CDM vs WDM

I have been a fan of CDM(Cold Dark Matter) since I learned about dark matter and and it's particle interpretation. By "Cold" it simply means non-relativistic at very early stage of the Universe ( = when it decoupled from the thermal equilibrium and got frozen whereas other standard model particles were still in equilibrium). Their mass can vary from a few GeV to TeV. People who love supersymmetry, would always bet for CDM since supersymmetric models naturally provide a CDM candidate without any ad-hoc assumptions. R-parity, a discrete symmetry which is ad-hoc in Minimal Supersymmetric Standard Model (MSSM), but naturally arises in gauge theories where B-L is a part of the gauge symmetry (Left-Right Symmetric Models for example) makes the CDM perfectly stable. Thus it can satisfy both cosmological relic density constraints as well as structure formation bounds.

Although CDM has been taken most seriously among other Dark Matter interpretations in the last 2-3 decades, there is no experimental evidence yet which can confirm CDM over others. And this slightly discouraging fact demands us to be a little open minded. I recently read a paper by Manfred Lindner et al (Physical Review D 2010) where they have talked about Warm Dark Matter in Left-Right Models (LRSM). Warm means slightly lighter than CDM (mass of the order of say keV) and hence they could be relativistic for a long time. And the most natural candidate is sterile neutrino which naturally arise in LRSM as a part of the Right handed doublet. But since, they are considering non-SUSY models, the stability of sterile neutrino is not guaranteed and hence they have to fit the parameters in such a way that the life-time of the lightest sterile neutrino exceeds the age of the Universe. Although this does not look elegant like in SUSY models, but this is a good alternative and it does not cost you too much. You have far less free parameters compared to SUSY models and it will be easier to rule out or confirm such models in the experiments. In this paper, the authors are focusing more on how the keV scale sterile neutrino DM can satisfy the cosmological bounds as well as neutrino oscillation data. And they have shown it does, although with some undesirable features like one active neutrino becomes too light ( 9 orders of magnitude smaller than 1 eV, but still allowed from neutrino oscillation data).

Apart from cosmological and neutrino oscillation bounds which are of course the most crucial test for a dark matter candidate, there should be a way to actually observe it. The ongoing direct detection experimental results, I don't think we can fit with a keV sterile neutrino. The indirect detection experiments like Positron excess will be even harder to fit with such WDM candidate. But in any case these indirect detection experimental results have other astrophysical explanations and the various direct detection experiments don't agree with each other. Even if LHC gives clue about such keV sterile neutrino, we still wont be able to say if it is the true dark matter candidate or not. We still have to rely on direct detection experiments, provided all of them agree with each other. Anyway as G. Bertone commented in one of his recent review (to be published in Nature), we can not keep on proposing more and more experiments to search for dark matter endlessly, if we do not get some positive signal in coming few years, may be there is a need of paradigm shift and we need to look for alternative scenarios. The worst such alternative to me would the MOND theories. They look so ugly to me that I wont ever be able to believe Nature surrendered herself to them!