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Space Science

Dark Matter's Profile Discovered? 83

pingbak writes "According to New Scientist, astronomers may have potentially discovered dark matter's EM profile (story). For the rest of us, this means astronomers may have just discovered all of the extra force holding the galaxy(-ies) together, which is not currently explainable though gravity and black holes at the center of universes alone. Since dark matter doesn't interact with ordinary matter, it's almost directly undetectable -- but now, physics and astronomy may just have had an awesome breakthrough. Nobel Prize material if it proves correct!"
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Dark Matter's Profile Discovered?

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  • by Randolpho ( 628485 ) on Thursday October 02, 2003 @09:06AM (#7113110) Homepage Journal
    Um... perhaps I'm very much misinformed, which is entirely possible, but the article submission makes the claim that Dark Matter doesn't interact with regular matter.

    WTF? I thought the reason we're looking for Dark Matter is because the matter we *know* about doesn't add up to cause the gravetic interactions that we can observe. I thought Dark Matter was just matter we couldn't observe just yet, not some exotic "doesn't work the same as other matter" matter.

    Am I totally wrong here?
    • by Anonymous Coward
      Am I totally wrong here?

      Probably not. It's a while since I did physics, but these dark matter theories to me sound like the epicyclic theories of the solar system of old. They are too convoluted to be natural IMHO. It's only in the last decade or so that telescopes (including the HST) have given us much insight into what's out there, along with the gamma ray observatories and orbiting infrared telescopes. Heck, we've only just figured out the Solar Neutrino Problem. I don't doubt the particle physicists wi

      • by CheshireCatCO ( 185193 ) on Thursday October 02, 2003 @10:16AM (#7113727) Homepage
        They've looked pretty hard and carefully for normal, baryonic matter to cause the effects. So far, little has turned up.

        On the other hand, it's pretty clear at this point that dark matter in *some* form must exist. It's just a simple grasp of gravity coupled with some weird observations that lead to this conclusion. It is, in fact, very similar to the way Neptune was discovered. First, notice something odd about Uranus's orbit, then realize that another planet at position X could explain it. Just do it with galaxies and clusters, instead, and you start to suspect there's dark matter out there. Do some surveys and find that there doesn't appear to be enough brown dwarfs and black holes to make up the needed mass.

        To be honest, while I'm a planetary scientist and thus obligated to make fun of cosmologists, I don't find dark matter, even heretofore undiscovered particles, that hard to believe. Not only is the evidence pretty good, it isn't difficult to imagine that we've only scratched the surface of what is out there. You suggest that we're just finding "what's out there" (a claim with which I might quibble). So why is hard to believe that we haven't found all of the particles in the subatomic zoo? Especially given that the ones we seek are, by definition, difficult to find.

        And if you want "too convoluted to be natural", study quantum mechanics. It seems the universe doesn't care what we consider to be "natural", after all.

        (And now, a few quibbles: the SNP was only recently really clinched with lab data, but people had speculated about the solution, neutrino oscillations, for quite a while before hand. The same is true of a lot of what HST and others have told us in the past decade: usually, they're helping refine our models and confirm our best guesses as to what's out there. So it isn't like astronomers a decade ago would be shocked at what we've learned.)
        • by SEE ( 7681 )
          In order for Newton's Law of Gravitation to work out with the observed orbit of Mercury, a new planet closer to the sun, generally called Vulcan, was postulated. It was never observed, but it had to be there, because of the gravitational effects.

          Then Einstein showed we didn't understand gravity sufficiently, and his General Relativity eliminated the need for Vulcan.

          In order for Einstein's General Relativity to work out for the observed motions of galaxies, dark matter and dark energy have been postulates
          • Of course that's always possible. That's always possible even when we understand everything as far as we know. That's science.

            But given that precision to which GR has been tested, something like to one part in ten to the eleventh, it's rather difficult for most physicists/astrophysicists to believe that we're so far off in our understanding of gravity. There are alternate models out there that seem to explain the observations without the dark matter, notably MOND.

            But you're given a choice: you're best
          • SEE asked:

            Will someone else come along and show that we don't understand gravity sufficiently, and postulate a theory that will eliminate the need for dark matter and energy?

            It is my impression that general relativity is extremely robust in its connection to a-priori mathematics, that it is very close to the inevitable properties of the richest mathematics which is unified in concept and also not self referential. So, an alternative theory must not, then, disturb this a priori property of Einstein's equa
          • Will someone else come along and show that we don't understand gravity sufficiently, and postulate a theory that will eliminate the need for dark matter and energy?

            When observation does not match theory, then either the observation is wrong, the thoery is wrong, or both are wrong. The observed mass of the Galaxy is not enough to do what is seen, based on current gravity theory. If we assume that the observed mass is wrong, and add in dark matter, everything seems to work out fine. The problem is that

        • Quatum mechanics? So maybe matter is simultaeously light and dark depending on how you measure it?
      • Actually. I think we are the exception. Almost a freak occurrence.

        God's probably having a good laugh.

        We're not made of the same stuff most of the Universe is.

        And many of us seem to think that life out there needs water just because we do, that life can only exist on planets similar to ours...

      • It's a while since I did physics, but these dark matter theories to me sound like the epicyclic theories of the solar system of old. They are too convoluted to be natural IMHO.

        Quite the opposite. Dark matter is one of the most obvious solutions. It would just be another particle out there that interacts with normal matter primarily, if not only, via graviation. Basically, a particle with zero electric charge and also no electroweak charge and no QCD color charge. Just mass.

        That's really not a very

    • by bscott ( 460706 ) on Thursday October 02, 2003 @09:24AM (#7113249)
      What they mean by "weakly interacting" is similar to how neutrinos are described - it doesn't have much of an electromagnetic impression, so it doesn't block light or smack into a detector in an earthbound observatory. Unlike neutrinos, it does posess a significant mass and is affected by gravity. And while that is "exotic", astrophysicists were only forced to consider this sort of thing when all previous efforts to explain some pretty obvious mis-matches in the numbers didn't work.

      Now I'll let someone else explain about "dark energy"...
      • Now I'll let someone else explain about "dark energy"...

        I'm not sure exactly what it is, but it is believed that copious amounts exist in Redmond and at the USPTO.

      • by Anonymous Coward
        Dark Energy (IIRC, IANAP, etc): A property of empty space that causes it to expand. Current theories suggest the universe will have a cold death instead of a heat death because unlike gravity, dark energy doesn't fall off at a distance. So, everything in the universe that isn't close enough to be held together by gravity is being accelerated away from everything else.
    • If it was ordinary matter, it ought to make stars, or smash into other bits, and then we'd see it. When they say "weakly interacting", that includes gravity but not electromagnetics or nuclear forces.

      The assumption that these photons have anything to do with dark matter, though, has more to do with fashion and funding than actual science. It's cool and helpful to have your new observation associated with something everybody's already keen on. What they do know, though, is that whatever's producing the

    • There are in fact many dark matter problems. This work deals with galactic dark matter only, which could be solved by normal matter. This is not the case for dark matter problems on larger scales.

      On scales larger than galaxies, we can see that galaxies and groups of galaxies appear affected by stronger gravitational forces than can be accounted for by visible matter, and on very large scales it appears that there is more gravity than can be accounted for by ordinary matter, period.

      There are strict limits
    • Scientific theories bounce back and forth between two possible fates for the universe. One possibility is that it expands forever, and all usable energy fades away into useless entropy. The other possibility is that the universe eventually collapses back into a singularity, and everything will explode back into space and time, ad infinium.

      Do the new revelations about dark matter now prove that there is sufficient mass (both regular matter and dark matter) to cause the universe to collapse back into a si

      • Do the new revelations about dark matter now prove that there is sufficient mass (both regular matter and dark matter) to cause the universe to collapse back into a singularity?

        Scientists for a few years have had a pretty good idea of the mass of the universe. That's why dark matter was postulated, the mass of the universe is vastly more than the mass that we see. Thus, finding dark matter is most likely just going to fill in the mass we suspect we should have instead of adding any additional mass to the
    • Dark matter doesn't exist. There is a small handful of galaxies that are eliptical that doesn't display the dark matter property.

      The simple existance of these galaxies rules out dark matter..

      Simon.

      • So how exactly do these exceptions disprove entirely dark matter? There may be something else going on in these rare eliptical galaxies, but do we really know enough about it to say dark matter can't exist anywhere because of them?
  • I, for one, welcome our new dark matters. Er, masters.
  • Profile (Score:5, Funny)

    by mopslik ( 688435 ) on Thursday October 02, 2003 @09:28AM (#7113279)

    In other news, dark matter's IM profile has also been found:

    Name: Matter, Dark
    Nick: d4rkm4tt3r
    Age: ~15 billion years
    Likes: Vast emptiness of the cosmos.
    Dislikes: Peeping-Tom astronomers.
    Bio: I generally keep a low profile, out of sight. Maybe one day, the matter of my dreams will see me for who I really am.

  • by bill_mcgonigle ( 4333 ) on Thursday October 02, 2003 @09:48AM (#7113426) Homepage Journal
    So, two leading theories on Dark Matter are:
    • it's a particle that's all around us, but has very strange energy levels and we've never detected it.
    • there's no dark matter, just gravity intruding into our 4D universe from other branes, which we can't detect and can barely imagine.

    Gotta love it. :)
  • by rsdavis9 ( 262908 ) on Thursday October 02, 2003 @09:52AM (#7113475)
    http://www.astro.queensu.ca/~dursi/dm-tutorial/dm0 .html

    From the link above there is:
    1. cold dark ordinary matter(baryonic)
    2. Non baryonic(exotic) dark matter both hot and cold

    The article seems to indicate wilp(weakly interacting light particles instead of (in addition to) wimps(weakly interacting massive particles. Wilp's are like neutrinos. We have not discovered any wimps yet.
  • Uh, how many universes are there?

    I didn't even know they'd found the center of ours.

  • Electronium? (Score:4, Interesting)

    by 4of12 ( 97621 ) on Thursday October 02, 2003 @10:05AM (#7113618) Homepage Journal

    IANAHEP, but is there anypossibility that an electron and a positron could orbit one another with a reasonably long half-life?

    • Re:Electronium? (Score:5, Interesting)

      by barawn ( 25691 ) on Thursday October 02, 2003 @11:06AM (#7114282) Homepage
      Positronium.

      It has a half-life of 0.1 uS. It's a relatively standard physics problem at the graduate school level to ask what the binding energy of positronium is.

      If it ever comes up, it's (1/2) the binding energy of a hydrogen atom. The reasoning is simple - a positron and a proton have the same charge, but a positron and an electron have the same mass, so the "reduced mass factor" is 1/2, rather than 1. (M_p/(m_e+m_p) ~= 1) vs (M_e/(m_e+m_e) = 1/2).
    • It's called positronium [wolfram.com], and it lasts 10^-10 seconds.
      • Crap, somebody beat me by a minute and with the right halflife, 10^-7 seconds. The halflife from Wolfram is incorrect.
        • Which leads to the inevitable conclusion: A 1x10-7 second here, a 1x10-7 second there, sooner or later we're talkin' real time!
        • Yah, someone with one letter difference in name. That's quite frightening.

          Actually, you're correct, and incorrect.

          If the spins are antiparallel, the half-life is 10^-10 seconds (because they have a higher wavefunction overlap, since the total spin is 0). If the spins are parallel, the half-life is 10^-7 seconds (less overlap, so the phase space is smaller).
  • I'm defniitely no expert on particle physics, but couldn't this just be another particle like the neutrino, only much more massive? What arguments would there be against this?
    • In the conclusions they appear to be saying that some new interaction is happening due to ('mediated by') exchange of a light gauge boson (translation: low-energy force-carrying integer-spin-particle)

      Alternatively a new heavy fermion (neutrinos are fermionic, spin-1/2) mediates in the interaction: their words "could be responsible". So you might not be far off (if there second guess is correct).

      Start talking Nobel prizes when CERN/Fermilab find either of these particles.

      [... I've not done any particle ph
    • There are several different observations of dark matter. If a spiral galaxy were like the Solar System, the stars on the fringes would orbit much more slowly than stars near the core. It turns out that spiral galaxies tend to spin like a freakin' phonograph record, which means there must be a lot of unseen mass in the spherical halo in which the spiral arm disk is embedded. Than you have galaxy clusters which tend to mill around much faster than the visible mass accounts for -- if those galaxy clusters a
      • Finally, Inflation Cosmology predicts that the Universe must have enough mass to keep it from flying apart forever, and that is even more mass.

        Inflation cosmology predicts that omega = 1 - that is, there's exactly the critical density in the universe. It does not predict that it's all matter, or what it is at all.

        Hence the reason we've got 70% dark energy, and 30% normal matter. Exactly the critical density, but it doesn't mean that we won't fly apart forever - if dark energy continues its wonderful neg

  • > Dark Matter

    Isn't the source of that somewhere in Redmond...???

  • Good luck 'proving' it correct. Though a very sound theory may come out of the whole thing.
  • http://www.arxiv.org/PS_cache/astro-ph/pdf/0309/03 09686.pdf [arxiv.org] ... this must be a draft there's a typo in their ackowledgments (I checked all the equations and they look OK though ;0)>
  • /. already knew this (Score:3, Informative)

    by darkpurpleblob ( 180550 ) on Thursday October 02, 2003 @12:15PM (#7115017)
    Dark Matter's profile. [slashdot.org]
  • I don't understand, unless my understanding of what "dark matter" is is seriously wrong. I thought that dark matter was simply non-light-emitting matter. Plannets, dust, rocks, the like. Not stars. And the problem is that we can't easily monitor dark matter, because it isn't emitting energy.

    All this "not interacting with regular matter" business comes off as completely strange to me.
    • Re:I don't get it (Score:4, Interesting)

      by Royster ( 16042 ) on Thursday October 02, 2003 @12:56PM (#7115471) Homepage
      There have been two leading candidates for dark matter: WIMPs and MACHOs. Each camp have had their proponents.

      WIMPS: Weakly Interacting Massive Particles. Neutrinos on steriods. Since they only interact through the weak and gravitational forces, they are by definintion dark in EM. But, we haven't found any in colliders.

      MACHOs: MAssive Cosmic Halo Objects. You're describing MACHOs. Ordinary, cold, dark matter. But there's probably too much of it to be this. It should have been swept up into stars.

      Frankly, I think that the energy levels detected will prove to be not what we're seeking here. It's too much of a coincidence that it is the e/e-bar annilation energy. OTOH, if there were a WILP which did have such a mass, we'd probably never see it thinking we were looking at e/e-bar reactions.
      • Frankly, I think that the energy levels detected will prove to be not what we're seeking here. It's too much of a coincidence that it is the e/e-bar annilation energy. OTOH, if there were a WILP which did have such a mass, we'd probably never see it thinking we were looking at e/e-bar reactions.

        Actually the article says they DO think they are seeing e/e-bar annilations (A.K.A. electrons and anti-matter electrons).

        I'm kind of sceptical about this particular story. Maybe their full research paper goes into
        • The problem I take it is that if they are e/e-bar collisions they are essentially at rest with respect to us the observes rather than e/e-bar collisions at an appropriate orbital speed for their distance from the galactic center. The energies should be spread from the e/e-bar value by an amount which represents the dispersion of velocities rather than a sharp peak.

          I assumed they meant by that that they were seeing some other anniliation with a different mass.

          Plus, there's no reason for there to be this nu
          • I assumed they meant by that that they were seeing some other anniliation with a different mass.

            According to the article, yes, but indirectly. It suggests an unseen dark-matter annilhilation which generates an essentially motionless e/e-bar pair that then generate the radiation they see.

            Plus, there's no reason for there to be this number of e/e-bar anniliations in these locations unless there's some weird physics going on.

            Yes, THAT is perfectly reasonable and worth investigating :) It's just their jum
        • The reasonning is:
          1. the repartition of the e/e-bar annihlations follows the mass repartition, not the repartition of the visible matter.
          2. they don't know what mechanism of ordinary matter could cause these e/e-bar annihlation, so they associate it with dark matter.

          I agree that this second hypothesis seems a bit risky, but it doesn't seems unreasonnable too, so calling it a "miracle step" is a bit too far IMHO: by definitions miracles are unreasonnable :-)
          • "then a miracle occurs" was a refference to a rather famous science cartoon. [sciencecartoonsplus.com]

            When reffering to someone else's work the phrase carries a specific implication that the final result is possibly correct or even probably correct. It just indicates there's an important gap in the middle. Even if the result happens to be correct, the calculation/logic is worthless until the gap is filled in.

            When a teacher or expert uses the phrase themselves in an explanation to a student/non-expert the implication is that it re
  • we were forced to consider a surprisingly light dark matter particle.

    Unfortunatly, all progress has come to a halt while physicists conduct the 'tastes great/less filling' debate. With opinions split nearly 50/50, this could take a while.

  • I thought that dark matter was supposed to exist not only at the center of galaxies but in a "halo" surrounding them. This dark matter halo would help explain the unusual rotational properties observed in most galaxies. If that is the case, did the researchers also see this signiture in the region of the supposed halo? If they did, that observation would indeed be interesting. If they only see it at the galactic core then I'm not convinced that they have identified a reliable indicator of dark matter's
  • See Mordechai Milgrom's modified Newtonian dynamics for an alternate explanation .
  • It's really heavy, smells really bad and a very small pellet can power interstellar spaceships.

Every little picofarad has a nanohenry all its own. -- Don Vonada

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