Slashdot Mirror

← Back to Stories (view on slashdot.org)

Dark Matter's Profile Discovered?

Posted by michael on from the dark-visions-in-daylight dept.

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!"

66 of 83 comments (clear)

  1. Re:Electrons? by KDan · · Score: 1, Insightful

    Yes, electrons are rather unlikely to be at rest at the centre of the galaxy. I could go into a long explanation of why this is so, but that would be wasted on you wouldn't it?

    Daniel

    --
    Carpe Diem
  2. Dark Matter Explaination? by Randolpho · · Score: 4, Interesting

    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?

    --
    "Times have not become more violent. They have just become more televised."
    -Marilyn Manson
    1. Re:Dark Matter Explaination? by Anonymous Coward · · Score: 2, Interesting
      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 will throw up some surprises eventually, but my hunch is that we still have a lot to discover with space-based telescopes across the whole range of the electromagnetic spectrum. This is wht orbiting gamma ray, x-ray, infra-red and optical observatories are important.

    2. Re:Dark Matter Explaination? by CheshireCatCO · · Score: 5, Interesting

      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.)

    3. Re:Dark Matter Explaination? by TheLink · · Score: 1

      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...

      --
    4. Re:Dark Matter Explaination? by radtea · · Score: 3, Informative

      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 on the amount of ordinary (baryonic) matter that come from primordial nucleo-synthesis. In the early seconds after the Big Bang, the quark-gluon plasma cooled down to form baryons (protons and neutrons, to you.) Eventually (some seconds or minutes later) the baryons cooled down to the point where their average energy was low enough that when they ran into each other they tended to stick together rather than flying apart again.

      The nuclei formed in these early times were: hydrogen (single protons), deuterium (p+n), 3He (p+n+n), 4He (p+p+n+n) and tiny amounts of lithium and above. As you can imagine, the ratios of hydrogen to everything else are critically dependent on the density of neutrons at the time these nuclei were forming: the more neutrons, the more heavy nuclei. We can infer from observations on stars fromed shortly (~millions of years) after the Big Bang what these ratios were, which tells us the density of neutrons and protons at that time. We also know the volume (curvature) of the universe at that time, and so can infer the total number of baryons in the universe.

      There are not enough baryons to account for the gravitational (or gravity-like) forces acting at the largest observable scales. Ergo, something else is happening (or the observations are wrong, which is always a possibility.)

      Given the number of dark matter problems, it is unlikely that one particle will solve them all, and quite possible that we are seeing the effects of quite different causes at different scales. This should not be a surprise. For example, we already see very different causes at nuclear scales vs. atomic scales vs planetary scales. It is possible that on galactic and size-of-the-universe scales different causes are at work as well, either due to hitherto undiscovered forces or equally unknown particles.

      --Tom

      --
      Blasphemy is a human right. Blasphemophobia kills.
    5. Re:Dark Matter Explaination? by SEE · · Score: 2, Interesting

      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. They've never been observed, but they have to be there, because of the gravitational effects.

      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?

    6. Re:Dark Matter Explaination? by Ckwop · · Score: 1

      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.

    7. Re:Dark Matter Explaination? by CheshireCatCO · · Score: 1

      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 model of gravity -- which predicted gravitational radiation, black holes, and a host of other extremely counter-intuitive phenonomenon -- is wrong or a large fraction of the universe isn't visible to you. Which do you chose? To most of us, the latter seems more likely.

      Remember, it wasn't just the perihelion shift of Mercury wasn't Einstein's only GR prediction. He made 3 of them, and it wasn't until the second was observed (the bending of light near the Sun observed during an eclipse in 1920, I believe, by Sir. Arthur Eddington) that people really bought into the theory. (The third was gravitational red-shift, or time dialation. This has been observed, but only significantly later.)

    8. Re:Dark Matter Explaination? by fluid_amp · · Score: 1

      Quatum mechanics? So maybe matter is simultaeously light and dark depending on how you measure it?

    9. Re:Dark Matter Explaination? by mburns · · Score: 1

      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 equation of gravity. (Thinking of general relativity as a-priori mathematics is equivalent to the Einstein-Davis theory that the classical universe is made out of curved spacetime.)

      This same equation of gravity also yields the conservation laws, even of other fields and forces, as a theorem. So, an alternative theory must not disturb this property of the theory of gravity.

      Alternative theories are very severely restrained, I think, by these considerations.

      --
      Mike Burns

      --
      Michael J. Burns
    10. Re:Dark Matter Explaination? by amRadioHed · · Score: 1

      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?

      --
      We hope your rules and wisdom choke you / Now we are one in everlasting peace
    11. Re:Dark Matter Explaination? by Mt._Honkey · · Score: 1
      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 (excepting this paper) searches for dark matter have thus far failed. If we assume that the theory of gravity is wrong, one can try to cook up new thoeries that make everything work out for the Galaxy. The problem is, or so a dark matter researcher told me, that so far those theories don't correctly explain what happens in groups of galaxies.

      So the end result will be that there is dark matter, general relativity is wrong or flawed, or some combination of the two. Observation, experiment, and theoretical work will decide the issue, that's what science is all about.
      --

      Don't Bogart the fish sticks
    12. Re:Dark Matter Explaination? by ekuns · · Score: 1

      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 convoluted idea.

      There are compelling reasons why baroyonic matter (ie, normal matter) cannot be more than a small percentage of the mass of the universe. The rest of the mass of the universe must be made up of SOMETHING! It's not just that dark matter is DARK, or non-glowing, and so difficult to detect. We can detect gas clouds just fine even when they do not glow. There are compelling reasons to believe that there is a lot of mass in the universe which does not interact electromagnetically at all.

      Of course, it's possible that we simply misunderstand how gravity works at very long distances. I am so far unimpressed with the theories I've seen for modified gravity.

  3. These are the jokes, folks... by GR|MLOCK · · Score: 2, Funny

    I, for one, welcome our new dark matters. Er, masters.

  4. Re:Electrons? by BigBir3d · · Score: 3, Informative
    I am sure that this is more due to the writer's lack of understanding of the subject, then to what appears to be grand jumps in logic. I am somewhat sure the language of the paper will be much more clear.
    However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy.
    Previous statement makes no sense until it is explained later that they started down the course of thinking dark matter has a mass far less then previously postulated.
    "Heavy dark matter particles would produce high-energy electrons," says Hooper. "Since it's difficult to imagine how they could be slowed to a standstill, we were forced to consider a surprisingly light dark matter particle."

    By "light", the researchers mean one to 100 megaelectronvolts, which is between 1000 and 10 times lighter than a proton.
  5. a little from column A, a little from... by bscott · · Score: 4, Interesting

    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"...

    --
    Perfectly Normal Industries
    1. Re:a little from column A, a little from... by SpaceLifeForm · · Score: 2, Funny
      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.

      --
      You are being MICROattacked, from various angles, in a SOFT manner.
    2. Re:a little from column A, a little from... by Anonymous Coward · · Score: 1, Interesting

      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.

  6. Profile by mopslik · · Score: 4, Funny

    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.

  7. Re:Electrons? by kalidasa · · Score: 1

    Somebody feel like recommending a book on dark matter that can catch the rest of us up to about 1999 or so?

  8. Gotta love Astro/Quantum physics by bill_mcgonigle · · Score: 2, Interesting
    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. :)
    --
    My God, it's Full of Source!
    OUTSIDE_IP=$(dig +short my.ip @outsideip.net)
    1. Re:Gotta love Astro/Quantum physics by Jerf · · Score: 2, Funny

      Yeah, gotta love it. These are the same people who snort at "intelligent design" but then join SETI@Home because they think they can empirically identify intelligence...

      The entrophy value for your message is much, much lower then the maximum it can have for that length. The probability of that occurring by natural chance is very low.

      When you figure out how those two sentences are related to your post, you'll understand the SETA@Home people. Enjoy learning. Warning: May be mind expanding.

    2. Re:Gotta love Astro/Quantum physics by Jerf · · Score: 1

      Hmm. Like the low entrophy value of a pulsar maybe?

      You may find this enlightening.

  9. good description of the kinds of dark matter by rsdavis9 · · Score: 4, Informative

    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.

    1. Re:good description of the kinds of dark matter by Anonymous Coward · · Score: 2, Funny
      We have not discovered any wimps yet.
      KERNEL PANIC -- Joke potential overload -- cannot continue
  10. center of universes? by Rick+the+Red · · Score: 1
    Uh, how many universes are there?

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

    --
    If all this should have a reason, we would be the last to know.
    1. Re:center of universes? by CheshireCatCO · · Score: 1

      And more to the point, how do they find the centers? (Given that there isn't such a creature and all.)

      I suspect that they meant "centers of galaxies".

    2. Re:center of universes? by Red+Rocket · · Score: 3, Funny


      And more to the point, how do they find the centers?

      You just keep licking them and you eventually get to the centers.

      --
      - Hail to our fearless misleader! Fool speed ahead!
    3. Re:center of universes? by damien_kane · · Score: 1

      You just keep licking them and you eventually get to the centers.

      Or you could just be impatient like the rest of us a bite... now, who here has a mouth big enough to bite down on the universe in its entirety?

    4. Re:center of universes? by ZerroDefex · · Score: 1

      Ozzy could do it, he can do anything.

    5. Re:center of universes? by dustmote · · Score: 2, Informative

      According to my friend the physicist, every point is the center, from that point's frame of reference. (I should point out, of course, that he's a relativist, rather than dealing too much with quantum scales)

      --


      -1, "1337" speak
  11. Electronium? by 4of12 · · Score: 3, Interesting

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

    --
    "Provided by the management for your protection."
    1. Re:Electronium? by barawn · · Score: 4, Interesting

      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).

    2. Re:Electronium? by barakn · · Score: 1

      It's called positronium, and it lasts 10^-10 seconds.

      --
      "I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
    3. Re:Electronium? by barakn · · Score: 1

      Crap, somebody beat me by a minute and with the right halflife, 10^-7 seconds. The halflife from Wolfram is incorrect.

      --
      "I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
    4. Re:Electronium? by zangdesign · · Score: 2, Funny

      Which leads to the inevitable conclusion: A 1x10-7 second here, a 1x10-7 second there, sooner or later we're talkin' real time!

      --
      To celebrate the occasion of my 1000th post, I will post no more forever on Slashdot. Goodbye.
    5. Re:Electronium? by barawn · · Score: 1

      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).

  12. Re:Dark Matter Explanation? by Euphonious+Coward · · Score: 2, Interesting
    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 photons is distributed like the galaxy's mass is, not like the visible stars are.

  13. Neutrino's Big Cousin? by jafuser · · Score: 1

    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?

    --
    Please consider making an automatic monthly recurring donation to the EFF
  14. Dark Matter by jo42 · · Score: 1

    > Dark Matter

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

  15. Re:Electrons? by Anonymous Coward · · Score: 2, Informative

    Some references...astronomy.net and the referred to article

  16. Nobel Prize by phlyingpenguin · · Score: 1

    Good luck 'proving' it correct. Though a very sound theory may come out of the whole thing.

  17. Re:Electrons? by pbhj · · Score: 1

    Well I'm sure there are enough astrophysicist and theoreticians of one bent or another ... gives us the gist ...

  18. the draft version of their paper by pbhj · · Score: 2, Informative

    http://www.arxiv.org/PS_cache/astro-ph/pdf/0309/03 09686.pdf ... this must be a draft there's a typo in their ackowledgments (I checked all the equations and they look OK though ;0)>

  19. Neutrino's Big Cousin -- conclusions by pbhj · · Score: 3, Interesting

    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 physics for 5 years so this could be baloney.]

  20. Re:Electrons? by KDan · · Score: 4, Informative

    There are many reasons, but one which you might look at is the large amounts of radiation coming from the galactic nucleus. As you may know, electrons absorb radiation and gain energy (velocity) when they do so.

    Another explanation, if you take away the radiation, would involve a huge fermi sphere of electons which would require that only very few of them are sitting in that big gravity well with no kinetic energy.

    There are other reasons why electrons would be very unlikely to be found at rest at the galactic core, but I think these will do.

    Daniel

    --
    Carpe Diem
  21. /. already knew this by darkpurpleblob · · Score: 2, Informative

    Dark Matter's profile.

  22. I don't get it by 0x0d0a · · Score: 1

    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.

    --
    May we never see th
    1. Re:I don't get it by Royster · · Score: 3, Interesting

      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.

      --
      I have discovered a truly marvelous sig, unfortunately the sig limit is too small to contain i
    2. Re:I don't get it by Alsee · · Score: 1

      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 more detail, but from the article I see reason for the leap to dark matter. I have no objecting to dark matter theories in general, I just don't see the link here. What they are actually seeing/reporting is the signal of ordinary electrons and anti-electrons colliding, then they have a "then a miracle occurs" step linking it to dark matter.

      -

      --
      - - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
    3. Re:I don't get it by Royster · · Score: 1

      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 number of e/e-bar anniliations in these locations unless there's some weird physics going on.

      --
      I have discovered a truly marvelous sig, unfortunately the sig limit is too small to contain i
    4. Re:I don't get it by Alsee · · Score: 1

      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 jump to dark matter that strikes me as very very weak.

      -

      --
      - - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
    5. Re:I don't get it by renoX · · Score: 1

      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 :-)

    6. Re:I don't get it by Alsee · · Score: 1

      "then a miracle occurs" was a refference to a rather famous science cartoon.

      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 really is correct. For example a math teacher might use the phrase to skip over an ugly calculus step when talking to pre-calculus students.

      -

      --
      - - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
  23. Unfortunatly by sjames · · Score: 1

    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.

  24. I'm a little confused by zejackal · · Score: 1

    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 presence.

  25. Dark Matter is Today's Ether by Detritus · · Score: 1

    See Mordechai Milgrom's modified Newtonian dynamics for an alternate explanation .

    --
    Mea navis aericumbens anguillis abundat
  26. Kinds of dark matter by Latent+Heat · · Score: 1
    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 are gravitationally bound, there must be even more hidden mass. Finally, Inflation Cosmology predicts that the Universe must have enough mass to keep it from flying apart forever, and that is even more mass.

    The hidden mass could be weakly interacting particles like neutrinos, which have either zero or very small rest mass, but have mass just like photons when they are zinging around according to E=mc^2. It could be stuff like frozen stars and rogue planets roaming around. The trouble with neutrinos is they are "hot" dark matter -- they contribute to gravity, but they are zinging around that they cannot congregate in a galaxy halo to get the phonograph record effect. The trouble with dark stars and rogue planets is 1) they are not turning up enough in searches, and 2) there are theoretical limits on the amount of baryonic matter (protons and sh*t made from protons like planets and stars and people and stuff) on account of the Big Bang and the abundance ratios of light elements.

    The $60,000 dollar question is whether cold dark matter exists -- particles much more massive than a neutrino so they aren't zinging around so much that they can congregate in galaxies, but non-baryonic (i.e. some form of matter unknown to physics) but also weakly interacting like neutrinos (really hard to detect).

    I always thought the notion that galaxies have strange orbital mechanics and that low and behold there is mystery matter was kind of bogus, but fellow Slashdotters tell me that MOND (a theory that there is no dark matter but instead a fudge factor in Newton's law at large distances) is weak against the data, and hey, the neutrino, a bookkeeping term to account for missing momentum in nuclear reactions was kind of made-up too, but apparently they can be detected.

    This business of the electron-positron energies is also an indirect observation just like the galaxy as a phonograph record, but it is a different data point, and hey, it may lead somewhere.

    1. Re:Kinds of dark matter by barawn · · Score: 1

      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 negative-pressure inducing fun, we most decidedly will fly apart forever. That being said, not having a damned clue what the hell dark energy is, we can't predict anything at all.

      The $60,000 dollar question is whether cold dark matter exists -- particles much more massive than a neutrino so they aren't zinging around so much that they can congregate in galaxies, but non-baryonic (i.e. some form of matter unknown to physics) but also weakly interacting like neutrinos (really hard to detect).

      The parent's statement (massive neutrino) is not totally nuts. In fact, it's actually quite possible. We know that there are 3 neutrinos, and only 3 neutrinos (there are experiments in particle physics which directly give the # of neutrinos), but as far as I know, that's energy-range dependent- you have to conserve lepton number (and family lepton number) so if there is a "super-tau-neutrino", it wouldn't appear in particle accelerators until they could generate the energy of a "super-tau". If there's a 4th generation of matter, and that neutrino is ridiculously heavy (MeV-ish), that could make up all the lost matter.

      That being said, it'd take a bit to figure out exactly *how* you make it so that it doesn't oscillate into other neutrinos, but the CKM matrix is a bit ad-hoc, so it's possible, I guess. Just not likely, at the moment. Besides, everyone loves axions and supersymmetric particles. Stuff like "gauginos" and "neutralinos" are too fun to say.

    2. Re:Kinds of dark matter by topynate · · Score: 1

      Neutrinos do oscillate into other neutrinos, don't they? (http://supernova.lbl.gov/~evlinder/umass/neu.html ) And that means they must have a (small) rest mass. This doesn't make up the missing matter entirely, but I think the figure is 20%.

  27. Thats nothing. Dark matter appears to be alive by WindBourne · · Score: 1

    http://slashdot.org/~darkmatter/

    --
    I prefer the "u" in honour as it seems to be missing these days.
  28. Key Question: Collapsing or Expanding Universe by reporter · · Score: 1
    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 singularity? Will SlashDot appear again in the new universe that emanates from the next Big Bang?

  29. We already knew it by sharkey · · Score: 1

    It's really heavy, smells really bad and a very small pellet can power interstellar spaceships.

    --

    --
    "Outlook not so good." That magic 8-ball knows everything! I'll ask about Exchange Server next.
  30. Re:Key Question: Collapsing or Expanding Universe by amRadioHed · · Score: 1

    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 universe. Anyway, so far as I've seen a cold death is still the most likely end to our universe.

    --
    We hope your rules and wisdom choke you / Now we are one in everlasting peace
  31. Cold Dark Matter by Latent+Heat · · Score: 1

    The deal is that "they" (those astrophysicists who worry about spiral galaxies revolving like phonograph records) need cold Dark Matter -- even neutrinos with mass are zinging around too much to be roped in to galactic halos.