Dark matter detection

  1. I hope this is in the right section of the forum. I figured astrophysics and cosmology could also be acceptable places to ask this.

    I was watching Lawrence Krauss's talk about everything from nothing, the one with the richard dawkins introduction. He talked about methods for trying to detect dark matter, and he mentioned this one method where germanium atoms were cooled down to a fraction of a degree above absolute zero. If the dark matter particles collided with the germanium atom it would raise the temperature and therefore could confirm the existence of a dark matter particle....or something along those lines.

    My question is though, if dark matter is able to go all the way through the planet without stopping(Krauss said this), and if the particles could go throughout our body even undetected, then how would it "hit" a germanium atom?

    I really know almost nothing about particle physics, so I'm sure theres a simple answer.
  2. jcsd
  3. phyzguy

    phyzguy 2,491
    Science Advisor

    The simple answer is that, while almost all of the dark matter particles go through the Earth, and through the detector, if you have a big enough detector, look very carefully, and wait long enough, eventually one of the dark matter particles will collide with a germanium nucleus and generate a detectable event. At least that is the hope. The hard part is separating out the few possible dark matter events from all of the natural background (natural radioactivity, cosmic rays, neutrinos, ...). There are quite a few detectors of different types around the world patiently looking to try and see dark matter events, but no confirmed events yet...

  4. Is it generally accepted that physicists believe dark matter would most likely be a subatomic particle? I mean I can see how this could be a possibility, but are there any other major experiments in progress that are hypothesizing dark matter to be something completely different than a particle?
  5. Weakly-Interacting Massive Particles (WIMPs) are the most popular candidates, since they're thought to exist for reasons outside of cosmology. Extensions of the Standard Model of particle physics, for example, have such particles.

    Another possibility are MAssive Compact Halo Objects (MACHOs). These are large (but mostly invisible) objects in galactic halos made up of ordinary matter. Efforts to directly detect these objects (through gravitational lensing, for example) have put serious constraints on MACHOs. Judging by the number of articles I see on the arXiv, the WIMPs are beating up on the MACHOs at the moment.

    Another possibility that has largely fallen to the wayside is MOdified Newtonian Dynamics (MOND), but observations of the bullet cluster struck a violent blow to that hypothesis, though there are still adherents.
  6. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Another contender in the particle category is the axion. The axion is a byproduct of Reccei-Quinn symmetry, which was proposed to solve the strong CP problem in particle physics.
  7. Nabeshin

    Nabeshin 2,202
    Science Advisor

    Most of the current effort is in confirming/denying the existence of the dark matter subatomic particle. Within the next 5 years we should know (or have placed very tight constraints!) on the existence of such a particle. Certainly within the next ten. People will become concerned and start devising serious experimental tests of other theories if these fail (but such experiments are comparatively easy to perform, so we pick the low hanging fruit first :)
  8. Remember that we really don't know what dark matter is, so much of the game involves trying to figure out what something is by figuring out what it isn't. One guess is that dark matter is some sort of unknown massive particle that interacts with matter through the weak interaction, and there is a class of theories that predict those particles. If you don't see those particles then it's not those particles and those theories are wrong. Here's a interactive plotter for detection limits...


    Something about dark matter is that there is so much of the stuff that you'd expect pretty large numbers of particles going through the earth.

    Also, there are known particles that behave in this way. When supernova 1987A went off, the earth was hit by a massive blast wave of neutrinos in addition to the rather large but constant neutrinos that the earth gets hit by the sun. We know that we got hit by a blast wave because there were a dozen neutrino detections within a few seconds of each other.
  9. No. We are just guessing.

    One problem with the other ideas about dark matter is that the theories aren't detailed enough for us to figure out what we are looking for. For example, MOND's just attempts to curve fit rotation curves to a weird gravity potential, without explaining what why those potential might be different.

    However, experiments have already put some limits on alternative to WIMP's. One alternative idea is that dark matter consists of large numbers of black holes, brown dwarfs, neutron stars, or something like that. These are called Massive compact halo object or (MACHO's).

    You can detect MACHO's by looking at the flicker of background stars as one passed in front of them. The experiments that have been done over the last decade have pretty conclusively established that MACHO's can't make up most of the dark matter.

    Also, there is another set of experiments that limit what dark matter could be. A lot of the explanations for dark matter involve "weird gravity." There are a ton of experiments that indicate that gravity doesn't behave weirdly at solar system distances, so if you have a theory of weird gravity, then you have to take that into account.

    Personally, I'm hoping that the WIMP detections find nothing. At that point we have a real puzzle, so this involves stepping back, looking at were we've looked, and thinking about what we've missed.
  10. phyzguy

    phyzguy 2,491
    Science Advisor

    The other strike against MACHOs is that the Big Bang Nucleosynthesis (BBN) models say pretty conclusively that dark matter cannot be made up of ordinary matter(protons and neutrons). If it were, then during the Big Bang much more Helium would have been produced through nuclear fusion than we observe.
  11. I think the big constraint is deuterium. See


    Helium-4 tends to be rather insensitive to the initial baryon concentration, whereas if you have lots of baryons then the deuterium burns out a lot faster, which means you have less of it.

    And then there is galaxy correlations and CMB fluctuations (i.e. lots of nails in the coffin).
  12. phyzguy

    phyzguy 2,491
    Science Advisor

    Your right of course. I oversimplified - my bad. Here's a nice graph showing the impact of more baryonic matter on the elemental abundances. As you said, the Deuterium concentration crashes quickly if the dark matter were baryonic.

    Attached Files:

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  13. Also, I find stuff like big bang nucleosynthesis much more interesting than string theory. You can take some math and physics that isn't that much more complicated than what you need to figure out what happens when you mix two chemicals in a chemistry set, and figure out the composition of the universe.
  14. zonde

    zonde 1,641
    Gold Member

    I would say that you can go for quite some time detecting nothing. It more like depends from viable alternatives at hand.
  15. To avoid natural radioactivity, cosmic rays, neutrinos...the detectors were deep under the ground. They detected 'something' after 1 year...
    Last edited: Dec 20, 2010
  16. Last edited: Dec 20, 2010
  17. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Considering how long it took for us to confirm the existence of neutrinos, dark matter is a worthy adversary. I think it will take us another 20 years to dig this devil out of the details. Astrophysicists have virtually proven it exists. It's up to the particle physicists to do the taxonomy part. What can I say? Astrophysics is easy, particle physicists get stuck with all the homework.
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