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Why is the dark matter: baryonic matter ratio about 5:1?

  1. Jan 5, 2010 #1


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    All the LCDM models with variations (see http://lambda.gsfc.nasa.gov/product/map/current/parameters.cfm ) have similar ratios of dark matter: baryonic matter density of very close to 5:1.

    Is this ratio determined from WMAP data such as angular size of a peak, or is it a not very tightly constrained variable or estimate taken perhaps from galactic rotation speeds?
    Last edited: Jan 5, 2010
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  3. Jan 6, 2010 #2


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    Our best estimate of the ratio between normal matter and dark matter comes from measuring the relative amplitudes of the first and second acoustic peaks in the CMB data. The basic idea is that before the emission of the CMB, the normal matter formed a plasma, which experiences pressure. When the normal matter fell into a potential well during this time, it would bounce back out. By contrast, the dark matter just falls in and stays there.

    This has the effect of making it so that the more dark matter you have, the more the even-numbered peaks are suppressed relative to the odd-numbered ones. We haven't been able to accurately measure the peaks beyond the second just yet, so nearly all of the constraint comes from just the first two.

    So, in the end, because the ratio of normal matter to dark matter is more-or-less directly constrained by the data, it doesn't vary much at all when you vary the model parameters.
  4. Jan 6, 2010 #3


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    An independent measurement is made by estimating the total (baryonic + dark) needed for the observed gravitational effect, while independently estimating baryonic matter total from the relative amounts of H1, H2, and He4 in the universe.
  5. Jan 6, 2010 #4
    The nuclear physics behind this is interesting.


    It turns out that He4 is very stable. You form protons and neutrons at a certain temperature. Almost all of the neutrons combine with the protons, and you end up pretty much the same amount of He4 no matter what happens. This is important because if we did see very different amounts of He4 in the universe, this would call into question the whole big bang picture. In fact one of the nice features of the BB is that it explains why there is so much He4 in the universe.

    The key element is deuterium. Deuterium is very fragile. The denser the universe, the more deuterium gets burned and the less of it sticks around. Since deuterium is fragile, the amount we see is the lower limit for the primordial abundance and you can go from that to the density of baryons.
  6. Jan 7, 2010 #5


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    The really interesting (and unknown) question is what physics is responsible for giving us the ~5:1 ratio? Of course we don't know what dark matter is, let alone how it formed/s so we can't answer this yet (and of course it remains possible that dark matter doesn't even exist).

    This is one of the tricky things that comes up when people discuss if say the Large Hadron Collider will 'find dark matter'. To convincingly do so, you not only have to discover some new particle (or particles) but you also have to really understand very well the production mechanism and probabilties such that you could include the appropriate 'dark matter synthesis' stage into our understanding of the early universe. This would take a lot of work, both in the lab as well as theoretical computations. Even if the LHC finds something promising, it will take a lot of collaborative work between particle physics, astrophysics and cosmology to work out the details to the point where we could really say that this new particle 'is' dark matter.

    It would be an exciting process to watch/experience/be part of though. Let's keep our fingers crossed!
  7. Jan 7, 2010 #6


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    I should add that most candidate models for dark matter come along with a production mechanism. By large, we only don't understand how dark matter is produced because we don't yet know what it is. When we have a good idea as to the particle properties of dark matter, we're likely to also have a good idea how it is produced.
  8. Jan 7, 2010 #7
    The fun thing about LHC (and the reason it's worth spending several billion dollars on) is that it will be extremely interesting if we run it for a few years and find nothing. It would be a lot like when Michelson-Morley did their experiments expecting to study the nature of ether, and found nothing.

    One thing that's useful about LHC is that it turns out that dark matter doesn't obviously fit into any of the known particles of the standard model. Even if it turns out that the LHC doesn't find the mystery particle, if it finds any new particles that will be extremely useful since it may turn out that dark matter is the found particle's cousin. By contrast, if we run LHC for a few years and find no new particles, that will also be interesting.

    The other thing is that there are lots of experiments that are being done which involve trying to detect the interaction between dark matter and normal matter. As with the LHC case, finding nothing puts limits on either how much dark matter there is or how much it interacts with normal matter so finding nothing is very, very useful.
  9. Jan 7, 2010 #8


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    Indeed, last I heard the DAMA collaboration (based in Italy) were still claiming to have detected dark matter in their direct detection experiment, although this is not generally an accepted result. They find a signal in regions of parameter space that all the other experiments have ruled out. I haven't heard much about it recently though.
  10. Jan 7, 2010 #9


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    There's been another potential detection by the CDMS collaboration:

    Basically they have two events that might possibly have been dark matter impacting their detectors, with about a one in four chance that they are background events. Certainly nothing to write home about, but potentially interesting. More data please!

    I should mention that the LHC is a really really really poor instrument for detecting dark matter. It is possible that it might detect the particle that makes up dark matter if said particle's properties are just right, but not very likely.
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