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Dark matter detected? (1 Oct. preprint)

  1. Oct 1, 2003 #1

    marcus

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    Joe Silk, who is world class, is one of the co-authors of this
    preprint

    http://arxiv.org/astro-ph/0309686

    "MeV Dark Matter: Has It Been Detected?"

    the paper is dated 1 October of this year.

    A sharp line of gammaray from near Milkyway center has been observed at 511 keV energy. A clear argument is given in this 4-page paper as to why this 511 keV line may result from decay of an as yet unobserved particle (DM) with mass-energy in the range 1-100 MeV. The argument is fascinating and worth downloading the paper for. I will try to give a simple condensed version for anyone who is interested buy doesnt want to bother downloading the paper itself.

    My first impression is that this could be it. Dark matter was bound to be observed some time---people have been estimating it and theorizing about it and trying to observe it for over a decade now. Dark energy is a different business and probably much harder to understand---it has only been on the table since 1998 and seems considerably more unfamiliar in its characteristics. But Dark Matter is somehow well just this matter that we havent seen yet. And now they have a suggesting 511 keV signal
     
  2. jcsd
  3. Oct 1, 2003 #2
    Sounds fascinating to say the least. This would be a giant step for astrophysics! Lets hope it is what it appears to be!
     
  4. Oct 1, 2003 #3

    marcus

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    what the 511 keV line says

    511 keV is the rest energy of an electron
    and it results from electron-positron annihilation

    but for it to be exact, the two particles cannot be moving much
    they have to be approximately at rest
    or else there is some extra kinetic energy in the picture and the photons will be different from 511 keV

    the decay of a DM particle in the MeV range (say 1 to 100 MeV)
    could produce some positrons that are initially moving but moving slow enough that they could be slowed down by a series of random close encounters with atoms in the dilute gas in the Milky bulge-----Joe Silk and the others calculate that the positrons could be "thermalized" or slowed down to just kind of drifting about after only traveling a parsec or so, in the Bulge.

    The Bulge near center is where they see this sharp 511 keV gamma line coming from. The line is so sharp that it even suggests that it results not simply from approximately-at-rest drifting about positrons and electrons, but that it results from
    positrons and electrons that have actually paired up to make "positronium"! That is, that they are as at rest relative to each other as they can get and are coallescing at rest rather than colliding.

    I've met and talked to Joe Silk and give him a lot of trust. If he and his friends say that quite possibly some DM particle in central Bulge cloud is decaying to make positrons that are coming nearly to rest (still within the central bulge) and annihilating with electrons to make this gamma then I tend to accept it as a very real possibility.

    This paper is rather nicely written.

    I like the way they calculate the stopping distance of a positron
    to be E24 or E25 centimeters
    and they calculate the radius of curvature of a path due to the magnetic field in the Bulge---it is E10 or so centimeters. So the path the thing travels as it is being slowed down by a series of ionizing collisions is a path made of a series of curved arcs
    (arcs with the so-called "Larmor" radius)
    It is accessible and transparent the way the best writing is
     
  5. Oct 2, 2003 #4

    chroot

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    So this hypothetical particle would decay into a positron only? Hmmm.. I haven't gotten my head wrapped around this yet.

    - Warren
     
  6. Oct 3, 2003 #5

    Ivan Seeking

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    from New Scientist

    http://www.newscientist.com/news/news.jsp?id=ns99994214
     
  7. Oct 4, 2003 #6

    Nereid

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    on to other experiments

    This should also stimulate folk to look for other footprints - something overlooked in collider data? Galaxy centre signals in other parts of the spectrum?

    Exciting times ahead. :smile:
     
  8. Oct 4, 2003 #7
    Re: on to other experiments


    I think there is a collaboration with CERN, I have seen relevant data that gives a good acount, no doubt the Comologists are working closely with HEP'ists?

    It may be the the Higgs Potential is what is being observed at our Galactic Core, makes a lot of sense really, take a look at some data here from the euphoria days late 2001.

    http://www-theory.lbl.gov/~ianh/nlc/talks/battaglia2_july16.pdf
     
  9. Oct 4, 2003 #8

    selfAdjoint

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    Well the paper says the 511 KeV signal pretty much limits the DM particles to have masses between 1 and 100 GeV, and that would rule out most people's idea of the Higgs. The authors give ingeneous arguments (based on prior theory) for the particles not being seen in accelerators.
     
  10. Oct 10, 2003 #9

    Nereid

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    IMHO, the last section "Possible Models and Particle Physics Constraints" gave good pointers to how existing data could be used to put some contraints on the parameter space. My guess is that a number of easy experiments, even data mining, based on the suggestions the authors made could quickly lead to further constraints.

    While the galactic bulge is certainly expected to have more dark matter than our quiet suburb, round here dm should be easier to spot. For example, if it's the 511 KeV line that's the marker, a long integration over lines of sight to the Moon, compared with a similar integration 5o away (say) should say something about local densities.
     
  11. Oct 13, 2003 #10
  12. Oct 13, 2003 #11

    selfAdjoint

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    So they have their holeum theory. Has anyone followed it up? It's kind of a stretch.
     
  13. Oct 13, 2003 #12

    Labguy

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    Yes, in the abundance mentioned, it shouldn't be too tough to detect, but it hasn't been done.

    Another consideration of the 511KeV, and the paper's comment that they (electrons, positrons) need to be roughly "motionless", leads me to the thought that (1) since they seem concentrated in our galactic core, (2) since they need to be motionless, (3) since it is known that the core contains a massive BH, (4) since some of the most common particles created by both accretion decay and Hawhing radiation are electrons-positrons, then why can't the 511KeV concentration simply be from activity around the core and have no bearing on "dark matter" at all?
     
  14. Oct 14, 2003 #13

    Nereid

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    IIRC there are no good models of the observed annihilation line emission. A massive BH at the galaxy centre can't be the observed source, it would have to be <1" in size, and the emission is diffuse, spreading over several degrees.
     
  15. Oct 26, 2003 #14
    hi, i'm a newbird but i'm hoping to know what this so call"dark matter"u'r saying is. Could someone pls tell me what that is!!!
     
  16. Oct 28, 2003 #15

    Nereid

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    Today's APOD (Astronomy Picture of the Day)* is a good place to start; the 'dark matter' links are informative, and the other links also good. If you click on Search (at the bottom of the page) and enter 'dark matter' you'll get lots of APOD pages about dark matter. The links should give you many hours of enjoyment and elucidation.

    *http://antwrp.gsfc.nasa.gov/apod/ap031028.html
     
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