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Looking at this link, CMB Anisotropy, I have two questions regarding the possible creation and properties of dark matter:

If dark matter has left it's imprint on the CMB, that to me, would imply that it was created during the time between the Big Bang and the time of recombination that we see reflected in the CMB.

  1. Are there any papers in the literature proposing a possible mechanism whereby all of the dark matter we that postulate currently exists in the universe was created during that epoch?

  2. That this second question is highly speculative I fully admit. Are there any papers suggesting that, a) as dark matter does not fit into the standard model, as far as I know, and b) because the Big Bang had potentially sufficient energy to create so far unknown particles, that dark matter may be indicative of the possible existence of a fourth generation of particles?

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  • $\begingroup$ As far as I understand the concordance model is that dark matter was created in the very early universe. When is impossible to say at this moment, since we are still lacking any knowledge about the microscopic properties of dark matter. At this point we are still struggling with the microscopic aspects of baryogenesis, although LHC should shed some additional light on that. Dark matter does fit into the standard model, but that's not particularly amazing, we can fit anything into into it that we want, it's simply a collection of phenomenological facts. $\endgroup$ – CuriousOne May 31 '15 at 15:19
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    $\begingroup$ Yes, I do follow that there may be no particular reasons why dark matter is still not being formed today. When you say it fits into the standard model, is it associated with the neutrino-ish side of things? $\endgroup$ – user81619 May 31 '15 at 15:28
  • $\begingroup$ We obviously do assume that dark matter annihilates and scatters on ordinary matter (otherwise it will be extremely difficult to detect!), so that means there have to be non-trivial production cross sections between ordinary matter and dark matter. Is it a neutrino? That's one of the models, but a neutrino assumes already more about the quantum numbers than we know and neutrinos don't show annihilation lines, either. What it is is a wide open question. Why do I say it fits into the standard model? Mostly because there is no divine law for what we can or can not call the SM. It's a choice. $\endgroup$ – CuriousOne May 31 '15 at 15:34
  • $\begingroup$ @CuriousOne The standard model models the "facts" we have up to now. Some guesses for dark matter can fit in the standard model, some require beyond the standard model exensions. It is a mathematical format after all. Not anything can be fit into it. $\endgroup$ – anna v May 31 '15 at 15:57
  • $\begingroup$ I'd just like to comment that a fourth generation of particles would be problematic in some aspects. Since we would expect it to interact gravitationally it would change the evolutionary history of the universe, like nucleossynthesis and the BAO. If I'm not mistaken there are some comments about this in the cosmology chapter of Wald's General Relativity book. $\endgroup$ – Gabriel Cozzella May 31 '15 at 19:06

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