal I'm sure. What I mean is that to understand the fusion of Dueterium and Tritium at least, and therefore nuclear fusion in general, is going to require more than computer models. The fudge factor you mention can be part of the kaleidoscope that many added "tweaks" at different stages can bring.

    For god's sake, we have String Theory, which you could argue is a triumph of computation, and an utter failure of physics. Your argument about the nature of computer modeling is practical in the field of nuclear fusion, but can be a slippery slope. Remember, as you said computers take xyz and crunch, they don't crunch a or b if you don't know to tell them to. Maybe you can make a realizable COMPUTER model by just tweaking x y and z, but if the issue really lies outside of those variables then we have a problem. A failed simulation is fine, but a success is necessarily questioned because it may be that it's just a toy model and not physical realizable.

    Now I can hear you saying that we have the physical models for a lot of this, and data from CMB to back up even more, and I agree. My point is that none of the methods of achieving progress that you describe match ones that have lead to breakthroughs such as the mapping of the CMB. In fact, it's the overuse of those methods that gives us ridiculous crap such as Dark Flow and other pseudoscience.

    A powerful tool, those computer models... but note that when we want to develop a truly novel package we seek to test. I refer to the so called "nuclear bunker buster" idiocy of the Bush Admistration (who apparantly had no concept that an uncontroled groundburst is BAD). I recognize that a majority of later testing in Russia and the USA was political theatre, and not legitimate research into the mechanics of nuclear weapons (although they may have been used as testbeds for high energy physics). That said, the basis of the computer models lies in an ability to test against a control so that you KNOW if you have a working fudge factor... or just a steaming pile of fudge.

    I agree that we'll probably never need to test for the sake of fissile weapons again, but when it comes to modeling the problem of why there is so much dueterium, I'll withold judgement on the incrementalism until I see a baby star many, many times.


    Oh. I did not know that. If it interacts with the weak then shouldn't it undergo beta decay?... is that the nature of the test?



    They understand the raw mechanics and what is REQUIRED to fuse deuterium and tritium from lithium deuteride, etc. They don't understand the full nature of what is occuring however, nor is that the goal. If you design bombs, your interest is in efficiency and the bomb working and being deployable. You are not concerned with a lot of other variable having nothing to do with the efficacy of the bomb. The type of tamper used varies, and sometimes to taste, as does the explanation ad hoc as to why certain efficiencies were reached. That should say all that needs saying.

    Agreed, unless of course we're completely wrong, which as you say is less likely than the existence of a form of matter with which we're not on speaking terms. I think the LHC was starting to get people to appreciate the study of the early universe, until the media discussion was sidelines (immediately) by harbingers of doom and whack-a-do theoreticians.
     
  19. Feb 26, 2010 #18

    jambaugh

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    Well the discussion has out-paced me but I would make a couple of comments.

    Point 1. With respect to cosmological models being simply a matter of "gas dynamics and nuclear chemistry", the thermodynamic assumptions implicit in simple gas dynamics are not the traditional equilibrium thermodynamics. There is significant recent developments into non-equilibrium thermodynamics which may not have filtered into the community of cosmologists. (Might be a good thesis topic!?)

    Point 2. FWIW One shouldn't identify exotic dark matter (e.g. monopoles, super-partners et al) with non-baryonic dark matter which may include non-exotic leptonic matter. If neutrino's have mass then they are indeed WIMPS. Not that this answers the deuterium abundance issues but it does alter some of the assumptions affecting the inferences made.
     
  20. Feb 26, 2010 #19
    Astrophysicists in general work very closely with physicists involved with physicists that specialize in hydrodynamics, non-equilibrium thermodynamics, and plasma physics, and in a lot of areas (such as the interstellar medium), non-local thermodynamic effects are important. However, as far as I'm aware of, no one has been able to come up with an argument that non-local thermal equilibrium effects are important on cosmological scales.

    You can usually figure out whether or not non-LTE effects are important by simple timescale arguments. You calculate the timescale it takes for something to go into thermal equilibrium, and you calculate the timescales that you are looking at. The problem with looking for non-LTE effects in cosmology is that you have billions of years for the gas to go into thermal equilibrium, which makes non-LTE effects hard to justify. It's very hard to get a non-LTE effect while most of the microwave background is close to a black-body.

    Now people have been looking at non-LTE effects in big-bang nucleosynthesis for years, and no one has come up with something that would change things enough to cause a major rethink.

    Also, in doing this sort of work you want to keep your arguments simple if you can. Simple arguments are easier to deal with. If you have gas that sits around for a million years, it's going to likely be in equilibrium so the behavior of said gas is likely to be simple.

    The problem is that you can use some very simple arguments to argue that non-LTE effects aren't important. (Basically, you have billions of years for the gas to reach local thermal equilibrium.) If you can come up with a reason why those arguments are questionable, that's worth an short journal article.

    The problem is that if you look at the question, and then you conclude that there is no way that it will work, then that's not publishable. A lot of science is like mining for gold or digging for oil. It's either there or it isn't, and there is no oil there, it doesn't matter how much you dig. You might find something else (i.e. you are looking for gold and find diamonds). No one has ever been able to figure out how to make deuterium, but people *did* figure out that you can make lithium and beryllium through some of these processes.

    This won't work. The problem with neutrinos is that you can calculate the number of neutrinos from nuclear reactions rates, and for each hydrogen atom, you will have N neutrinos. If the neutrinos have any substantial mass, then the universe would have collapsed billions of years ago. If the neutrinos don't have much mass, then what happens is that they will move energy from dense regions to non-dense regions, and this will wipe out any galaxy structure. It's also that we have limits on the mass of the neutrino, and the experimental limits mean that they don't have enough mass to be WIMPS.

    You can get clever (what happens if the neutrinos decay, well we should so some sort of glow, what happens if the nuclear reactions don't produce the necessary neutrinos, well then that means that certain conservations law don't work and that causes other problems). People have been playing this game for about three decades, and it's pretty settled that if the WIMPS are neutrinos, then there is something basically wrong with our understanding of particle and nuclear physics. Again, it's possible that people have missed something, and after reading up on all of the ideas that have been tried, someone comes up with some new and creative idea how WIMPS could be massive neutrinos, then that's worth a journal article.

    After thinking about new, clever, and creative ideas for thirty years, people just run out of them. Just to use the gold analogy. Maybe someone did miss a spot, but if you go into a mine and you see holes everywhere and no one has struck gold, then you start thinking that maybe there isn't any gold there, and you should look somewhere else.

    And if they are not neutrinos, then there is nothing else in the standard model will work. So if dark matter does exist. it's going to be something we don't understand. Period.
     
  21. Feb 26, 2010 #20
    Point 1 reminds me of attempts to 'construct' a naked singularity (gravitational) in a star undergoing inhomogeneous collapse, add in gas pressure... and yeah, you COULD maybe have that naked singularity.

    This is then held as an example for the victory of the naked over clothed singularity, when really it does nothing to explain much larger Black Holes, or those formed by accretion of a dense body, etc. In other words, very interesting, but unconvincing as a natural principle.
     
  22. Feb 26, 2010 #21
    There are statistical tests that you use in order to calculate the "clumpiness" coefficient. Basically, given a galaxy in location X and Y, what is the probability of seeing a galaxy in location Z. You run that statistical test on real data and against your computer simulations, and this tells you if things fit or they don't.

    It seemed likely in 1983, and I think in 1983, you'd probably have more people lean toward hot dark matter than cold dark matter. It was when people did large scale galactic observations in the late-1980's that hot dark matter started to die, and COBE and WMAP just nailed everything in the coffin pretty good.

    You run experiments and then write computer programs to replicate the results. The point of the computer is just to say, if I assume X, I get result Y. You need result Y in order to see what that means.

    Exactly, that's why they are useful. It's like the hitchhikers guide to the galaxy. We know the answer is 42. The thing we are trying to do is to phrase the question so that we get 42 for an answer. If you do a computer simulation of the sun, you know what the final result should be. If you put in factors X, Y, and Z and you don't get the right answer, you win because you know you are missing something.

    Success means understanding you didn't know before. If the model fails, then you win. Also, if you do get reasonable results out of a toy model, you also win. It's often not obvious what factors are important in the outcome of a simulation and which one's aren't, and if you discover that only one thing matters in the outcome, you win.

    I'm not sure what your point is. Sure you have to have data, but you also have to have theory. Also breakthroughs are far, far overrated. Most science doesn't happen through breakthroughs.

    You need both theory and observation to figure out that you have too much deuterium. You observe that there is this much deuterium in the universe. Theory tells you that if you assume X then you should get Y. The numbers don't much. Also for this particular calculation you don't need a computer as you can calculate this out by hand, which people did in the early-1970's.

    Acoustic peak calculations do require a computer, but they don't require a particularly powerful computer. You can run the necessary calculations via the web

    http://lambda.gsfc.nasa.gov/cgi-bin/cmbfast_form.pl

    No test. I'm just looking at the amount of data coming in, what people are working on, and I'm guessing that by 2025, we should have a much better idea of what dark matter is, because by then we'll have more data on what it isn't.

    I didn't say that it was less likely. What I can say is that given the current state of the data, dark matter is less weird than the alternatives. That doesn't mean that it's more likely.
     
  23. Feb 26, 2010 #22
    You've definitely clarified the point about computer models, and I realize that 'weird' doesn't mean less-likely, just as 'elegant' doesn't mean 'assured'. I should add however, that even in the early 80's WIMPs were by no means trumped by the Heavy Tau, they were just less popular.

    One thing I have to say about breakthroughs, beyond the obvious that they are rare events... they may be rare, but they tend to propel a humanity at an astonishing rate for a while. Centuries or even millenia of dogma or incremental change can be tipped by (usually the same) breakthrough made several times. I say several, becaue repetition is necessary, and because historically (and I'm not talking about 1700+ for physics/math) the first person to make a breakthrough isnt necessasarily able (or in a position) to propogate it.

    One other element of 'the breakthrough' is that you see change in your lifetime. Discover a new method of forming the simple wooden peg (for use in construction pre nails/screws) changed lives in ways that historians are still examining. The combined breakthroughs which led to the Manhatten Project and subsequent events have changed how we live in a century more than humanity has changed since we started to live in primitive city-states.

    So, while the incremental work is a necessity, like the human computers used in 1970 and before to do the grunt calculations for PDEs and such, it is the breakthrough that gives you the cell-phone... it's the incrementalism which gives you the iPad. Ok... that was unfair, and I'm kidding.

    The counterargument I'd make in your case is: Ok, you have a choice: "Wait for a breakthrough to make you immortal, or accept the incremental changes of medicine and live a longer healthier life than your parents, and their parents ad infinitum." I want to make it clear I'm not glorifying breakthroughs more than they deserve, nor am I disparaging the other 99.99999999999999999% of us. I'm just demonstrating an appreciation that even dead ends have to be discovered for some clever di**... I mean genius :smile: to find the right model.
     
  24. Feb 26, 2010 #23
    I need to look at the papers to figure out who said what and when, but the argument that heavy tau particles could not be dark matter (at least not easily) is something that someone could have figured out in the late-1970's.

    One thing that is important about the original CDM papers, is that they emphasize that the identity of the particle doesn't matter. This is important because in 1980, one of the hot theories at the time was supersymmetry which predicts lots and lots of new particles. In 2010, supersymmetry is much less popular because we haven't found any trace of those particles.

    Also I don't think that they are that important in the grand scheme of things.

    My reading of economic history (which is very heavily influenced by Richard C. Allen) suggests that "scientific breakthoughs" aren't that important for economic growth.

    And what you'll find is that science isn't the most important factor in health and medicine. All the high technology advances in the world won't help you if you don't have health insurance, and plumbers and flush toilets have saved more lives than trauma surgeons.
     
  25. Feb 26, 2010 #24
    True... but how many did Pasteur save all told, and his legacy? How about the first 'scientist' who realized that "nightsoil" was unfit as compost? The peg example I gave is a real one by the way, although as a sociological proposal its necessarily vague and messy compared to a 'hard' science.

    True, the everyday work of people, their access to care, nutrition, etc... all critical and definitely = more life-years. That said, the things they do are defined by the incremental gains made on the "the shoulders of giants". Economics can be very interesting, and I might argue that breakthroughs such as double-entry bookkeeping have had impacts that shape the very society you've described some benefits (or not if you don't have them) of.

    The first person to defeat Foot and Mouth Disease, or identify the means by which glia can be 'reset' so as to defeat opiate dependance... or the person who invented crack.... they will or have had MAJOR impacts. A billion people could vanish from the earth and after settling in, life would only improve as scarce resources were shared among fewer people. To me, a breakthrough is the action of one or a group which competes with that relative benefit.

    The defeat of smallpox isn't a breakthrough, but if you consider it an incremental extension of the breakthrough of vaccines... and that from understanding microbiology... you see where I'm headed?

    I'll grant you that what I'm saying is more a matter of what "frame" of history you isolate and examine, or if you do so as a continuum.

    By the way, if you find those papers I've love to read them. I remember reading about WIMPs in some rag of a scifi book when I was young, so it would be pleasantly nostalgiac as well as informative.
     
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