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View Full Version : How Important is CDM is the new candidate for cosmology warm dark matter?



mikeg64
2012-Mar-30, 10:00 PM
I heard an interesting presentation today suggesting that cold dark matter is insufficient to explain the distribution of dwarf galaxies. To explain the structure of the observable universe the prime candidate is warm dark matter.
CDM seems to do quite a good job for explaining the structure and distribution of galaxies but now it fails with dwarf galaxies.

A couple of questions.
The observed power spectra from cosmological simulations indicate a preference for WDM rather than CDM, is this correct? As my background is not cosmology I'm having difficulty in understanding this interprettation of the power spectra.

e.g. see
The power spectrum dependence of dark matter halo concentrations. (http://dro.dur.ac.uk/1530/)

surely we're not ruling out CDM (it explains the clumping of galaxies quite well)?
Particle physics hasn't indicated that the candidate is CDM or WDM why can it not be both, after all there
is a range of excitations or supersymmetric candidates or QHD (http://arxiv.org/pdf/1201.2512.pdf) excitations, technicolour excitations etc.... which could form the WDM or CDM candidates.

A couple of links (one quite fun)
Build a universe with CMB (http://map.gsfc.nasa.gov/resources/camb_tool/index.html)
Lambda CMD (http://en.wikipedia.org/wiki/Lambda-CDM_model)

antoniseb
2012-Mar-30, 10:16 PM
... surely we're not ruling out CDM (it explains the clumping of galaxies quite well)?...

No, CDM is observed. By cold, we simply mean gravitationally bound to galaxies and clusters. A significant amount of DM is CDM. There is also HDM (neutrinos and perhaps other stuff), which is far too energetic to be gravitationally bound. Maybe there is something in the middle, and perhaps that stuff can help explain the distribution of dwarf galaxies better than without it in our simulations.

mikeg64
2012-Mar-31, 10:41 AM
CDM is observed but indirectly ... the claim is that it does NOT explain the structure of the observable universe.
Distribution of dwarf galaxies.... and evidence from simulations indicate that CDM is insufficient, HDM ruled out ( back in the 80's and as you say it is too energetic to be gravitationally bound)

Why is CDM insufficient to explain this more detailed structure?

Your response indicates that there is a spectrum of DM candidates not just CDM but WDM as well?

antoniseb
2012-Mar-31, 01:31 PM
CDM is observed but indirectly ... the claim is that it does NOT explain the structure of the observable universe.
Distribution of dwarf galaxies.... and evidence from simulations indicate that CDM is insufficient, HDM ruled out ( back in the 80's and as you say it is too energetic to be gravitationally bound)

Why is CDM insufficient to explain this more detailed structure?

Your response indicates that there is a spectrum of DM candidates not just CDM but WDM as well?

Let's be clear. We have observed where and how much CDM is bound to quite a few galaxies and clusters. This has been done through weak lensing, where we observe the distortion of the shape of distant background elliptical galaxies. This observation is backed up by observation of the xray temperature of intracluster gasses (plasma), and by the intracluster velocities of member galaxies, and in some cases by strong lensing observations of well-placed background galaxies. This is pretty solid information. We know how much Cold Dark Matter there is. The whole issue with Warm Dark Matter is that it is being suggested by modelers as a way to explain their inability to get the right number of dwarf galaxies. Are their models right? Are their initial conditions and assumptions right? Frankly, that is less certain than our observations of Dark Matter... Now, I'm not saying that the CDM we see and the normal matter we see, and the Dark Energy we think we see are enough to cause there to be fewer dwarf galaxies formed than what the modelers come up with. However, I AM saying that the models we're talking about are about event in the first billion years of the universe, and a lot of what they are starting with in their models is speculative.

I am also saying that if their assumption about Warm Dark Matter proves correct, it was a factor more than 13 billion years ago, and doesn't even have to exist today. CDM is here now, regardless of the number of dwarfs that haven't yet been absorbed into larger galaxies.

publiusr
2012-Apr-01, 08:37 PM
Warm dark matter--can we use it for a blankie? ;)

mikeg64
2012-Apr-02, 11:28 AM
Thanks for the explanations about CDM.

One of the things I did not understand were these curves presented by the modellers in which we observe that the spectra for HDM falls away quickly and does not produce the required long range structure?
16663
http://archive.ncsa.illinois.edu/Cyberia/Cosmos/Images/PowerSpectrum.jpg

You have cited many convincing lines of evidence for (C)DM, I would be uneasy about claiming that it has been observed. The body of evidence enables us to infer it. I remember an experiment as an undergraduate in which we made measurements of galaxy rotation curves, tedious but I remember being quite amazed by the results.

mikeg64
2012-Apr-02, 11:30 AM
Personally I prefer CDM ( which is chocolate here) :p