Tuesday, October 6, 2009

Be Prepared For Posts On Dark Matter.

I am not finished with all evidence for the Big Bang but I decided to start another series on dark matter. I will still finish the Big Bang stuff but, to be honest, as a blogger you have to write about the things most on your mind at the moment otherwise your posts begin to reek. (Sorry if they already do.)

First I want to talk about what the evidence is for dark matter.  Specifically I want to discuss:
  1. Galactic rotation curves.
  2. Clusters and lensing.
  3. The power spectrum of the Cosmic Microwave Background.
  4. The formation of large scale structure.
  5. The famous Bullet Cluster collision.
I then want to go over the best candidates for dark matter:
  1. Neutrinos
  2. The lightest supersymmetric particles (LSP).
  3. Axions
  4. Machos
  5. Wimps in general
Just to preempt future posts, dark matter is almost assuredly real.  In fact, the picture above  is the famous Bullet Cluster collision where dark matter was extracted from the rest of the visible matter.  The blue region you see is the gravitational lensing of the dark matter which has separated from the visible baryonic (normal) matter shaded red.

Though we don't know exactly what dark matter is, we know it is some type of matter that is effectively collisionless and doesn't have a lot of kinetic energy (It moves slowly).  Through numerical simulations we know if we could see it, on really large scales, we would see this:

What is amazing about this picture is this is exactly the cob-web pattern we see galaxies forming into in the real universe.  This is evidence that the dark matter, more than anything else, governs how large scale structure such as clusters and galaxies form.

Anyways, I have skipped over all the details but just know future posts will make up for it.  Dark matter is cool, and if nothing else you will see interesting pictures/plots and have any questions you are will to ask answered. (So please ask.)

Therefore, please start leaving questions so I know what needs to be covered specifically.


  1. What darkness will the LHC shed on this matter?

  2. Stan,

    I will give the short answer now, but will try to work in more details later.

    None of the particles of the standard model can be dark matter. (With the possible exception of the neutrino but for a variety of reasons that seems very unlikely).

    We know, for various reasons, that there has to be new physics at the 1 TeV scale which isn't covered by the standard model alone. The hope is that this new physics, which will be explored at the LHC, will give off signatures of the types of particles that make good dark matter candidates.

    If the light supersymmetric particles can be created at 1 Tev or something like the axion could be detected then we will immediately know what the dark matter probably is.

    However, in many ways the most promising dark matter experiments are not those associated with the LHC.

  3. By the way, if subpersymmetry is detected at all the odds are overwhelmingly that dark matter is the lightest and or next to lightest supersymmetric particle.

    The reasons is a ton of particles at high energies would decay into these guys who cannot decay further for symmetry reasons nor interact with the standard model particles save through gravity. They would be floating around in really high abundances minding their own business. Hmmm...

  4. This is an understanding coming from the actual system. Otherwise is dark matter the same thing as dark energy, because in your last comment you did write that dark matter is the lightest or next to lightest supersymmetric particle or you wanted to write that from dark matter is...?

  5. Cartesian, good question. Dark matter cannot be the same thing as dark energy. Dark matter, like all matter, has the property that it tries to pull things toward it. First, this makes it clump up. Second, given enough dark matter the entire universe would collapse being pulled together by the dark matter.

    Dark energy does not seem to clump up, as far as we can tell, and has the opposite effect: it pushes the universe apart.

    Now, I know this sounds crazy: using words like "dark this" and "dark that" and assigning bizarre properties to them. But believe me, not only is there experimental evidence for this stuff, theoretically you should expect this stuff to be their even if you never took a measurement.

    Dark energy is what arises if you don't set Einstein's famous cosmological constant to zero. Naively, you should be skeptical why such a constant should work out to be exactly zero. As for dark matter, both with neutrinos, supersymmetric candidates and other theoretically appealing things you might naively expect to see something like dark matter.

    Now that's just to clear up confusion: I didn't try to say dark matter was the lightest supersymmetric particles. Only that if supersymmetry exists the odds are overwhelmingly in its favor.

    We have no idea what dark matter is other than: it is composed of collision-less matter that doesn't seem to have a lot of kinetic energy.

  6. Interesting. I was actually about to finish working on a post I had about dark matter which explains why people who claim they have found the silver bullet for explaining rotation curves without dark matter have not 'solved' dark matter. Even disregarding rotation curves, dark matter is a component of many theories and observations in many realms of our universe: lensing, galaxy mergers (bullet cluster), CMB, and power spectrum (BAO).

    I look forward to hearing what you have to say about the best candidates for dark matter which is a topic I know considerably less about. I understand there is a sort of standard model for relic DM particles that have self annihilated to the present equilibrium density; is this correct, and could you give a solid description of this model?

    Also, what about models of DM where DM particles have others forces that are not included in the standard model?

  7. The Astronomist, I'm excited to read this given you do good work.

    I will try to answer your questions in future posts adequately. I guess short answers are: Yes, mainstream theory is particles decayed into present day dark matter. (I will try to give sufficient details.)

    And second, there is a growing amount of literature on the idea that dark matter may have forces not effecting other particles. This, one would hope, would give rise to some new decay or cross section or something that would show up in some experiment. However, as you probably know, given this stuff seems so collision-less there have to be large constraints on such forces. If they heavily interacted with each other they should not have so freely passed through each other in the Bullet Cluster collision.

  8. The problem is that the particles about which you are relating seem to not be matter, but energy.
    See :

  9. I was searching some other information and came across your post.Its interesting and so i would like to know more about it i will keep visiting often.


To add a link to text:
<a href="URL">Text</a>